Addressing Challenges for Eco-Friendly and Sustainable Wastewater Treatment Solutions Using Extremophile Microorganisms

Abstract

The pressure on the environment from wastewater has been increasing in line with industrialization and urbanization, thus calling for better and eco-friendly solutions for wastewater treatment. Extremophilic microorganisms, which can grow in extreme conditions including high salinity, acidity, and temperature, can be applied in wastewater bioremediation. This review assesses the various functions of extremophiles, halophiles, thermophiles, alkaliphiles, and acidophiles in the treatment of organic and inorganic pollutants. They are capable of catabolizing a wide range of hazardous chemicals, such as polycyclic aromatic hydrocarbons, phenolic compounds, and heavy metals. Moreover, extremophilic microalgae, like Galdieria sulphuraria, have been effective in nutrient removal, biosorption of heavy metals, and pollutant conversion into valuable biomass. This dual-functioning, therefore, helps not only in wastewater treatment but also in the production of biofuel and biofertilizer, making the process cost-effective. The use of extremophiles in biofilm reactors improves pollutant removal, with less energy input. Extremophilic microorganisms can, therefore, be used to revolutionize wastewater management by providing green solutions to current treatment approaches. This review discusses the existing drawbacks of wastewater treatment along with the additional requirements needed to enhance the capability of bioremediation and potential future research.

1. Introduction

The increasing population of the world, intertwined with intensive agriculture and swift industrial expansion, has led to a marked surge in urban wastewater, presenting a serious environmental challenge to mankind. Many developing countries grapple with a lack of drinking water, often stemming from subpar wastewater treatment or the discharge of hazardous effluents into the rivers [1]. However, according to a WHO report, 2.2 billion people could not access clean drinking water in 2022, especially in countries with dense populations, like India, Kenya, Ethiopia, and Nigeria, which face alarming issues related to access to clean drinking water due to inadequate wastewater treatment and the discharge of hazardous effluents [2]. Water bodies near urban and industrial zones are often contaminated with a blend of organic and inorganic matter, including micropollutants, heavy metals, and an excess of nutrients, arising from industrial, agricultural, and residential waste [3,4]. Wastewater from various sources carries a plethora of contaminants, ranging from industrial by-products and pathogens to heavy metals and pesticides, each with well-documented effects on human health [5]. The use of fertilizers and pesticides poses another significant source of contamination. In urban areas, commercial activities, mining operations, and waste disposal sites further contribute to the global wastewater burden [6]. This results in waterborne diseases, such as diarrhea, which account for approximately 1 million deaths per annum, and Shigellosis causes the deaths of 150,000 individuals each year [7].

For wastewater treatment, various physical and chemical methods are used. Among these methods, reverse osmosis [8,9], UV disinfection [10,11], and coagulant methods [12] are widespread. During reverse osmosis, a membrane filtration is used that operates by exerting pressure to force water through a semi-permeable membrane [13]. This method effectively removes contaminants, such as dissolved salts and organic matter [14]. In contrast, the coagulant method uses different chemical coagulants, such as aluminum sulfate [15] and iron salts [16], for water treatment. The UV method is a chemical-free disinfection technique that inactivates many microorganisms, though its effectiveness can be reduced by water turbidity [17].

Apart from these, biological methods, such as the activated sludge process [18,19], biological nutrient removal [20,21,22,23], and the membrane bioreactor [24], play an important role in wastewater treatment. The activated sludge process involves proper aeration in order to mix microbes and facilitate the breakdown of organic matter [18]. However, biological nutrient removal targets the reduction of nutrients, like nitrogen and phosphorus, in wastewater using microbes [25,26]. It prevents nutrient pollution by converting nitrogen compounds into nitrogen gas and phosphorus in order to precipitate. However, the membrane bioreactor incorporates biological treatment with membrane filtration for enhanced solid–liquid separation [24]. Microorganisms treat wastewater in a biological reactor and membranes help to separate solids from treated water. The wastewater is released from industries creating extreme conditions, such as high salt concentration and low pH [27,28,29]. To counter this, extremophilic microbes are effective, because these microbes can survive in extreme physiochemical conditions [30]. The extremophiles have the property of efficacy in addressing different types of wastewater, ranging from industrial effluents to agricultural runoff. Their unique metabolic pathways and enzymatic activities allow them to enzymatically transform or degrade pollutants [31]. There are different types of extremophiles, such as acidophiles, alkaliphiles, halophiles and thermophiles, which are considered effective in bioremediation [29,32].

These extremophiles, through their highly specialized metabolic functions, play a pivotal role in the remediation of contaminated water bodies by breaking down and transforming specific pollutants into less harmful or non-toxic forms. Halomonas species and other extremophiles show special metabolic functions that help them survive harsh conditions while they clean up pollution. These microorganisms produce specialized enzymes that withstand high salt levels and assist in breaking down organic contaminants, such as hydrocarbons and chlorinated solvents. By synthesizing exopolysaccharides and metal-binding proteins, these microbes help neutralize heavy metals from the environment as they trap hazardous metals, like lead, cadmium and chromium. These microbes protect their cell processes by making compatible solutes, including ectoine and trehalose, which keeps them working in salt-heavy environments. The special ways in which extremophiles process materials make them essential for treating saltwater waste and environmental toxins under tough conditions.

For instance, extremophiles thriving in saline environments, such as Halomonas spp., demonstrate remarkable potential in treating saline wastewater. These microorganisms possess unique enzymatic pathways and physiological adaptations that enable them to tolerate high salt concentrations while metabolizing organic pollutants, heavy metals, or other hazardous substances typically found in such environments [33]. This effectively reduces the concentration of dissolved salts, such as cadmium, zinc, lead, nickel, and associated contaminants [33]. Industrial mining is the producer of highly acidic wastewater. For treatment, Acidithiobacillus is an acidophilic bacteria capable of oxidizing sulfur compounds. Furthermore, the ability to oxidize sulfide minerals and generate acidity can contribute to the remediation of acidic mine waters [34,35].Alkaliphilic bacteria, like Natronobacterium and Thioalkalivibrio spp., treat wastewater by breaking down organic pollutants, oxidizing sulfur compounds, and facilitating ammonia removal under high-pH conditions. They also precipitate heavy metals by altering pH, making them effective in remediating industrial effluents [36,37,38].

Among industrial wastewaters, palm oil mill effluents are a significant source of concentrated organic toxic and nitrogenous compounds. The pollution from palm oil mill effluents needs special treatment methods because these liquids contain high levels of organic and nitrogenous chemicals. Ferroplasma acidiphilum and Ferroplasma acidarmanus species excel at cleaning harmful substances in acidic wastewater environments. Their special metabolic properties, such as iron processing and the capacity to break down complex organic materials, make them perfect for treating POME and similar wastewater effluents. These microbes show us how to handle industrial wastewater with difficult chemical properties [37]

The genus Ferroplasma has demonstrated excellent potential in degrading these pollutants. Species like Ferroplasma acidiphilum and Ferroplasma acidarmanus are particularly effective in treating acidic wastewater environments, making them promising candidates for the remediation of such industrial effluents [39].

On the other hand, Galdieria sulphuraria has a natural tolerance against high temperatures and hot climates [40]. This genus has been reportedly used for treating municipal wastewater using the hydrothermal liquefaction method [32,41]. Moreover, Dunaliella salina, Galdieria sulphuraria, Cyanidium caldarium, and Chlorobium tepidum are examples of microalga that have been used in the treatment of wastewater [42,43,44].

In this review, we discuss the potential of extremophiles to survive in extreme conditions and explore their applications in the treatment of wastewater from diverse sources. Our hypothesis suggests that extremophiles, with their unique adaptations to extreme conditions, offer promising prospects for efficient, low-energy, and sustainable wastewater treatment strategies. This review, through a comprehensive analysis, aims to shed light on the multifaceted roles of extremophiles, addressing their survival capabilities and their relevance in contemporary wastewater treatment practices.

2. Methodology

2.1. Literature Search and Data Collection

Research over the last 50 years (1973 to 2023) is discussed on the utilization of extremophilic microorganisms for wastewater treatment. A systematic literature review was conducted in Scopus. Search terms were “extremophiles”, “wastewater bioremediation”, “halophiles”, “thermophiles”, “alkaliphiles”, “acidophiles”, and “microalgae in wastewater treatment”. These were chosen to reflect studies of the adaptation mechanism, metabolic pathways, and biotechnological applications of extremophiles in the treatment of municipal and industrial wastewater.

Moreover, studies of biodegradation of organic pollutants by extremophiles, the removal of heavy metals from wastewater, and overall purification by extremophiles constituted the inclusion criteria. Studies that were not concerned with wastewater treatment, or which did not offer experimental data on extremophilic application, were excluded. We then analyzed trends in keywords, publication volume, country contributions, and collaborative research networks by applying this refined dataset.

2.2. Data Analysis and Visualization

Major research themes and their relationships were identified using keyword co-occurrence analysis performed using the VOS viewer software (Version 1.6.20), R studios and Excel. In Figure 1, we proved that the terms ’wastewater treatment’, ’microbial community’, and ’bioreactor’ were at the center of the network, implying that the body of publications in this field is themed together. In addition, we identified distinct groups of bacterioplankton associated with extremophilic adaptations (e.g., halophiles, thermophiles) or operational parameters (e.g., chemical oxygen demand, temperature). Insights into focus areas in extremophilic wastewater bioremediation were obtained from this approach.

Data from the publications were also analyzed to identify the leading institutions, countries, and primary published journals within the field. As seen in Figure 2, the major affiliations are Wageningen University, leading with 52 affiliations, followed by Chongqing University at 47, and Aalborg University at 40, which contribute most of the publications in this research area.

Figure 3 presents the process of scientific production, which is the process of generating, disseminating, and using scientific information. It includes a range of elements, such as financing, authorship, and research impact, and shows the geographic distribution of publications, in which China, the United States, Japan, and Korea are among the top countries in publication volume due to their industrial applications and regional environmental needs. China leads the list with 732 publications followed by Japan with 331, USA with 305 and South Korea with a 189.

2.3. Geographic and Institutional Contribution to Research

A citation analysis was performed to evaluate research impact and collaboration. China, Germany, and Japan emerged as the most frequently cited countries in this field (see Figure 4). China demonstrated a high publication output and significant citation impact, which has grown steadily since 2015, reflecting the country’s investments in environmental biotechnology. China led with 4153 citations, followed by Germany with 3037 and Japan with 2979.

Figure 5 shows that the publications have been categorized into single country publications (SCP) and multiple country publications (MCP). Strong SCP output was observed in the United States, Japan and Korea; it is believed that European countries, like Germany and the Netherlands, use a high proportion of MCPs pointing towards active international collaboration in research.

2.4. Analysis of Publication Trends in the Time

Temporal publication trends (Figure 6) are analyzed to understand the evolution of research interest in extremophilic wastewater treatment, revealing a significant number of publications in recent years, primarily from China. The increasing upward trend, on the other hand, is indicative of the shift in global perspective towards sustainable and innovative strategies in wastewater management, against rising environmental pressures.

3. Extremophilic Microorganisms in Wastewater Treatment

Extremophilic microbes survive under very harsh conditions. They are generally found in environments that are unfavorable for most life forms, including high salinity, low pH, high temperature, and acidic areas. These are of different types, i.e., halophiles, alkaliphiles, thermophiles, and acidophiles. Extremophiles are considered a possible source of wastewater bioremediation due to their specificity and versatility [45]. There are some extremophilic algae which can degrade numerous pollutants, such as organic material and heavy metals due to their exceptionally high metabolism and adaptability, hence making them valuable options in wastewater treatment [44]. The subsequent sections consist of a description of the various categories of extremophiles and their relation to wastewater treatment.

3.1. Acidophilic Microorganisms

Acidophiles are a major group of extremophiles that naturally thrive in low-pH environments. They are categorized based on their optimal pH. There are four classes of acidophiles: (i) extreme acidophilic organisms that effectively grow at a pH < 3, (ii) moderate acidophilic organisms that grow at a pH range of 3–5, (iii) acid-tolerant species that can tolerate pH conditions > 5 and (iv) hyper-acidophilic organisms that thrive in environments with a pH close to or below 1 [46]. Some characteristic mechanisms enable acidophilic microorganisms to survive in very acidic conditions. They have highly impermeable cell membranes to prevent further influx of protons into the cell and transporters that expel the excess protons present within the cell. Moreover, acidophiles carry buffering chaperone proteins within the cytoplasm to protect their DNA and proteins from acidic environments. Many acidophiles possess flagella with which to move in response to chemical signals and can form biofilms to enhance survival in acidic environments [47] (Figure 7). Aside from their ecological versatility, acidophiles are characterized into different types, like chemosynthetic or photosynthetic depending on the light intensity (Galdieria sulphuraria or Cyanidium caldarium), heterotrophic, which metabolize organic chemicals (Acidiphilium and Leptospirillum), and lithotrophic, which are dependent on an inorganic compound as carbon source (Acidithiobacillus species) [47].

Acidic pH is widely varied in both natural and anthropogenic environments [48]. Acidophiles are frequently found in geothermal sulfur-rich acidic environments, e.g., Acidianus sulfidivorans from Lihir Island, Papua New Guinea, Sulfolobus solfataricus from Yellowstone National Park, and Sulfolobus hakonensis from Hakone, Japan, but they also thrive in human-made environments, such as acid mine drainage (AMD).

Conversely, human activities, such as mining operations and industrial processes, contribute considerably to the development of acidic environments. In abandoned mines, microbial activity leading to the dissolution of pyrites and other sulfides results in significant amounts of AMD. Such metal-rich, acidic drainage is a major contributor of water pollution [49]. These waters cannot support most life forms, but acidophilic microorganisms can thrive in such environments. Acidophilic microorganisms are adapted to a low pH environment and have a narrow range of substrates. The rate of growth is slow and they are generally sensitive to changes in pH; thus, any significant change in pH can result in reduced activity or even the death of the microbes. In some wastewater, toxic compounds or heavy metals may inhibit the growth of these microorganisms. Some metals, such as copper or lead, when present at excessively high levels can prove toxic to these microorganisms [50]. Optimal operating conditions for acidophilic microorganisms are often specific and hard to achieve, particularly in large-scale treatment plants. Hybrid systems can integrate acidophilic microorganisms with other types of microorganisms in a multi-stage treatment process; thereby, the spectrum of pollutants can be degraded over a range of pH values. For instance, in a two-stage system, acidic waste could be handled by acidophilic microorganisms, while neutral or basic components could be treated by other microorganisms. Advanced hybrid systems could be defined by their significant use of advanced monitoring and control technologies to manage environmental conditions for acidophilic and other microbial species involved in the treatment process [51]. These sensors, for example, might monitor pH, temperature, and oxygen levels, and real-time feedback loops could utilize these parameters to maintain optimal environmental conditions [52].

These microorganisms display wide physiological and phylogenetic diversity across bacteria, archaea, fungi, and the alga Chlamydomonas acidophila. Examples include Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, Acidithrix ferrooxidans, Ferrimicrobium acidiphilum, Acidiphilium, Acidocella, Acidicaldus, and Acidomonas [53,54,55,56,57].

3.1.1. Acidophilic Microorganisms for Organic Wastewater

Acidophilic bacteria have an excellent ability to degrade numerous organic pollutants present in the acidic environment. They contain specialized metabolic systems that degrade toxic chemicals, like phenol, benzene, and toluene, into less harmful by-products. The research in [58,59] discussed an acidophilic bacteria, Acidithiobacillus ferrooxidans, that can break down polycyclic aromatic hydrocarbons (PAHs), which are harmful organic compounds found in coal tar and petroleum [60], also confirming that this strain has the ability to biodegrade low and high molecular weight PAHs, such as anthracene, phenanthrene, naphthalene, pyrene, and benzo (e) pyrene at a pH of 2 [59].

Acidophiles are highly effective at degrading phenol and its derivatives, which are abundant in nature, having a carcinogenic property. The authors of [61] have described acidophilic yeast strains, such as Rhodotorula, which are used for the partial degradation of phenol. [62]. Moreover, [62] obtained a Zymomonas mobilis 1 (ZM1) strain from areas endemic for AMD, observing that ZM1 reduced phenol concentration by up to 1100 mg/L within 120 h, with a pH of 3 [62]. Similarly, the archaeon strain Sulfolobus solfataricus 98/2 works effectively at high temperature of 80 °C and a low pH of 3.2. It quickly metabolizes phenol into biomass and CO2 [63].

Acidocella species, such as WJB-3, play a role in wastewater treatment. They use the aliphatic hydrocarbons as a carbon source [64]. The yeast Candida digboiensis TERI ASN6 in oily sludge-polluted soils can grow and degrade n-alkanes in the presence of TPH in media with a pH of 3.0 [65]. Due to this property, they are used in oil-polluted acidic territories to detoxify oil pollution.

Currently, advanced genetic engineering of acidophiles has contributed to optimizing wastewater treatment and neutralization processes. Researchers have genetically modified Pseudomonas putida S16 strains to produce genes for glutamate decarboxylase (GAD) and irrigator E (IrrE) to enhance the efficiency of wastewater treatment [66]. These strains can decrease concentrations of nicotine and benzoate by up to 50% over 2 days of treatment at a pH of 5 [66]. Many acidophile species contribute to the biodegradation of a wide range of organic pollutants, including phenols, PAHs, and hydrocarbons, as discussed in

Table 1. Examples of different acidophiles in organic pollutant treatment.

Domain	Acidophiles	Organic Pollutants	Degradation %	Duration	References
Bacteria	Acidocella sp. WJB-3	Aliphatic acid (dodecanoic acid)	85%	7 days	[67]
	Pseudomonas putida S16(genetically modified)	Benzoate	90%	5 days	[66]
	Acidosphaera sp. C197	Dodecane, Hexadecane	75%	10 days	[68]
	Acidocella sp. LGS-3	Dodecane, hexadecane	78%	10 days	[69]
	Acidocella sp. IS10	Naphthalene	80%	6 days	[70]
	Stenotrophomonas maltophilia strain AJH1, Acidiphilium	PAHs	82%	12 days	[59]
	Acidiphilium, Acidocella, Acidisphaera	Petroleum oil	87%	14 days	[71]
	Mycobacterium montefiorense	Phenanthrene, pyrene	85%	8 days	[72]
Archaeon	S. solfataricus 98/2	Phenol	70%	5 days	[63]
Yeast	Zymomonas mobilis (ZM1)	Phenol	72%	4 days	[62]
	Candida digboiensis TERI ASN6	Total petroleum hydrocarbons (TPHs)	90%	15 days	[65]
Fungi	Exophiala oligosperma	Toluene	80%	6 days	[73]
	Exophiala jeanselmei	Styrene	85%	7 days	[74]
	Paecilomyces variotii	Toluene	83%	6 days	[75]
	Phanerocheate chrysosporium	Toluene	88%	8 days	[76]
	Exophiala lecanii-corni	Toluene, ethylbenzene, benzene, styrene	90%	10 days	[77]

.

3.1.2. Acidophilic Microorganisms for Inorganic Wastewater

It has been found that acidophilic microbes can be useful in the removal of solid inorganic pollutants. These organisms can detoxify and eliminate heavy metals from wastewater (Table 2).

Acidophiles primarily incorporate strategies to remove heavy metal toxicity [78]. They can dissolve and effectively precipitate the solids separated from wastewater containing heavy metals, converting them into insoluble forms. In this course, ferric iron undergoes reduction to ferrous iron, followed by the precipitation of metal hydroxides and sulfides [79]. For example, Acidithiobacillus ferrooxidans are bacteria that are mainly involved in the oxidation of ferrous iron, which results in the formation of a non-tangible compound called ferric hydroxide. Since iron acts as a precipitating agent, Acidiphilium species can reduce ferric iron to ferrous iron, aiding in the dissolution of ferric minerals Scientists have identified acidophilic bacteria capable of removing up to 90% of metals, such as iron, zinc, and copper, from acid AMD [80].

The bioremediation process is further improved by the concept of symbiotic bioremediation, in which bacterial consortia work with bio-augmentation. There is an example of Acidithiobacillus spp. with Leptospirillum ferrooxidans that could speed up the rate of heavy metal removal from sediment [58]. This confirmed that co-culturing these bacteria, such as Bacillus subtilis, with other species, like Pseudomonas aeruginosa or Aspergillus niger, resulted in significantly higher heavy metal extraction efficiency. The bioaugmentation process leveraged the metal-binding proteins and biosurfactants produced by P. aeruginosa in conjunction with the bioaccumulation capabilities of B. subtilis. Symbiotic association was ensured by optimizing pH (7.0–7.5), temperature (30–37 °C), and nutrient supplementation (e.g., glucose and nitrogen sources), as reported in studies highlighting the mutual metabolic benefits between these species [80].

It has been shown that integrating acidophilic bacteria with other wastewater treatment technologies can be improved by combining their abilities to break down organic and inorganic contaminants. This can result in more efficient wastewater treatment and lower energy use [81].

Table 2. List of inorganic pollutants along with relevant acidophiles.

Domain	Acidophiles	Inorganic Pollutants	Removal %	Duration	Pollutant Initial Concentration	References
Heavy Metals
Bacteria	Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, cidithiobacillus ferrooxidans, Acidomicrobium ferrooxidans, Ferroplasma, Alicyclobacillus, Acidiphilium spp., Sulfobacillus spp., Acidocella spp.	Iron Reduction	85–90%	10–15 days	500 mg/L	[82,83,84]
	Desulfovibrio, Desulfomicrobium, Desulfobulbus, Desulfosarcina, Desulfobacter, Desulfotomaculum, Desulfosporosinus, Thermodesulfobacterium, Thermodesulfovibrio, Desulfovibrio desulfuricans, Desulfomicrobium baculatum	Sulfate Reduction	70–80%	12–20 days	300 mg/L	[85,86,87]
	Desulfurella	Sulfur Reduction	75%	14 days	200 mg/L	[88,89]
	Acidithiobacillus spp., Leptospirillum ferrooxidans, Acidiphilium cryptum	Symbiotic Role	N/A	N/A	N/A	[90,91,92]
Algae	Anabaena, Cladophora, Oscillatoria, Phaeodactylum, Scenedesmus, Spirulina sp.	Bio-sorption and Bioaccumulation	60–85%	7–14 days	100 mg/L	[92,93,94]
Radioactive Pollutants
Bacteria	A. ferrooxidans, Cupriavidus metallidurans	Bio-sorption	80–90%	10 days	50 mg/L	[95,96]
	Sulfolobus metallicu, Acidithiobacillus sp., Sulfolobus metallicus	Bioaccumulation	85%	12 days	70 mg/L	[97,98,99]

Table 2. List of inorganic pollutants along with relevant acidophiles.

Domain	Acidophiles	Inorganic Pollutants	Removal %	Duration	Pollutant Initial Concentration	References
Heavy Metals
Bacteria	Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, cidithiobacillus ferrooxidans, Acidomicrobium ferrooxidans, Ferroplasma, Alicyclobacillus, Acidiphilium spp., Sulfobacillus spp., Acidocella spp.	Iron Reduction	85–90%	10–15 days	500 mg/L	[82,83,84]
	Desulfovibrio, Desulfomicrobium, Desulfobulbus, Desulfosarcina, Desulfobacter, Desulfotomaculum, Desulfosporosinus, Thermodesulfobacterium, Thermodesulfovibrio, Desulfovibrio desulfuricans, Desulfomicrobium baculatum	Sulfate Reduction	70–80%	12–20 days	300 mg/L	[85,86,87]
	Desulfurella	Sulfur Reduction	75%	14 days	200 mg/L	[88,89]
	Acidithiobacillus spp., Leptospirillum ferrooxidans, Acidiphilium cryptum	Symbiotic Role	N/A	N/A	N/A	[90,91,92]
Algae	Anabaena, Cladophora, Oscillatoria, Phaeodactylum, Scenedesmus, Spirulina sp.	Bio-sorption and Bioaccumulation	60–85%	7–14 days	100 mg/L	[92,93,94]
Radioactive Pollutants
Bacteria	A. ferrooxidans, Cupriavidus metallidurans	Bio-sorption	80–90%	10 days	50 mg/L	[95,96]
	Sulfolobus metallicu, Acidithiobacillus sp., Sulfolobus metallicus	Bioaccumulation	85%	12 days	70 mg/L	[97,98,99]

3.1.3. Potential Impacts of Acidophiles

Acidophilic bacteria applied in wastewater treatment can simultaneously bring about benefits and problems regarding environmental and health aspects. Overcoming such challenges by developing appropriate sustainable and eco-friendly technologies for treatment will be critical in ensuring that safety and effectiveness continue during the process.

Acidophilic microbes in wastewater can unintentionally release low-pH water, damaging aquatic ecosystems and affecting biodiversity. To prevent environmental harm, pH levels must be monitored and adjusted before reuse. Hybrid systems can handle acidic waste, followed by a neutralization phase to restore pH before discharge. Natural or synthetic pH buffers can also help neutralize excess acidity [100].

Poorly managed wastewater treatment procedures may not completely eradicate hazardous pathogens, especially in situations where the microbial population is not working effectively. In this respect, acidophilic bacteria are effective for some contaminants but may not remove all and can potentially harm human health. Integration of acidophilic bacteria with other methods, besides the use of hybrid systems (e.g., UV disinfection, membrane filtration), can develop mitigation strategies for better pathogen elimination efficiency [101].

Hybrid systems combining acidophilic microbes with other organisms can enhance pollution removal and reduce environmental concerns. These systems are durable, sustainable, and improve treatment outcomes. A circular economy can reduce environmental impact by reusing waste products, like wastewater, for irrigation, industrial operations, or energy production. Ecological design approaches, like nature-based solutions, wetland treatment systems, and integrated artificial wetlands, can enhance system sustainability [81].

3.2. Alkaliphile Microorganisms in Wastewater Treatment

Alkaliphiles are capable of growing in an alkaline environment that has a pH greater than 8, and the best growth rate is usually seen at a pH of 9 and 10. These organisms have been reported to have an important application in treating wastewater containing inorganic pollutants, mainly because these microorganisms can survive and effectively carry out their metabolic processes at high pH levels (Figure 8) [36]. Mostly, alkaliphilic microorganisms facilitate wastewater treatment via the production of bio-flocculants. These bio-flocculants are natural, biodegradable polymers, which coagulate and sediment the suspended and dissolved materials and other contaminants in wastewater [102]. The bio-flocculants produced by Bacillus licheniformis have proved to be very effective in treating tannery wastewater by flocculating the suspended particles and thereby enabling their settling [103].

Alkaliphiles synthesize bio-flocculants by secreting polysaccharides, proteins, and lipids that help in the removal of contaminants in wastewater through processes such as bridging, charge neutralization, and adsorption. The performance of these bio-flocculants depends on parameters such as molecular weight and functional groups of the EPS, and environmental factors, such as pH and ionic strength [103,104].

The use of bio-flocculants is preferable over the use of chemical flocculants in many ways. They do not pollute the environment, are non-toxic and biodegradable. The use of bio-flocculants from alkaliphilic bacteria is also economical, since substrates that support bacterial growth and EPS formation are cheap. This cuts down the overall cost and environmental burden of wastewater treatment methods [105].

Alkaliphilic bio-flocculants are more effective at treating industrial effluents, for example, from the textile, food and tannery industries. In the context of tannery wastewater, bio-flocculants are quite effective in the removal of heavy metals, organic compounds, and other pollutants. This improves the quality of the treated water and reduces the detrimental effect of industrial emissions on the environment [106]. More studies are being conducted to enhance the synthesis and efficiency of bio-flocculants through genetic manipulation and the enhancement of fermentation processes. Using mixed cultures of alkaliphilic microorganisms can produce different types of bio-flocculants that improve the efficiency of wastewater treatment processes [107].

3.2.1. Alkaliphiles for Organic Wastewater

Alkaliphiles are also characterized by their distinctive biochemical processes, through which they can preserve their internal balance in very adverse conditions. The hydroxide ions are highly reactive, but they cannot easily pass through the adapted cell walls of alkaliphiles, which provide protection to the cells. Such organisms also have very efficient ion pumps to remove any excess hydroxide ions from the cytoplasm and thus maintain a near-neutral cytoplasmic pH. The enzymes of these organisms have evolved to work optimally at high pH conditions, facilitating efficient metabolism [108].

Alkaliphilic microorganisms are favorable options for the treatment of organic wastewater from different industries, like textiles, tanneries, and paper mills. These alkaliphilic bacteria present in bioreactors can metabolize the organic pollutants in the wastewater and adsorb the heavy metal ions, thus minimizing the concentration of the toxic components in the wastewater [109]. Interestingly, Bacillus licheniformis has been proven to have the potential to treat tannery effluents in terms of the removal of chemical oxygen demand (COD), biological oxygen demand (BOD), and total chromium [38,103].

Alkaliphilic bacillus and Pseudomonas metabolize and degrade various complex compounds (dyes and hydrocarbons) present in industrial effluent, to a great extent. These bacteria can decompose and demobilize pollutants in the environment and, hence, lessen the impact of pollution due to organic substances hazardous to living organisms [110].

Moreover, alkaliphilic microorganisms can transform metal ions into insoluble compounds, which are less soluble than initial metal ions. Bacillus reduces soluble Cr⁶⁺ ions into relatively less soluble Cr³⁺ ions, thus assisting in the removal of the former from effluent. This process thus improves the efficiency of heavy metal elimination from contaminated water streams [111].

The mutualistic interaction between alkaliphilic bacteria and fungi improves bioremediation efficiency. For example, bacteria and fungi can synergistically or antagonistically act to break down organic matter and withstand heavy metals in order to enhance the processes of wastewater treatment [36]. Alkaliphilic microbes catabolize polymeric organic molecules into simpler molecules, such as carbon dioxide and water. This mineralization process is vital for regulating persistent and bio-accumulative substances like phenols and PAHs in industrial wastewater. Ref. isolated an alkaliphilic enzyme, phenol hydroxylase, which operates optimally at high pH to degrade phenolic compounds. Many alkaliphiles have been included and listed in

Table 3. Examples of different alkaliphiles and their pollutants.

Domain	Alkaliphiles	Pollutants	Removal %	Duration	References
Organic Pollutants
Bacteria	Citricoccus alkalitolerans CSB1	Tannery effluents (COD, BOD, total chromium)	80–85%	10 days	[38]
	Bacillus sp.	Phenol	90%	7 days	[112,113,114]
	Pseudomonas and Bacillus strains	PAHs (Polycyclic Aromatic Hydrocarbons)	75–88%	12–15 days	[115,116,117,118]
	Halanaerobium lacipiscis	NOx-N, NH3-N	80%	14 days	[119]
	Halomonas, Marinobacter	High salinity and heavy metals	70–85%	12 days	[36]
	Pseudomonas putida	o-nitro-benzaldehyde (ONBA)	85%	8 days	[120]
	Alkaliphilic Bacillus sp.	Chromium (Cr VI)	90%	6 days	[121]
	Halomonas campisalis	Benzoate and Salicylate	88%	10 days	[122]
	Bacillus sp. Bacillus circulans	Textile dyes	80%	12 days	[123]
Archaea	Alkaliphilic Pseudomonas sp.	Industrial effluents	75%	14 days	[124]
Bacteria	Thioalkalivibrio spp. (Thioalkalivibrio halophilus, Thioalkalivibrio versutus)	Sulfides, polysulfides, thiosulfates, tetrathionates, thiocyanates	80–85%	15 days	[125,126]
	Ectothiorhodospira magna, Ectothiorhodospira shaposhnikovii	Zinc, lead, copper (via sulfide oxidation to sulfur)	90%	7 days	[127,128]
	Bacillus spp.	Cr6+ (Chromium reduction to Cr3+)	80%		[129]
Archaea	Methanobacteria and Methanomicrobia	Methanogenesis in high-strength organic wastewaters	75% CH4 yield	20 days	[130]
	Methanosarcina	methanogenesis and ammonia oxidation	18 days	400 mg/L COD	[131]

, which represents each alkaliphile and its pollutants.

3.2.2. Alkaliphiles for Inorganic Pollutant Wastewater Treatment

Another characteristic of alkaliphilic microorganisms is the ability to perform the bio-oxidation of inorganic sulfur compounds. This feature is very effective in the treatment of industrial waste pollutants, like sulfide, polysulfide, thiosulfate, tetrathionate, and thiocyanate [132].

Species such as Thioalkalivibrio, Thioalkalivibrio halophilus, and Thioalkalivibrio versutus have been shown to be highly effective in sulfide oxidation under alkaline conditions. These bacteria are halophilic and alkaliphilic, meaning that they can survive in salty and alkaline conditions. Moreover, they are used in the oil refining and gas production industries.

Ref. [133] described the process of thiopaq, which employs these bacteria in 99% H2S reduction from biogas streams, thus revealing the efficiency of alkaliphiles in gas desulfurization processes.

Moreover, alkaliphiles are involved in the bio-amalgamation and biomineralization of heavy metals, like zinc, lead, and copper. Microbes, such as Ectothiorhodospira magna and Ectothiorhodospira shaposhnikovii, remove heavy metals, like zinc, lead, and copper, by oxidizing sulfides to sulfur which then reacts with the metals to produce sulfides. This process effectively scrapes out heavy metals from the liquid phase and hence plays a role in minimizing water pollution [134,135].

The application of alkaliphilic bacteria has been studied in the treatment of arsenic contamination. Certain forms of arsenite oxidase can metabolize As (III) to As (V), making removing arsenic from water easier. This capability is invaluable when it comes to treating industrial effluents with arsenic as one of the contaminants [136].

To optimize the growth and activity of alkaliphiles in bioreactors, it is crucial to adjust environmental conditions, bioreactor design, and operational strategies. Key approaches include pH control, temperature optimization, and nutrient supply. Alkaliphiles thrive in high pH environments, so maintaining an optimal pH is essential to prevent stress and maximize enzymatic activity. Maintaining optimal temperatures enhances metabolic rates and pollutant degradation efficiency. Balancing nutrients prevents metabolic bottlenecks [137].

Mitigating inhibitory compound effects involves pre-treatment of wastewater, stepwise loading, and using protective carriers. Encapsulation or immobilization of alkaliphiles on carriers like alginate beads, activated carbon, or biofilm matrices can shield them from toxic compounds and stabilize their growth [138].

Optimizing bioreactor design involves reactor configuration, mixing and aeration, and retention time. Advanced designs, like biofilm reactors, fluidized bed reactors, and membrane bioreactors, provide higher surface area and retention time, enhancing microbial activity. Efficient mixing and oxygen supply ensure even distribution of nutrients and inhibitory compounds, reducing localized stress on microorganisms. Prolonged retention times allow alkaliphiles to metabolize inhibitory compounds gradually, increasing degradation efficiency [139].

3.3. Halophilic Microorganisms in Wastewater Treatment

Halophilic bacterial species are grown, reproduced, and function properly in conditions of high salinity and can also handle the process of metabolism in such conditions; hence, they can be used for the purification of water from industries that produce high-salt wastewaters and other hazardous organic compounds, including the petroleum, textile, and tanning industries [140].

Halophiles are categorized based as (i) slight halophiles that grow in 1–3% NaCl, (ii) moderate halophiles in 3–15% NaCl and (iii) extreme halophiles in 15–30% NaCl [141,142]. Halomonas, Haloarcula, and Natrinema are widely known genera of halophiles and play important roles in the bioremediation of wastewater. In particular, Halomonas species play an important role in phenol and other aromatics degradation in environments with high salinity [142,143].

Halophiles have adopted certain mechanisms in order to live in high-saline environments. They store compatible solutes, like glycine betaine, betaine, and ectoine, in the cell in order control the osmotic pressure without affecting the cellular processes. These enzymes are tolerant to high salinity and their efficiency increases with an increase in salinity; therefore, they are appropriate for bioremediation [144]. These adaptations enable the halophiles to perform metabolic functions essential in extreme circumstances, and therefore they are useful in treating saline industrial wastewater (Figure 9) [145,146].

3.3.1. Halophilic Organic Pollutant Wastewater Treatment

Many industries generate saline wastewater, such as those involved in textile manufacturing, food processing, and petrochemical production [147]. These waters not only have high levels of salinity but also contain high concentrations of organic compounds, such as aromatic hydrocarbons and phenolic compounds [148]. The petroleum industries generate water with high salinity, hydrocarbons, and phenolic matter. Moreover, in the textile industry, salt is a major ingredient in dyeing processes, so it discharges dye effluents with high salinity [149].

Halophilic bacteria can function in highly acidic or alkaline conditions, where there is a reduced presence of other microorganisms in the treatment of industrial wastewater (Table 3). Marinobacter sp., a bacterial species, ferments ethanol from glucose and cellulosic materials in unsterilized seawater and algal-contaminated wastewater. This is advantageous in the biorefining industries, as it eliminates the need for sterile water and allows the use of low-quality, inexpensive water sources [140]. Moreover, they are used for the breakdown of hazardous organic compounds, like phenol and benzene. Some of the halophilic strains isolated from oil-contaminated environments have been found to have the potential for complete degradation of phenol at high levels within 8–144 h depending on the conditions. They can use a wide range of biochemical reactions to break down and metabolize the complex organic substrates into simpler and non-toxic products. Phenol hydroxylase and catechol 1,2-dioxygenase enzymes are highly involved in the first steps of the phenol biodegradation process [150,151].

3.3.2. Halophilic Inorganic Pollutant Wastewater Treatment

Apart from organic pollutants, halophilic bacteria have been known to show high efficiency in the bioremediation of heavy metals from wastewater (Table 4). The two bacterial species, Ectothiorhodospira magna and Ectothiorhodospira shaposhnikovii, precipitate heavy metals, such as zinc, lead, and copper, by converting sulfides to sulfur, which in turn reacts with the metal ions to produce sulfides. This process immobilizes the heavy metals in the aqueous phase and, hence, decreases their pollution potential [145].

Halophilic bacteria are also applied in the biodegradation of pollutants with arsenic. Some strains of thiomonas can convert arsenite As (III) into the less toxic arsenate As (V), thus helping in the removal of arsenic from water. This feature is helpful in treating industrial wastes, in which arsenic is a frequent constituent [145].

Halophilic bacteria have been used in commercial and large-scale wastewater treatments in bioreactors. Some of these systems employ the use of activated sludge with halophiles to treat the pollutants. The activated sludge from marine mud was able to remove more than 80% of phenol in a batch experiment and bioreactor within 48 h [152]. Activated sludge is used in wastewater treatment systems because it contains a high concentration of microorganisms that help break down organic pollutants. In the process, wastewater is aerated to encourage the growth of bacteria, protozoa, and other microorganisms in a suspended state, known as activated sludge. These microorganisms degrade organic matter through biochemical reactions, effectively reducing pollutants in the water. Halophilic mixed cultures have also been successfully applied in anaerobic MBR systems for treating saline wastewater with pollutants of phenylphenol and bisphenol A [153].

These characteristics make them suitable for use in long-term treatment in the WWTPs; they do not require frequent replacement owing to microbiological fouling. The Halomonas species has a very low contamination level and is very effective in treatment processes and can even work in open bioreactors [145,154]. The Halomonas family lives in wastewater treatment plants without causing problems. Its species thrive in salty wastewater with colony-forming unit counts below 10⁵ per milliliter and higher than 10³ per milliliter, which shows that the population appears cleaner than other bacteria discovered. Organisms that flourish in extreme conditions and keep their dirt levels low can be important bioremediation tools.

Moreover, the sulfur-oxidizing halophilic bacteria Thioalkali vibrio is used in bioreactors to eliminate sulfur from biogas in the thiopaq process. This process reacts with hydrogen sulfide (H₂S) and produces elemental sulfur at an alkaline pH, which makes the treatment of biogas cost-effective, along with its purification [155,156].

Table 4. List of inorganic pollutant treatments with relevant halophiles.

Domain	Halophiles	Pollutants	Mechanism	Removal %	References
Organic Pollutants
Bacteria	Halomonas spp.	Phenol and other aromatic hydrocarbons	Biodegradation	85%	[157]
	Bacillus marmarensis	Ethanol fermentation	Fermentation	90% ethanol yield	[158]
	Halophilic strains from oil-contaminated environments	Complete phenol degradation	Biodegradation	95%	[159]
	Mixed halophilic cultures	Phenyl-phenol, bisphenol A (in anaerobic MBR systems)	Biodegradation in MBR systems	80–85%	[160]
	Halophilic bacteria from marine mud	Phenol degradation (more than 80% removal in 48 h)	Biodegradation	80%	[161]
	Bacillus spp.	Organic matter, hydrocarbons	Biodegradation	85%	[162]
	Ochrobactrum, Marinobacter, Bacillus	Fish market wastewater (COD, TSS removal, energy production in MFC)	Biodegradation and energy production	90%	[154]
	Lysinibacillus fusiformis and Providencia stuartii	Proteins, lipids, mucopolysaccharides from tannery wastewater	Biodegradation	88%	[163]
Archaea	Haloarchaea	Carbon, nitrogen, phosphorus, sulfur, and heavy metal metabolism in hypersaline wastewater	Metabolism and remediation	75–90%	[164]
Inorganic Pollutants
Bacteria	Ectothiorhodospira magna, Ectothiorhodospira shaposhnikovii	Zinc, lead, copper (via sulfide oxidation to sulfur)	Sulfide oxidation to sulfur	85%	[165]
	Thiomonas spp.	Arsenic (As (III) to As (V) conversion)	Oxidation	90%	[166]
	Thioalkalivibrio spp.	Sulfur (H2S removal from biogas in the thiopaq process)	Oxidation	88%	[155]
	Halophilic bacteria	Heavy metals, arsenic	Bioaccumulation and biosorption	80%	[167]
	Ochrobactrum, Marinobacter, Rhodococcus, Bacillus	Seafood and pharmaceutical wastewater treatment with energy production	Biodegradation and energy generation	85%	[168]
	Exiguobacterium mexicanum	Nitrogen removal from saline wastewater via heterotrophic nitrification and aerobic denitrification	Nitrification and denitrification	78%	[169]

Table 4. List of inorganic pollutant treatments with relevant halophiles.

Domain	Halophiles	Pollutants	Mechanism	Removal %	References
Organic Pollutants
Bacteria	Halomonas spp.	Phenol and other aromatic hydrocarbons	Biodegradation	85%	[157]
	Bacillus marmarensis	Ethanol fermentation	Fermentation	90% ethanol yield	[158]
	Halophilic strains from oil-contaminated environments	Complete phenol degradation	Biodegradation	95%	[159]
	Mixed halophilic cultures	Phenyl-phenol, bisphenol A (in anaerobic MBR systems)	Biodegradation in MBR systems	80–85%	[160]
	Halophilic bacteria from marine mud	Phenol degradation (more than 80% removal in 48 h)	Biodegradation	80%	[161]
	Bacillus spp.	Organic matter, hydrocarbons	Biodegradation	85%	[162]
	Ochrobactrum, Marinobacter, Bacillus	Fish market wastewater (COD, TSS removal, energy production in MFC)	Biodegradation and energy production	90%	[154]
	Lysinibacillus fusiformis and Providencia stuartii	Proteins, lipids, mucopolysaccharides from tannery wastewater	Biodegradation	88%	[163]
Archaea	Haloarchaea	Carbon, nitrogen, phosphorus, sulfur, and heavy metal metabolism in hypersaline wastewater	Metabolism and remediation	75–90%	[164]
Inorganic Pollutants
Bacteria	Ectothiorhodospira magna, Ectothiorhodospira shaposhnikovii	Zinc, lead, copper (via sulfide oxidation to sulfur)	Sulfide oxidation to sulfur	85%	[165]
	Thiomonas spp.	Arsenic (As (III) to As (V) conversion)	Oxidation	90%	[166]
	Thioalkalivibrio spp.	Sulfur (H2S removal from biogas in the thiopaq process)	Oxidation	88%	[155]
	Halophilic bacteria	Heavy metals, arsenic	Bioaccumulation and biosorption	80%	[167]
	Ochrobactrum, Marinobacter, Rhodococcus, Bacillus	Seafood and pharmaceutical wastewater treatment with energy production	Biodegradation and energy generation	85%	[168]
	Exiguobacterium mexicanum	Nitrogen removal from saline wastewater via heterotrophic nitrification and aerobic denitrification	Nitrification and denitrification	78%	[169]

3.4. Thermophilic Microorganisms in the Treatment of Wastewater

Nowadays, organisms that function at temperatures above 45 °C are increasingly used in wastewater treatment due to their high efficiency under thermophilic conditions. Thermophilic microorganisms demonstrate exceptional suitability for wastewater treatment in warm industrial settings due to their metabolism, which functions best at temperatures above 45 °C. Five major industrial sectors, i.e., paper and pulp, food processing, chemical manufacturing and textiles regularly produce wastewater exceeding temperatures of 45 °C. Solar thermal microorganisms eliminate the need for cooling infrastructure, which results in both decreased operational expenses and reduced environmental impacts. Studies indicate that thermophilic anaerobic digesters produce higher biodegradation outcomes and methane production levels than mesophilic systems, making them a perfect solution for treating hot industrial discharge streams [170,171].

Thermal resistance, along with anti-fouling capabilities and metabolic flexibility under variable environmental conditions, make thermophiles an ideal choice for warm wastewater systems. Two studies, Thermophilic Wastewater Treatment: A Review on Applicability and Process Stability and Microbial Strategies for High-Temperature Wastewater Treatment present extensive practical research on thermophilic organisms. Laboratory research demonstrates that Methanothermobacter thermautotrophicus performs efficiently for biogas production from organic waste while functioning at high temperatures [170,171].

Research should examine the relationship between thermophilic systems and mesophilic systems through assessments of pollutant removal capabilities and system energy performance. Precise knowledge of thermophilic reactor design combined with nutrient management and inhibitory compound influences at elevated temperatures represents foundational information for future scientific exploration. These expanded research areas will enhance the direction of analysis towards practical applications of extremophiles for warm wastewater management [Table 5] [170,171].

3.4.1. Thermophilic Organic Pollutant Wastewater Treatment

Thermophilic microbes possess active and heat-resistant enzymes that hydrolyze protein, lipids, and polysaccharides into simpler forms of organic polymers using extracellular enzymes. These monomers are then subjected to fermentative thermophiles to produce volatile fatty acids, hydrogen, and carbon dioxide. This phase is significant because it produces methanogenic substrates needed for the last stage of anaerobic digestion and can effectively break down proteins, lipids, and carbohydrates present in food items [172]. Intermediate products are converted to methane and carbon dioxide by thermophilic methanogens. The rate of this process is faster at high temperatures in thermophilic digesters, hence increasing the production of biogas [116,117]. Moreover, when the process of ozonation is combined with thermophiles, it significantly enhances the removal of COD and chromaticity in effluents from high-temperature pulping processes. This integrated method can achieve COD and chromaticity removal efficiencies of up to 90% [172,173]. Some thermophilic microorganisms, such as Chloroflexus, Meiothermus, and Roseiflexus species, comprise a large part of the microbial community in thermophiles. These bacteria are efficient in breaking the lignin and other macromolecules, which are the most common effluents in the pulp industry. Ref. [173] found that the insertion of planktonic thermophiles in biofilm systems coupled with ozonation improves COD and chromaticity removal, thus enhancing biodegradation of the effluent [173,174]. Mostly, thermophiles are useful in anaerobic digestion processes for the stabilization and treatment of sewage sludge and high-concentration industrial waste effluents. This involves the mechanism of Thermophilic Anaerobic Digestion (TAD), where anaerobic enzymes work optimally at higher temperatures to degrade the waste into biogas with very low CO₂ emissions. This process also reduces the production of volatile organic compounds in the sludge, improves dewatering, and enhances biogas production [175].

Table 5. Different types of thermophilic microbes and their role in wastewater treatment.

Domain	Thermophiles	Pollutants	Removal %	References
Organic pollutants
Bacteria	Chloroflexus sp.	Lignin and macromolecules	75–80%	[176]
	Meiothermus sp.	Lignin and macromolecules	80%	[177]
	Roseiflexus sp.	Lignin and macromolecules	85%	[174]
	Thermophilic methanogens	Proteins, lipids, carbohydrates	70–85% CH4 yield	[178]
	Thermoanaerobacterium sp.	Sewage sludge, industrial effluent (TAD process)	75–88%	[179]
	Clostridium thermocellum	Organic matter, cellulose, complex biopolymers	80–85%	[180]
	Thermoanaerobacterium thermosaccharolyticum	Carbohydrates, volatile fatty acids	78%	[181]
	Caldicellulosiruptor saccharolyticus	Polysaccharides, organic matter	85%	[182]
Archaea	Thermophilic Anaerobic Digesters	Sewage sludge, high-concentration industrial waste	80%	[183]
	Methanothermobacter marburgensis	Sewage sludge, high-organic waste	85% CH4 yield	[130]
	Methanosaeta sp., Methanosarcina sp.	Industrial wastewater (organic pollutants)	75–85% CH4 yield	[184]
	Methanothermobacter thermautotrophicus	Sewage sludge, organic waste	80% CH4 yield	[130]
Inorganic pollutants
Bacteria	Sulfur-oxidizing thermophiles (Thermothrix sp. (sulfur-oxidizing, Thermothrix thiopara)	Sulfides and sulfur-containing compounds	85–90% 80–85%	[185]
	Iron-oxidizing thermophiles	Iron (ferrous to ferric)	80–85%	[186,187]
Archaea	Acidianus brierleyiSulfolobus acidocaldarius	Sulfides	90%	[188]

Table 5. Different types of thermophilic microbes and their role in wastewater treatment.

Domain	Thermophiles	Pollutants	Removal %	References
Organic pollutants
Bacteria	Chloroflexus sp.	Lignin and macromolecules	75–80%	[176]
	Meiothermus sp.	Lignin and macromolecules	80%	[177]
	Roseiflexus sp.	Lignin and macromolecules	85%	[174]
	Thermophilic methanogens	Proteins, lipids, carbohydrates	70–85% CH4 yield	[178]
	Thermoanaerobacterium sp.	Sewage sludge, industrial effluent (TAD process)	75–88%	[179]
	Clostridium thermocellum	Organic matter, cellulose, complex biopolymers	80–85%	[180]
	Thermoanaerobacterium thermosaccharolyticum	Carbohydrates, volatile fatty acids	78%	[181]
	Caldicellulosiruptor saccharolyticus	Polysaccharides, organic matter	85%	[182]
Archaea	Thermophilic Anaerobic Digesters	Sewage sludge, high-concentration industrial waste	80%	[183]
	Methanothermobacter marburgensis	Sewage sludge, high-organic waste	85% CH4 yield	[130]
	Methanosaeta sp., Methanosarcina sp.	Industrial wastewater (organic pollutants)	75–85% CH4 yield	[184]
	Methanothermobacter thermautotrophicus	Sewage sludge, organic waste	80% CH4 yield	[130]
Inorganic pollutants
Bacteria	Sulfur-oxidizing thermophiles (Thermothrix sp. (sulfur-oxidizing, Thermothrix thiopara)	Sulfides and sulfur-containing compounds	85–90% 80–85%	[185]
	Iron-oxidizing thermophiles	Iron (ferrous to ferric)	80–85%	[186,187]
Archaea	Acidianus brierleyiSulfolobus acidocaldarius	Sulfides	90%	[188]

3.4.2. Thermophilic Inorganic Pollutant Wastewater Treatment

Consequently, sulfur and iron metabolism in wastewater treatment are associated with thermophiles. Sulfur-oxidizing thermophiles help to decrease the toxicity of wastewater through the oxidation of sulfides and sulfur-containing compounds to sulfate. Similarly, iron-oxidizing thermophiles convert ferrous to ferric, and then ferric is deposited and settled out of the liquid effluent. These metabolic activities are very useful in the treatment of mines and industrial metallurgical effluents with high sulfur and iron content [189].

In practice, many bioreactors with thermophilic bacteria are applied in the treatment of industrial wastewater. Thermophilic bioreactor systems combined with ozonation units provide better treatment for the thermophilic effluents. This combined system uses the high degradation capability of thermophiles and the strong oxidative force of ozone to enhance COD and chromaticity removal rates. For example, in the case of pulping wastewater treatment, the thermophilic biofilm system has been found to reduce COD by 59% [189].

Moreover, the use of thermophilic conditions in wastewater treatment has several advantages, including a high reaction rate, low biomass yielding, and effective pathogen die-off. These are the advantages of thermophilic technologies, and therefore are useful in the treatment of high-strength industrial effluents with high concentrations of organic matter. Moreover, thermophilic systems have a higher load ability and higher rate of biodegradation compared to mesophilic systems and are thus fit to be used in industries that demand fast and effective ways of treating their wastewater [190].

3.5. Extremophilic Microalgae

Extremophilic microalgae have gained attention for their application in wastewater treatment due to their efficiency in growing in extreme conditions of salinity, temperature, and pH. These characteristics make them suitable for industrial effluent. It is crucial to pay attention to the possibilities of their application in the improvement of efficiency and sustainability of the treatment plant [Figure 10] [191]. Extremophilic microalgae have generated research interest for wastewater treatment because they survive well in extreme salinity, temperature, and pH conditions. These special characteristics help them clean industrial wastewater better, because such effluents exist in tough environments. Our studies into their application help us develop better wastewater plants that work efficiently and last longer. By leveraging their adaptability, these microalgae could contribute to improved pollutant degradation, resource recovery, and overall operational resilience, paving the way for more sustainable treatment solutions.

3.5.1. Organic Pollutant Removal

Among extremophilic microalgae, Galdieria sulphuraria are very efficient in treating wastewater containing high levels of organic matter. Galdieria sulphuraria is a thermo-acidophilic red alga that has the ability to grow at a very low pH value of 1.8 and at a temperature of 56 °C, thus being suitable for treating acid and warm industrial effluents. This alga is capable of decreasing the following parameters in wastewater: organic carbon by 46–72%, ammonium by 63–89%, and phosphate by 71–95% (Table 6) [41].

3.5.2. Heavy Metal Removal

It should also be noted that these microalgae can filter out heavy metals from the water. Galdieria sulphuraria has been reported to have good biosorption potential for heavy metals, such as vanadium, uranium, and titanium [192]. The presence of special binding sites on the surface of microalgal cells enables the adsorption of heavy metal ions, as a result of which the concentration of the pollutant substances in wastewater decreases. This feature is especially effective for the treatment of wastewater containing industrial metals as hazardous impurities [192,193].

3.5.3. Nutrient Removal

Extremophilic microalgae are already known to uptake nutrients, such as nitrogen and phosphorus, which are commonly present in high concentrations in agricultural and industrial wastewaters [191]. Scenedesmus and Nannochloropsis are some of the microalgae that are capable of filtering large quantities of nitrogen and phosphorus from water and therefore prevent cases of eutrophication in water bodies. This process in the conservation of water ecology systems and the protection of the environment is very important [28].

3.5.4. Integration with Bacterial Systems

Immobilization of extremophilic microalgae in bacterial systems enhances the overall efficiency of the wastewater treatment system. In these biofilms, the microalga photosynthesizes oxygen, which is in turn relied on by the bacteria to break down organic matter aerobically [194]. This system improves the efficiency of pollutant removal in water and reduces the mechanical aeration energy consumption used in standard wastewater treatment plants. According to the literature, this integration can assist in increasing the effectiveness and efficacy of the treatment and its maintenance [194].

New bioreactor designs including Algal Turf Scrubbers and rotating algal biofilm reactors have been introduced to enhance the use of extremophilic microalgae in treating wastewater. Such systems are efficient in nutrient and heavy metals removal with a marked performance in rotating algal biofilm reactors in sulfate-rich mining effluents treatment. Research shows that rotating algal biofilm reactors can remove up to 46% of sulfate and 87% of total nitrogen, demonstrating their effectiveness in treating sulfate-rich mining effluents and other industrial wastewater [195].

As a result, the application of extremophilic microalgae in wastewater treatment has the advantage of transforming pollutants into useful biomass. This biomass is used for the production of a plethora of bioproducts, including biofuels, animal feed, bio-fertilizers, etc., which enhance the circular bioeconomy. Moreover, G. sulphuraria creates phycocyanin, which is used in food processing and cosmetics, to name but a few areas, thus making the treatment process more profitable [196]. Thus, these extremophilic microalgae, which have the dual advantage of pollutant removal and biomass production, can be used for wastewater treatment, making for a sustainable and cost-effective method [197,198]. Table 6 provides a list of certain types of extremophiles, along with pollutant treatments.

Table 6. Different types of extremophile microalgae and their role in wastewater treatment.

Pollutant Type	Pollutants	Extremophilic Microalgae	Mechanism	Removal %	References
Organic Pollutants	Organic matter	Galdieria sulphuraria	Biodegradation of organic load	85–90%	[32]
	Pharmaceuticals	Scenedesmus sp.	Adsorption and biodegradation	75–85%	[199]
	Textile dyes	Dunaliella salina	Decolorization and degradation	80–88%	[200]
	Industrial effluents	Nannochloropsis gaditana	Reduction of organic and toxic load	78–85%	[201]
	Organic pollutants (general)	Spirulina platensis	Biodegradation	80%	[201]
Inorganic Pollutants	Nutrient Removal (Nitrogen, phosphorus)	Scenedesmus sp., Nannochloropsis	Assimilation and nutrient uptake	75–90%	[202]
	Nutrients (Nitrates, phosphates)	Galdieria sulphuraria	Nutrient assimilation	85%	[191]
	Heavy Metals (Aluminum (Al), Nickel (Ni), Copper (Cu), Vanadium, Uranium, Titanium	acidophila, Galdieria sulphuraria	Biosorption and bioaccumulation	70–85%	[203]
	Sulfates	Microalgae in rotating biofilm reactors	Sulfate reduction	80–90%	[27]
Emerging Contaminants	Pharmaceuticals and personal care products (PPCPs)	Microalgae consortia	Biodegradation and biosorption	75–85%	[204,205,206]

Table 6. Different types of extremophile microalgae and their role in wastewater treatment.

Pollutant Type	Pollutants	Extremophilic Microalgae	Mechanism	Removal %	References
Organic Pollutants	Organic matter	Galdieria sulphuraria	Biodegradation of organic load	85–90%	[32]
	Pharmaceuticals	Scenedesmus sp.	Adsorption and biodegradation	75–85%	[199]
	Textile dyes	Dunaliella salina	Decolorization and degradation	80–88%	[200]
	Industrial effluents	Nannochloropsis gaditana	Reduction of organic and toxic load	78–85%	[201]
	Organic pollutants (general)	Spirulina platensis	Biodegradation	80%	[201]
Inorganic Pollutants	Nutrient Removal (Nitrogen, phosphorus)	Scenedesmus sp., Nannochloropsis	Assimilation and nutrient uptake	75–90%	[202]
	Nutrients (Nitrates, phosphates)	Galdieria sulphuraria	Nutrient assimilation	85%	[191]
	Heavy Metals (Aluminum (Al), Nickel (Ni), Copper (Cu), Vanadium, Uranium, Titanium	acidophila, Galdieria sulphuraria	Biosorption and bioaccumulation	70–85%	[203]
	Sulfates	Microalgae in rotating biofilm reactors	Sulfate reduction	80–90%	[27]
Emerging Contaminants	Pharmaceuticals and personal care products (PPCPs)	Microalgae consortia	Biodegradation and biosorption	75–85%	[204,205,206]

4. Conclusions

Extremophilic microorganisms possess a high resistance power in order to work under stress conditions, making them suitable for treating industrial wastewater, particularly from the petrochemical, mining, and textile industries, which emit wastewater with high salinity, heavy metals, and organic toxins. However, there are several problems that need to be addressed to enable the large-scale use of extremophiles in wastewater treatment. The slow growth rates of these microorganisms under production demands delayed treatment processes, while creating mandatory energy costs for maintaining the correct environmental parameters. The microorganisms used in aquaculture cannot easily adjust to changing wastewater content or eliminate new pollutants, including microplastics and pharmaceutical substances. To overcome these limitations, it is crucial to focus on improving bioreactor designs that optimize energy efficiency and scalability, increasing the ability of organisms to assimilate new and future pollutants, and refining genetic engineering to generate strains with higher efficiency and broader applicability. In particular, employing advanced omics technologies and CRISPR-based techniques could accelerate strain development and enhance pollutant degradation capabilities. Resolving these challenges will require cross-disciplinary efforts in biotechnology, nanotechnology, and systems biology to expand the application of extremophiles in wastewater treatment. Moreover, the potential of extremophilic organisms can be combined with other advanced technologies for the treatment of wastewater, including biogenic nanoparticles and biofilm reactors. Furthermore, extremophilic microalgae have the capability to generate biofuel feedstock during the treatment of wastewater, offering the hope of a closed-loop system, where the removal of pollutants is linked with the generation of useful bio-products. This approach not only enhances sustainability but also reduces the overall environmental impact of wastewater treatment systems.

There are several problems that need to be solved in order to use extremophiles on a large scale in wastewater treatment.

Environmental and Ecological Risks. Impact on Local Ecosystems: Extremophiles may disrupt local biodiversity if introduced unintentionally. Mitigation involves rigorous monitoring and hybrid treatment systems. Invasive Species: Certain extremophiles could become invasive, necessitating the use of non-invasive, well-studied strains and strict containment measures.

Limitations of Extremophile-Based Treatment. Specificity of Conditions: Extremophiles require precise environments, challenging scalability. Hybrid systems may enhance adaptability. Slow Growth: Extremophiles often grow slower, reducing efficiency. Optimized reactor designs or integrating fast-growing microbes can help.

Regulatory and Safety Challenges. Regulatory Hurdles: Lack of established standards for extremophile use. Clear frameworks are needed to ensure environmental and public health safety. Health Concerns: Potential toxic byproducts require thorough risk assessments and health protocols.

Cost and Technological Barriers. High Costs: Specialized infrastructure increases initial investment. Research into cost-effective solutions is essential. Technical Expertise: Deployment requires specialized knowledge, calling for operator training and expert support.

Public Perception and Acceptance. Concerns about Safety: Public skepticism may arise over safety and environmental impacts. Transparent communication and public education can build trust. This includes improving the design of bioreactors for industrial application, increasing the ability of the organisms to assimilate new and future pollutants, and refining genetic engineering for the generation of strains with higher efficiency. It is, therefore, important to resolve these challenges through cross-cutting studies in biotechnology, nanotechnology and systems biology to enhance the application of extremophiles in wastewater treatment.

5. Future Prospect

The potential of extremophilic organisms can be combined with other advanced technologies for the treatment of wastewater, including biogenic nanoparticles, and biofilm reactors. Furthermore, extremophilic microalgae have the capability to generate biofuel feedstock during the treatment of wastewater, which offers the hope of a closed loop system, where the removal of pollutants is linked with the generation of useful bio-products. Thus, with the further development of extremophilic microorganisms and partnership between different fields, wastewater management can be revolutionized, which will help to improve the quality of the future. By tapping into these strengths and combining these with advanced technology, it will be possible to come up with systems that are efficient in wastewater treatment, friendly to the environment and economical in the long run, thus playing a part in the conservation of the environment. The authors advocate for comprehensive research and exploration of the potential of extremophilic organisms in wastewater treatment by addressing certain pertinent problems, which will facilitate the advancement of these methodologies. There are a number of information gaps that need to be filled in order to progress the field of treating extremophilic wastewater: How can researchers and scientists address the challenges associated with large-scale implementation of extremophile-based wastewater treatment? A critical evaluation of the scalability and feasibility of these approaches would be needed. How can scalability, cost-effectiveness, and environmental sustainability be taken into account when optimizing extremophilic bacteria for practical wastewater treatment applications? What are the possible hazards and unforeseen repercussions of using extremophilic bacteria to clean wastewater, and how may these be avoided? How can the development and application of extremophilic wastewater treatment technology be accelerated through interdisciplinary cooperation among microbiologists, engineers, legislators, and industry stakeholders? Finally, a thorough examination of underlying biochemical mechanics of extremophilic organisms is required. Thus, with further development of extremophilic microorganisms and partnerships between different fields, wastewater management can be revolutionized, improving the quality of future treatment systems. By tapping into their strengths and combining them with advanced technologies, it will be possible to develop systems that are efficient in wastewater treatment, environmentally friendly, and economical in the long run, thus contributing to environmental conservation.