Harnessing an Algae–Bacteria Symbiosis System: Innovative Strategies for Enhancing Complex Wastewater Matrices Treatment

Abstract

Complex wastewater matrices hinder the efficacy of conventional treatment methods due to the presence of various inorganic and organic pollutants, along with their intricate interactions. Leveraging the synergy between algae and bacteria, algal–bacterial symbiosis (ABS) systems offering an evolutionary and highly effective approach. The ABS system demonstrates 10–30% higher removal efficiency than conventional biological/physicochemical methods under identical conditions, especially at low C/N ratios. Recent advances in biology techniques and big data analytics have deepened our understanding of the synergistic mechanisms involved. Despite the system’s considerable promise, challenges persist concerning complex pollution scenarios and scaling it for industrial applications, particularly regarding system design, environmental adaptability, and stable operation. In this review, we explore the current forms and operational modes of ABS systems, discussing relevant mechanisms in various wastewater treatment contexts. Furthermore, we examine the advantages and limitations of ABS systems in treating complex wastewater matrices, highlighting challenges and proposing future directions.

1. Introduction

Aquatic ecosystems face severe threats from human activities [1], resulting in widespread contamination by nutrients, heavy metals (HMs), persistent organic pollutants (POPs), and emerging contaminants [2]. The diverse forms and complex interactions of pollutants from multiple sources have exacerbated remediation challenges, underscoring the need for advanced, multifaceted water treatment technologies. Among existing methods, bioremediation has gained considerable attention due to its cost-effectiveness, efficiency, and environmental sustainability, achieved through the metabolic or co-metabolic activities of microorganisms [3]. For example, Actinobacteria such as Nocardia and Streptomyces genera effectively remediate polycyclic aromatic hydrocarbon (PAH)-contaminated water via oxidoreductases and biosurfactants [4]. Similarly, Pseudomonas species, particularly Pseudomonas aeruginosa and Pseudomonas putida, are well known for their ability to degrade various organic pollutants through enzymatic processes involving monooxygenases [5]. Microorganisms can convert pollutants into less toxic or non-toxic compounds, ultimately yielding carbon dioxide (CO₂) and water. Nevertheless, selecting appropriate microbial treatment systems remains critical to addressing the growing complexity of wastewater matrices.

Microalgae play a crucial role in global carbon cycling, sequestering approximately 1.83 gigatons of carbon annually [6]. Their rich metabolic pathways and high specific surface area make them highly effective for wastewater purification and nutrient recycling through processes such as assimilation, biodegradation, and adsorption [7,8,9]. However, wastewater composition is typically highly complex, containing multiple interacting pollutants that can severely impair the efficiency of microalgae-based bioremediation systems designed for single contaminants. Additionally, toxic pollutants—including heavy metals (HMs) [10], chlorinated hydrocarbons (CHs), and nanoparticles [11]—can disrupt indigenous microbial communities [12], suppress metabolic activity, and impair enzymatic functions [13]. Globally, an estimated 359 billion cubic meters (Bm³) of wastewater are generated each year, yet only 20% undergoes treatment and reuse [14]. Given the rising complexity of wastewater matrices and the urgent need for resource recovery, transitioning from conventional methods to advanced biological treatment technologies has become essential.

The algae–bacteria symbiotic (ABS) system, which leverages synergistic interactions between algae and bacteria, has emerged as a promising technology that continues to evolve with ongoing advancements [15]. In this system, bacteria utilize oxygen generated through algal photosynthesis to support respiratory metabolism and enhance contaminant removal. Simultaneously, algae assimilate the CO₂ produced by bacterial activity, creating a mutually beneficial cycle. This synergistic metabolic relationship enables different microorganisms to capitalize on their respective metabolic strengths, collectively improving wastewater treatment efficiency [16]. The ABS system has demonstrated remarkable effectiveness in removing diverse pollutants and has been successfully implemented in real-world applications [17,18,19,20]. Extensive research has documented its dual benefits in wastewater purification and the production of high-value bioproducts, including lipids, biogas, and bioethanol [21,22,23,24,25,26,27,28].

The algae–bacteria symbiotic (ABS) system demonstrates superior performance to conventional wastewater treatment methods (e.g., activated sludge) in terms of capital expenditure (CAPEX), operating expenditure (OPEX), and production of high-value byproducts. Unlike traditional systems, ABS implementation eliminates the need for energy-intensive aeration equipment, requiring instead photobioreactors and algae harvesting systems. This configuration reduces unit construction costs by 30–50%. From an operational perspective, ABS systems offer multiple advantages: significantly lower aeration requirements, reduced nutrient demands, and complete elimination of sludge treatment needs. Notably, these systems exhibit a 48.6% lower global warming potential (GWP) compared to conventional activated sludge processes [29], thereby decreasing associated carbon compliance costs. Furthermore, ABS technology enables valuable byproduct generation. During wastewater treatment, Chlorella biomass production increases by 4.71-fold, yielding biofuel suitable for biodiesel production with a market value of USD 300–800 per ton [30]. The system’s photosynthetic carbon fixation capability also generates emission reduction credits valued at USD 20–50 per ton [31].

Current reviews of ABS systems primarily concentrate on wastewater with single-pollutant compositions [32,33], including municipal wastewater, seawater effluent, pharmaceutical wastewater, and dye wastewater. However, real-world wastewater typically exhibits complex compositions, and there is growing research interest in treating and recycling such complex wastewater streams. While some studies have reported ABS applications in complex wastewater treatment and resource recovery [34], a comprehensive systematic review and critical discussion of this topic remains lacking.

The ABS system continues to evolve dynamically alongside technological progress. Given the rapid developments in this field, conducting timely reviews of research advances becomes crucial for evaluating the system’s application potential and future trends in complex wastewater treatment. Accordingly, this review systematically examines four key aspects: (1) fundamental concepts and classification of ABS systems; (2) their applications in complex wastewater treatment and associated influencing factors; (3) abiotic and biotic mechanisms governing algae–bacteria interactions; and (4) current challenges and future research directions.

2. Basic Concept and Classification of the ABS System

2.1. Basic Concepts

The ABS system enables complex algae–bacteria interactions via three key mechanisms: nutrient exchange, signal transduction, and gene transfer. In wastewater treatment applications, this system achieves comprehensive pollutant removal (including nutrients, antibiotics, and heavy metals) while enabling resource recovery and biomass cycling of carbon, nitrogen, and phosphorus (Figure 1). These synergies maintain system stability and functionality, where photosynthetic oxygen acts as a crucial electron acceptor for bacterial respiration. This process boosts bacterial metabolism, enabling aerobic denitrification, calcium precipitation, and nutrient removal [35,36]. Meanwhile, algal chloroplasts assimilate bacterial CO₂ via Rubisco-mediated fixation [37,38], enhancing photosynthesis and biomass production. ABS systems occur globally in freshwater, marine [39], and terrestrial ecosystems [40]. ABS systems play vital roles in global carbon/nitrogen cycling and primary productivity [41]. Their ecological ubiquity and functional versatility make them promising for wastewater treatment applications, which require further research [42].

2.2. Interaction and Mechanism of the ABS System

Nutrient Exchange Mechanisms: The ABS system facilitates a mutualistic nutrient exchange cycle wherein microalgae convert light energy into chemical energy through photosynthesis, providing organic substrates essential for bacterial metabolism. Concurrently, metabolic byproducts derived from bacterial activity are assimilated by microalgae, thereby completing a sustainable nutrient recycling loop. Furthermore, the dynamic interactions between microalgae and bacteria—encompassing horizontal gene transfer (HGT) as well as organic and inorganic substance exchange—confer enhanced stress resistance to the ABS system under extreme conditions, including high acidity, elevated salinity, extreme temperatures, and heavy metal exposure (Figure 2) [43]. This symbiotic relationship is further enhanced when bacteria reciprocate by producing essential growth factors for algae, including nitrogen compounds and B vitamins [44]. Studies of algal–fungal consortia demonstrate this synergy clearly: through carbon-source sharing, enzymatic hydrolysis, and coupled photosynthesis–respiration processes, these systems achieve synchronous peaks in both biomass and chlorophyll-a (Chl-a) production, along with 75% ammonium nitrogen (NH₄⁺-N) removal efficiency [45]. Remarkably, even after enzymatic activity and biomass decline, the system maintains high removal efficiencies, indicating the structural stability and sustained functionality of the initially formed algal–fungal aggregates.

Signal Transduction Pathways: Inter-organism communication in ABS systems primarily occurs through quorum sensing (QS) molecules [46], which regulate gene expression and physiological processes in both partners. Research has revealed a sophisticated signaling network where the bacterial secondary messenger cyclic diguanylate (c-di-GMP) coordinates exopolysaccharide (EPS) production, thereby promoting algal–bacterial aggregation. Simultaneously, the autoinducing peptide (AIP) enhances inter-kingdom signaling, up-regulating chlorophyll biosynthesis genes. When these signaling pathways synergize at optimal concentrations (3.85 μg/g c-di-GMP and 2.81 mg/L AIP), the system achieves peak removal efficiencies: 86.55% for soluble chemical oxygen demand (sCOD), 88.95% for total dissolved nitrogen (TDN), and 80.28% for total dissolved phosphorus (TDP) [47].

Research has shown that in a normally functioning ABS system with efficient material exchange and signal transduction, the growth rate of algae is 2.3 times higher than that in a mono-algal system [48], while chlorophyll content reaches 2 times that of the mono-algal system [49]. Additionally, the Cd removal efficiency was 91.3% (compared to 87.5% in the mono-algal system) [50], and thermal stress survival rates increased significantly [51]. However, when vitamin B12 synthesis was disrupted, algal growth dropped to just 15% of normal levels [52], indicating the critical role of bacteria-derived B12 in algal–bacterial material exchange. Furthermore, blocking indole-3-acetic acid (IAA) signal transduction reduced algal lipid accumulation to 30% of normal levels [53]. Depriving the system of a carbon source led to a 70% decrease in bacterial attachment [54], resulting in the failure of ABS system establishment. These findings demonstrate the essential roles of material exchange and signal transduction in the ABS system, with material transfer having a particularly significant influence.

Utilizing the QS mechanism can enhance the operational performance of the ABS system. For instance, N-acyl-homoserine lactones (AHLs) can stimulate algal morphogenesis, while compounds from Asparagopsis taxiformis extracts exhibit QS inhibition properties that suppress bacterial biofilm formation [55,56,57,58]. In the ABS system, both the exogenous addition of signal molecules and internal interactions can enhance the concentration of signal molecules, thereby improving treatment efficiency by affecting microbial activity (Figure 2) [59]. Liu et al. [60] demonstrated that AHL concentrations in both water and EPS phases increased significantly, particularly the C6-HSL and C8-HSL concentrations, which enhanced the activity of nitrite-oxidizing bacteria, thereby reducing the effluent nitrogen concentration. The enhancement of interspecies communication also strengthens the stability of the system. Notably, nanoparticles demonstrate context-dependent modulation of bacterial QS systems, exhibiting either enhancing or inhibitory effects.

Nanoparticle-Mediated Quorum Sensing Modulation: Research has demonstrated that metal oxide nanoparticles differentially regulate QS systems. Copper oxide (CuO) and zinc oxide (ZnO) nanoparticles effectively suppress QS by down-regulating related genes and reducing signal molecule concentrations (e.g., AHL and Autoinducer-2 (AI-2)), consequently inhibiting biofilm formation [61]. Conversely, silver nanoparticles (AgNPs) exhibit concentration-dependent stimulatory effects, with 10.8–21.6 mg/L concentrations up-regulating QS genes in Pseudomonas aeruginosa. Similarly, cerium dioxide (CeO₂) nanoparticles enhance AHL molecule production in wastewater biofilms [62,63].

Extracellular Vesicle-Mediated Communication: Beyond QS molecules, extracellular vesicles (EVs) serve as crucial mediators of intercellular communication. These membrane-bound particles, produced by organisms across all domains of life, facilitate the transport of diverse biomolecules including DNA, antimicrobial peptides, and QS signals. EVs significantly influence microbial community dynamics and ecosystem functions, playing particularly important roles in cellular signaling and the regulation of large-scale marine algal blooms [64,65].

HGT enables non-sexual genetic exchange between bacteria and algae (Figure 3). Algae such as green algae acquire nitrogen metabolism functional genes from bacteria, significantly enhancing both organisms’ adaptability and survival in extreme environments through improved tolerance to harsh conditions, including temperature extremes, high salinity, acidic pH, and heavy metal contamination [66,67]. A notable example is Galdieria sulphuraria, which has acquired bacterial genes via HGT to thrive in extreme habitats, underscoring the evolutionary importance of genetic exchange in algae–bacteria systems [68]. However, bacterial–algal interactions exhibit complex dynamics beyond mutualism; while bacteria often utilize algal-derived organic matter without reciprocal benefits, some parasitic bacteria actively harm algae by producing cell wall-lysing enzymes [69,70]. Additionally, resource competition for nutrients and light frequently occurs between these microorganisms [71].

2.3. Taxonomy of the ABS System

2.3.1. Classification by Symbiotic Bacteria

The high species diversity of both algae and bacteria provides a robust foundation for developing versatile symbiotic systems capable of addressing diverse pollutants. This biodiversity enhances system stability through complex ecological interactions, with specific bacterial groups offering targeted wastewater treatment solutions. Consequently, classifying ABS systems based on functional bacterial groups yields valuable operational insights.

(1)

Organic pollutant-degrading bacteria

These specialized bacteria play a dual role in ABS systems by (i) breaking down complex organic contaminants into bioavailable carbon sources for algal metabolism [73], and (ii) supplying essential nutrients through their metabolic activities to promote algal growth and improve treatment efficiency [74]. Notably, they mitigate the inhibitory effects of certain organic pollutants on microalgae [75]. In reciprocation, microalgae support bacterial communities by secreting growth-promoting extracellular compounds, including dissolved organic carbon (DOC) and complex lipids [76].

(2)

Heavy metal removal bacteria

Algae demonstrate particular vulnerability to HM toxicity, which manifests through multiple mechanisms [77,78,79,80]; this ultimately damages cellular integrity and metabolic functions, leading to significant reductions in both chlorophyll production and photosynthetic capacity [81]. In contrast, HM-resistant bacteria employ multiple detoxification strategies, including bio-adsorption, biochelation, metal exclusion, intracellular sequestration, and enzymatic transformation [82,83]. Within ABS systems, these bacteria play a protective role by mitigating HM toxicity toward algae, while benefiting from the favorable microenvironment created by algal growth. A notable example is the Pseudomonas putida–Chlorella vulgaris consortium, where bacterial activity neutralizes Cd (II) and Pb (II) toxicity, enabling algal survival under metal stress [84]. This mutualistic relationship can be further enhanced through quorum sensing mechanisms, as demonstrated by Agrobacterium sp. and Ensifer adherens symbionts with Chlorella vulgaris. These AHL-producing bacteria significantly improve HM removal efficiency by stimulating biomass production and increasing protein synthesis. Specific AHL molecules (3OC8-HSL and 3OC12-HSL) have been shown to boost both symbiotic growth and Cd (II) accumulation capacity [84]. Functional materials offer additional performance enhancements for ABS systems. For instance, activated carbon supplementation has been demonstrated to simultaneously increase microalgal biomass and lipid production while achieving >91% Cd (II) removal efficiency [85].

(3)

Nitrogen-converting bacteria

Nitrogen management through recovery and fixation represents a crucial aspect of wastewater treatment, serving dual purposes of eutrophication prevention and N₂O emission reduction. This process is particularly facilitated by symbiotic relationships between cyanobacteria (blue-green algae) and nitrogen-fixing bacteria [86]. Under nitrogen-limited conditions, algae form mutually beneficial associations with diazotrophic bacteria: photosynthetic algae provide essential carbohydrates as carbon sources to support bacterial growth and nitrogenase activity, while the bacteria fix atmospheric nitrogen for algal utilization.

The oxygen generated through algal photosynthesis creates a dynamic microenvironment that simultaneously benefits both nitrogen-fixing and nitrifying bacteria, thereby enhancing ammonium nitrogen (NH₄⁺-N) removal efficiency [87]. Within ABS systems, algae play a protective role by shielding nitrifying bacteria from environmental stressors, reducing bacterial mortality, and consequently improving overall nitrogen removal capacity. This synergistic interaction establishes an efficient coupled nitrogen–carbon cycle, optimizing wastewater treatment performance [88].

(4)

Phosphate-solubilizing bacteria

The biological removal of phosphorus fundamentally depends on phosphorus bioavailability, a critical nutrient for algal growth [89]. Phosphate-solubilizing bacteria play a pivotal role in this process by converting insoluble phosphates (including calcium phosphate and organic phosphorus compounds) into bioavailable orthophosphate (PO₄³⁻). This transformation establishes a mutualistic nutrient exchange between algae and bacteria: algae provide organic matter to support bacterial growth, while the bacteria mineralize phosphorus into forms readily assimilated by algae, creating a positive feedback loop that benefits both organisms [90].

A notable example is the introduction of Paenibacillus xylanexedens into ABS systems, which secretes beneficial compounds such as phosphatases, organic acids, proteins, and polysaccharides [91]. These secretions significantly enhance Chlorella pyrenoidosa biomass production and improve overall wastewater treatment efficiency. This symbiotic relationship becomes particularly vital in phosphorus-limited environments, where algae experience impaired photosynthesis and growth, while bacteria struggle to maintain essential metabolic functions.

2.3.2. Classification by Operational Mode

As mentioned above, the classification of functional bacteria provides a clear framework for us to understand the specific role of microorganisms in wastewater treatment, but the interaction between these microorganisms and their performance under different environmental conditions can be further explained by the operation mode of the system. The operation mode not only determines the cooperative relationship between microorganisms, but also affects the distribution of resources and the flow direction of metabolites, thus playing a decisive role in the efficiency and stability of the whole system. Next, we will further discuss the diversity and complexity of algae–bacteria symbiotic system from the key dimension of system operation mode, and reveal the dynamic changes inside the ABS system under different operation modes and its influence on wastewater treatment effects.

(1)

Batch and fed-batch operation:

Batch operation represents a discontinuous production mode where the ABS system completes a full treatment cycle from initial material input to final harvest without intermediate additions or removals. This self-contained approach offers several advantages such as simplified process control and parameter regulation, making it particularly suitable for laboratory research and small-scale applications [92]. Ayre et al. [93] demonstrated the effectiveness of batch operation in treating piggery anaerobic digestate, where optimization of the algae–bacteria ratio and digestate dilution achieved remarkable removal efficiencies of 94.3% for NH₄⁺-N and 87.2% for COD. Zheng et al. [94] developed algae–bacteria symbiotic granules in a photo-sequencing batch reactor and found that the removal rates of total nitrogen and phosphate were enhanced with higher algae biomass in the PSBR. The construction of an algae–bacteria granule system within this reactor demonstrates superior nutrient removal performance and enhanced residual biomass recovery value compared to conventional activated sludge processes [95]. However, inherent limitations remained: (1) unsuitability for industries requiring constant production rates, (2) reduced overall efficiency due to system cleaning and restart, and (3) higher operational costs associated with these periodic interruptions.

Thus, fed-batch systems have been proposed to address batch models’ limitations by (1) extending the productive growth phase of both algae and bacteria, (2) increasing biomass yields, and (3) allowing dynamic control of growth rates and biological composition through strategic nutrient addition [96,97]. Fields et al. [98] demonstrated that fed-batch modes significantly enhanced total biomass and recombinant protein density compared to traditional batch cultivation. These characteristics make fed-batch operations particularly valuable for biomass and biofuel production applications.

(2)

Continuous operation:

In the continuous operation model, raw materials and nutrients are continuously fed into the system while treated water and biomass are simultaneously harvested, which enables uninterrupted production without frequent shutdowns [99]. This model can maintain stable operation for extended periods, which is particularly suitable for large-scale applications, allowing for steady production of valuable outputs such as biofuels, proteins, and other bioproducts. Purba et al. [100] demonstrated that the ABS system maintains excellent stability in continuous-flow reactors, even when operating with double the organic and nutrient loadings. However, this method demands more sophisticated control systems and increased maintenance efforts, and requires stable influent conditions to ensure long-term system stability. For instance, Altimari et al. [101] developed mathematical models that provide valuable guidance for bioreactor design and control in continuous ABS systems, contributing significantly to their stable operation.

(3)

Semi-continuous operation:

Semi-continuous operation combines features of both batch and continuous systems. In this hybrid approach, the culture system undergoes periodic replacement of the partial medium, which sustains metabolic activity and reduces resource waste. Amini et al. [102] reported that semi-continuous algal–bacterial photobioreactors achieved the highest removal efficiencies of 94% for NH₄⁺ and 80% for PO₄³⁻. Research has demonstrated the effectiveness of semi-continuous cultivation. Yu et al. [103] achieved a 289.19% increase in biomass production of Synechocystis sp. PCC 6803 compared to conventional batch culture by implementing a growth model based on volume average light intensity (VALI). In another successful application, Fernández [104] cultivated algal symbionts in a 200 L open pipe pond using piggery wastewater (containing NH₄⁺-N 80 mg/L) as culture medium through semi-continuous operation. This system achieved exceptional treatment performance, with nearly complete NH₄⁺-N removal (100%), over 89% total nitrogen (TN) removal, more than 88% COD reduction, and greater than 80% phosphate elimination. The semi-continuous approach proves particularly advantageous for microorganisms and algae requiring extended cultivation periods, offering significant improvements in overall production efficiency compared to traditional batch systems.

2.3.3. Classification by System Configuration

The operational mode does not exist in isolation; it is closely related to the configuration of the system. System configuration, including the design of reactor, the setting of operating conditions, and the initial composition of the microbial community, provides a basic framework for the implementation of the operation mode. Reasonable system configuration can create a suitable environment for the symbiotic system of algae and bacteria, so as to give full play to the advantages of the operation mode and realize efficient wastewater treatment. Therefore, we will discuss the configuration of algae–bacteria symbiotic systems and analyze the influence of different configurations on the operation mode.

(1)

Open systems

Open systems are able to maintain continuous matter exchange with their surrounding environment, such as rivers and lakes. As a result, they are cost-effective in construction and operation, making them particularly suitable for large-scale ecological restoration and wastewater treatment applications. A classic example is the open pond system, which represents the most fundamental cultivation approach where algae grow under conditions mimicking their natural environment. In such open systems, the mutualistic interactions between algae and bacteria enhance ecological stability through increased biodiversity, particularly in critical functions like nutrient cycling and organic matter degradation [105]. The dynamic nature of rivers and stratified characteristics of lakes make species diversity in ABS systems particularly valuable for environmental adaptation. Various algal and bacterial species occupy distinct ecological niches, forming complementary metabolic networks that improve nutrient processing capacity. This biodiversity also strengthens system resilience, enabling maintenance of water quality despite external disturbances like pollution and eutrophication [106]—a crucial factor for effective river and lake management.

However, open systems are more vulnerable to contamination by undesirable microorganisms compared to closed reactors. To enhance pollution resistance, several strategies can be employed: early detection and monitoring systems, physical/chemical treatments, and application of biocidal agents when necessary.

(2)

Closed systems

Closed systems, exemplified by photobioreactors, function within fully controlled environments that enable precise regulation of growth conditions to maximize algal and bacterial productivity [107]. In general, closed systems demonstrate superior efficiency compared to open systems due to the higher anti-contamination capacity, which is more appropriate for the production of high-value compounds [108]. However, these advantages come with notable limitations, including substantial capital investment requirements, higher operational expenses, and dependence on sophisticated technical infrastructure.

(3)

Hybrid systems

The hybrid system combines the advantages of both open and closed systems by incorporating internal material circulation while allowing controlled external material exchange [109]. This integrated approach typically employs a closed photobioreactor during the initial culture phase to optimize environmental conditions, followed by transfer to an open system for large-scale production [110]. Hybrid systems effectively simulate natural ecosystems while maintaining adjustable environmental control, offering exceptional flexibility in practical applications where the extent of material exchange can be precisely regulated. These systems are particularly suitable for applications requiring a balance between natural conditions and controlled intervention, such as semi-natural ecological restoration projects.

To address the biomass harvesting issues in hybrid systems, innovative hybrid solutions like biofilm reactors (BRs) have been proposed. BRs function as attachment-based growth systems where algae, bacteria, and fungi form biofilms on solid supports, allowing for efficient harvesting through scraping [111]. A notable example is the vertical rotating algae–bacterial symbiotic biofilm reactor developed by Yu et al. [112], which achieved remarkable removal efficiencies of 95.6% COD, 96.1% TN, 97.6% TP, and 100% NH₄⁺-N when treating mixed municipal and soybean-processing wastewater. When combined with a high-yield algae pond, this system produced approximately 2.6 times more biomass than conventional systems, with a harvesting efficiency of 61%—nearly triple the 22% efficiency of traditional methods.

3. Applications and Influencing Factors of the ABS System

3.1. Applications

The application of the ABS system includes both laboratory-scale and full-scale implementations, addressing various types of wastewaters such as industrial, municipal, and aquaculture wastewater [113].

Table 1. Comparison of the ABS system and single biological treatment efficiency.

Types of Wastewater	Algae Species	Bacterial Species	Pollutants and Removal Efficiency	Application Characteristics	Innovation	References
Lab-scale HM wastewater	Chlorella pyrenoidosa	/	Cu 80%	Low treatable concentration. The HM removal type is single.	/	[114]
	Chlorella vulgaris	/	Cd 40–70%	Low removal efficiency. The HM removal type is single.	/	[115]
	Scenedesmus obliquus	/	Cd 60–75%		/
	Fucus vesiculosus	/	Cd 143.2 ± 7.5 mg/g, Ni 70.1 ± 1.9 mg/g, Pd 516.3 ± 12.5 mg/g	Low adsorption efficiency. Greatly influenced by the environment.	/	[116]
	/	Aspergillus niger	Pb 85.6%, Cd 80%	Low treatable concentration. Poor adaptability of strain	/	[117]
	/	Trametes versicolor	Cu 88.35%, Cd 87.69%, Pb 96.8%, Zn 91.9%	Long incubation time of strain. Low removal efficiency.	/	[118]
	Chlorella vulgaris	Enterobacter sp. Mn17	Cu 90%, Cd 85%	Environmentally friendly. High value-added products can be produced.	Synergistic interaction—Bacteria enhance heavy metal adsorption via EPS, while Chlorella further degrades or immobilizes the metals. Engineering potential—Simultaneous pollutant elimination and resource recovery.	[119]
	Chlorella pyrenoidosa	Bacillus subtilis	Cu 72.9%, Cd 70% Zn 73%	High biological tolerance. Low cost and sustainability.	Non-sterile synergistic treatment: Indigenous bacteria secrete EPS to promote algal aggregation and sedimentation, enabling synergistic heavy metal removal and low-cost harvesting. Dilution strategy optimization: Ten-fold dilution with deionized water reduces heavy metal toxicity to microalgae, preserves bacterial growth promotion, and brings inorganic nutrients closer to discharge standards. Heavy metal tolerance mechanism: Microalgae chelate heavy metals, dilution reduces ROS, and bacteria acidify the medium, forming a feedback loop between HMs, microalgae, and bacteria.	[120]
	Navicula seminulum	Alcaligenes faecalis	Cr 71.8%, Hg 74.8% Pb 79.6%, Cd 72.5%	High removal efficiency. Low cost and sustainability.	Immobilization innovation: A co-immobilized microalgae-bacteria consortium was engineered into stable microbeads, significantly enhancing HM removal compared to free-cell systems. Synergistic mechanism enhancement: Microalgal–bacterial interactions overcome the inherent limitations of free-cell configurations, such as biomass washout and poor shock resistance. In situ remediation capability: The immobilized beads, designed for aquaculture sediments, work without aeration or renewal, offering a cost-effective, sustainable solution.	[121]
Multi-POPs wastewater	Chlorella pyrenoidosa	/	Nonylpheno l90.9%	Dependent metabolic enhancement. Metabolite increase. The POP removal type is single.	/	[122]
	Eucheuma cottonii	/	Congo red 91%	Decline in regeneration efficiency. Complex preparation process of composite materials.	/	[123]
	Chlorella vulgaris	/	PAHs 38–96%	Strict nutritional conditions. Unstable removal efficiency.	/	[124]
	/	Acinetobacter Stenotrophomonas Comamonas	Phenanthrene 88.67%, Pyrene 36.42%	Environmental condition limitation. Synergistic reagents lead to system alkalinity problems.	/	[125]
	/	Pseudomonas aeruginosa	Bipheny l95%	Low removal efficiency. Complex practical application.	/	[126]
	/	Alcaligenes faecalis	Tetracycline 68.78%	Low removal efficiency. Time consuming.	/	[127]
	Tetradesmus obliquus	Alcaligenes faecalis	Tetracycline 93.87%	High removal efficiency. Various ways of removal.	Triple-synergistic immobilization strategy: Tetracycline-degrading bacteria and microalga were co-immobilized in a PVA–SA–PDI matrix, with PDI enhancing tetracycline transfer and supporting consortia formation. Photosynthetic O2-CO2 reciprocity loop: Photosynthetic oxygenation by the microalga sustains bacterial respiration, while bacterial CO2 fuels algal carbon fixation, creating an “oxygen-carbon” loop that outperforms single-species systems. Robustness under extreme contamination: The consortium removes >77% tetracycline and 79–85% organic carbon from complex wastewaters, showing high adaptability to multi-pollutant environments.	[127]
	Chlorophyta Bacillariophyta	Fee-living biosphere	PFBA, PFPeA, PFHxA, PFHpA, PFOA, PONA, PFDA, PFUdA, PFDoA, PFTeDA, PFBS, PFHxS, PFOS	High efficiency PFASs removal ability. Multi-biosphere synergy. Alleviation of oxidative stress	PFAS transport pathway: Chain-length-dependent migration (chemosphere → phycosphere → biosphere → sand → silt-clay) with SEM-verified fluxes. Algae–bacteria-sediment eco-coupling mechanism: The consortium enhanced PFAS removal via ROS signaling, with pigments counteracting oxidative stress and bacteria modulating pollutant transformation. Carbon-chain-community turnover hypothesis: Chain-length heterogeneity steered stochastic vs. deterministic community assembly (βNTI), reshaping micro-ecosystem structure and function.	[128]
	Chlorella sorokiniana	Bacteria in goldfish culture pond	Sulfamethoxazole 54.34% ± 2.35% Partial CECs removal capacity	Excellent biological synergy. Propagation of low antibiotic resistance gene (ARGs). Multi-pollutant removal potential.	Synergistic metabolic activation: The study elucidates the synergistic mechanism of “photosynthetic oxygen production by microalgae and co-metabolic degradation by bacteria”. Complex matrix adaptability: The study breaks through the limitations of traditional simplified systems by achieving a SMX removal efficiency of 54.34% in real secondary effluent from a WWTP. Functional community homeostasis: Under SMX stress, the consortium maintains homeostasis through synergistic ROS buffering by microalgal pigments and bacterial proliferation, offering a new strategy for co-removal of antibiotics and ARGs.	[129]
POPs-HMs Composite wastewater	Chlorella sorokiniana	/	Zn 86.14%, Etrone 84.96%	Poor long-term operation stability. Sensitive to environmental conditions.	/	[130]
	Chlorella vulgaris	Bacteria in aerobics activated sludge	Cu 66.7% Sulfadimidine 91.3%+	High removal efficiency. High biomass and oil content. Low ability to spread antibiotic-resistant genes.	Enhanced algal growth and lipid accumulation: Bacteria secrete extracellular substances promoting algal growth and supplying essential nutrients, leading to higher lipid yields. Selective symbiotic colonization: Microalgae select dominant symbiotic bacteria, resulting in a stable, efficient microbial community with fewer species and improved system stability and pollutant removal. Robustness under stress conditions: Microalgae–bacteria consortium shows strong resistance to heavy metals and antibiotics, adapting to Cu (II) and SM2 stress while maintaining higher biomass and lipid content under composite pollution.	[131]
	Chlorella sp., Scenedesmus obliquus, Stichococcus strains, Phormidium sp.	Rhodococcus sp. Kibdelosporangium aridum	Zn 90%, Cu 62%, Ni 62%, Mn 70%, Fe 64%, BOD 97%, Oil leakage 96%	High pollutant removal efficiency. Stable symbiotic system. High economic benefit. Environmentally friendly. Excellent sustainability.	Stabilization of microbial community: Fixing algae and bacteria on carriers constructs a stable microbial community. Algal secretions fix bacteria and prevent washout, enhancing system stability and impact resistance.	[132]
Real wastewater	Scenedesmus Obliquus	/	COD 86.4%, TN 93.4%, TP 93.4%	Wastewater needs pretreatment. Strict operating conditions. Low economic benefit.	/	[133]
	/	Trichoderma harzianum	COD 55.1%, BOD 40.8%, NO3-N 33.9%	Low removal efficiency. Poor long-term stability.	/	[134]
	Rhodotorula, Apiotrichum	Acidocella, Enterobacter, Delftia, Macellibacteroides	NH4+-N 96.4%, TN 97.3%, COD 98.6%	High removal efficiency.	Electrogenic synergy—Delftia YP-3 enhances algal–bacterial cooperation and nitrogen metabolism through extracellular electron release. Metabolic boost—YP-3’s superior electron transfer elevates microbial activity and community stability. Community engineering—YP-3 drives functional microbe enrichment via electron transfer, optimizing system nitrogen removal.	[135]
	Scenedesmus, Auxenochlorella, Nitzschia	Candidatus Competibacter, Candidatus Accumulibacter, Dechloromonas, Thauera, Ferribacterium et al.	NH4+-N 99%, TN 78%, PO43-P 95%, COD 92%	Fast start-up speed. High removal efficiency. Resource recycling. Environmentally friendly. Good adaptability.	Efficient utilization of actual municipal wastewater: adding exogenous organic carbon (e.g., sodium acetate) to municipal wastewater significantly boosts Chlorella vulgaris growth and oil productivity, offering a new approach to replacing artificial media with complex wastewater in microalgae culture. Optimization and synergy of organic carbon sources: The optimum sodium acetate concentration of 1 g/L increased Chlorella vulgaris dry weight and oil productivity by 2.40 and 2.44 times, respectively, while enhancing nitrogen and phosphorus removal by 1.75 and 2.23 times, highlighting the synergistic potential of organic carbon in the ABS system. Nutrient removal ability of ABS system: The ABS system effectively removes organic pollutants and reduces nitrogen and phosphorus in municipal wastewater, achieving both biofuel production and nutrient recovery, offering a new approach for wastewater resource utilization.	[120]

Note: The / form indicates an algae-free and bacteria-free system.

presents a comparative analysis of algae-only, bacteria-only, and ABS systems for various wastewater types.

3.1.1. Laboratory-Level Multi-HM Wastewater

The interaction between algae and bacteria could enhance the removal efficiency of 20–30% of HMs through biosorption and bioaccumulation with extracellular polymers (EPS) [136]. System structure, environmental factors, and coexisting substances influence the HM’s removal efficiency. For instance, the increase in the Aspergillus oryzae ratio obviously enhanced the removal rate of Cu in ABS [137]. The pH adjustments also optimize the fixation effect of algae on HMs, and thus improve the removal rate [138]. This synergistic process enables microalgae to effectively fix and precipitate HMs, which improves the removal efficiency. Similarly, in the co-culture of Chlorella vulgaris and Bacillus subtilis, bacteria significantly enhanced the adsorption capacity of microalgae to Cd (II) and Pb (II) by secreting organic acids and EPS [139].

3.1.2. Laboratory-Level Multi-POP Wastewater

The ABS system effectively treats POP wastewater by utilizing mechanisms such as biosorption, bioaccumulation, and biodegradation. Degradation of POPs is a complex process, and bacteria and algae typically play roles at different stages. Li et al. [140] constructed two series of ABS systems with different sequences: system A used microalgae for preliminary treatment followed by sludge, while B reversed the order. The initial degradation of anthraquinone dyes by algae in system A led to a better perform of COD removal and decoloration. Another important removal pathway is the adsorption through the functional groups of EPS secreted by algae and bacteria, as well as bioaccumulation through active transport. In addition, algae and bacteria can continuously eliminate antibiotics through dissolution, enzymatic degradation, and cell phagocytosis, preventing their accumulation in cells [141,142].

3.1.3. Laboratory-Level POP-HM Composite Wastewater

POPs often coexist with HMs in wastewater. The interactions between POPs and HMs, combined with their distinct impacts on microbial communities, significantly increase the complexity and challenge of simultaneous removal. The synergistic relationship between algae and bacteria enhances the overall remediation efficiency for both POPs and HMs. Ethiraj et al. [143] highlighted the potential of ABS in treating composite wastewater containing both POPs and HMs. This system effectively utilizes the complementary roles of algae and bacteria to degrade organic compounds while simultaneously absorbing and accumulating HMs. Algae can effectively transform toxic substances through diverse nutritional modes (heterotrophic, mixotrophic, and phototrophic) using processes such as absorption, adsorption, and metabolism. POPs are converted through co-metabolism processes, and bacteria exhibit similar effects. Moreover, certain bacterial groups undergo redox cycling in ABS (light/dark cycle), which may accelerate the removal of pollutants. Eheneden et al. [20] found that, under aerobic conditions, bacteria oxidize antibiotics using enzymes or cofactors in the presence of oxygen, while under anaerobic conditions, antibiotics are reduced by enzymes or cofactors produced during microbial treatment of another major substrate. The co-metabolism process can enhance the degradation efficiency of antibiotics, and when co-substrates such as sodium acetate or glucose are added, the removal rate of specific antibiotics is significantly improved.

Micro-pollutants should not be ignored in POP-HM composite wastewater. The presence of micro-pollutants poses challenges by exerting physiological and biochemical stress on algal cells, which can lead to decreased chlorophyll and carotenoid levels, thereby affecting photosynthetic efficiency and growth. Micro-pollutants can also impact the symbiotic relationship between algae and bacteria by altering bacterial attachment behaviors, influenced by changes in algal surface structures and excretions. The ABS system facilitates the removal of complex contaminants, including endocrine disruptors and pharmaceuticals [144,145]. Although the composition of micro-pollutants varies widely and removal efficacy can fluctuate with environmental factors like pH and temperature, the ABS system provides a cost-effective, eco-friendly solution to meet the growing demands in wastewater management [146].

3.1.4. Wastewater Under Extreme Conditions

The ABS system demonstrates robust performance even under extreme environ-mental conditions, including extremely low carbon-to-nitrogen (C/N) ratios, high salinity, and extreme pH levels. Under low C/N conditions (C/N < 3.5), the ABS system achieves 10–30% higher TN and NH₄⁺-N removal efficiencies compared to conventional activated sludge, A/O systems, and membrane aeration biofilm reactors—all without requiring supplemental carbon sources or mechanical aeration [147,148,149].

Studies on salinity effects indicate that the ABS system performs optimally at 0.5 ppt (parts per thousand), with 10 ppt representing a critical threshold beyond which performance gradually declines. At 15 ppt, pollutant removal efficiency decreases significantly, and at 20 ppt, the system experiences substantial inhibition [150]. When compared to other biological and physicochemical methods, such as algal–bacterial granular sludge (ABGS), membrane photobioreactors (MPBRs), activated sludge (AS), and electrochemical oxidation, the ABS system exhibits the widest operational salinity range. It also demonstrates superior COD removal (outperforming ABGS and AS), comparable TN removal to MPBR, and significantly higher TP removal efficiency than all other methods [130,151,152].

Under highly acidic conditions (pH = 3.5), the ABS system maintains stable performance, with only an 8% reduction in NH₄⁺-N removal and a 64% decrease in Chl-a content compared to neutral pH conditions. Remarkably, TP degradation remains highly efficient at 92% [153].

Although extreme environments inevitably impose some stress on the ABS system, it consistently outperforms other biological and physicochemical wastewater treatment methods, demonstrating superior adaptability and removal efficiency across diverse challenging conditions.

The ABS system demonstrates superior adaptability and stability when exposed to antibiotic stress. Research indicates that while standalone microalgae systems experience growth inhibition or even cell death at high antibiotic concentrations (>100 mg/L), the ABS system exhibits enhanced resilience. This improved performance stems from two key mechanisms: bacterial communities share the antibiotic degradation burden, thereby reducing direct toxic exposure to algae, and co-metabolic interactions between algae and bacteria significantly decrease antibiotic retention time in the system [154].

3.1.5. Actual Wastewater

Actual wastewater, such as livestock wastewater, typically contains a high concentration of NH₄⁺-N, which could inhibit nitrification and other biological treatment processes. Huang et al. [155] achieved the effective removal of COD, N, and P from piggery wastewater by integrating anaerobic ammonium oxidation (Anammox) granular sludge with the ABS system. Biogas technology generates large amounts of anaerobic digestion effluents (ADEs). The ABS system efficiently removed COD, TN, NH₄⁺-N, and TP in ADEs, achieving removal rates of 73.78%, 80.67%, 89.74%, and 95.39%, respectively [156]. Phenol is a typical wastewater pollutant and has complex toxicity to microalgae [157]. Transcriptome analysis revealed that Chlorella sp. L3 is tolerant to high-concentration phenol, which is associated with the up-regulation of genes related to oxidative stress, pigment synthesis, and photosynthesis in phenol-tolerant strains [158]. Traditional wastewater treatment methods often fail to kill several pathogens. However, the ABS system shows considerable potential for inactivating pathogens, including bacteria and viruses from wastewater. The pathways include gravity sedimentation [159], adsorption and encapsulation, as well as microbial metabolism and decomposition [160]. The ABS system demonstrates effective application in constructed wetlands for antibiotic wastewater treatment. By incorporating gravel matrices, the system mitigates antibiotic-induced stress on microalgae and microorganisms, thereby enhancing overall treatment efficiency [161]. Some researchers also realized more advanced treatment of mariculture wastewater or poultry wastewater [162] by adjusting the proportion of algae inoculation in ABS systems.

The practical application of the ABS system still faces limitations and bottlenecks. A significant challenge lies in bridging the gap between laboratory research and engineering practice, as real-world conditions often differ substantially from controlled experimental environments. Its adaptability to diverse pollutant types and concentrations is also constrained, particularly under fluctuating or extreme conditions. Moreover, the downstream processing and resource recovery of biomass generated within the ABS system remain unresolved [163]. Additionally, the intricate interactions between microorganisms within the symbiotic system introduce a level of complexity that is not yet fully understood, posing difficulties for system control and optimization [164]. Various strategies have been employed to improve the ABS system efficiency, such as adding external carbon sources and using novel immobilization materials. By optimizing the external carbon source, the lipid accumulation and self-flocculation efficiency of ABS in wastewater treatment can be significantly improved, thus enhancing the economy and resource recovery potential of the system [165]. Formate was found to be a favorable carbon source to effectively promote biomass production and carbon capture [166]. Xu et al. [167] constructed an immobilized ABS biofilm reactor using pink-fluorescent filler, which significantly enhanced both the N and P removal efficiency of the ABS system and the vitality of the algae.

3.2. Influencing Factors

Operation of the ABS system relies on several factors, including the algae–bacteria inoculation ratio, hydraulic retention time, aeration, light, and inorganic carbon (IC) levels (Figure 4). Optimal conditions are necessary to maintain the balance between algae and bacteria, thereby promoting system stability. For example, temperature serves as the critical fulcrum balancing oxygen supply and demand in ABS systems. When the temperature is lower than 35 °C, the activity of algae and bacteria will double for every 10 °C increase in the environment [168]. An optimal temperature range (like 30–35 °C) maximizes algal oxygen release, precisely meeting the elevated oxygen demand of heterotrophic bacteria during COD and ammonia nitrogen degradation. This thermal optimization enables highly efficient wastewater treatment with minimal external energy input. The complex wastewater matrix often has fluctuating environmental conditions, and the factors that will interfere with the system are not single. A study has integrated hydraulic retention time and carrier dosage into the validated central composite response surface model, and optimized the interdependent operating variables at the same time, and confirmed the robust optimal value (1.4 d, 0.18 g/L) through experiments, which can ensure the pollutant removal rate of >94% and prevent the system from collapsing under fluctuating wastewater load [169]. Leveraging a “multi-model–multi-parameter–global optimization” paradigm, another study employed advanced machine learning techniques to simultaneously calibrate the tightly coupled variables of pH, temperature, contact time, and algae-to-bacteria ratio within the algal–bacterial symbiotic system. This integrative approach secured maximal pollutant-removal performance while proactively safeguarding the system against operational collapse [170].

3.2.1. Proportion of Inoculation of Algae and Bacteria

The algae–bacteria inoculation ratio is a critical and controllable factor in designing an ABS system [172]. The inoculation ratio determines the balance between nutrient supply and demand, which subsequently affects the overall metabolic efficiency. The treatment performance serves as the most direct indicator for determining the optimal inoculation ratio. Research indicates that maintaining an optimal algae–bacteria ratio (1:1) maximizes bioremediation efficiency for Pb (II) and Cd (II) removal [173]. An appropriate inoculation ratio of 90%:10% (C. vulgaris/NAS, w/w) facilitates the development of bacteria–algae flocs, enhancing pollutant adsorption and biodegradation through the secretion of EPS [17]. A proper inoculation ratio boosts system stability and stress resistance by enabling microbial communities to compensate for each other’s weaknesses under adverse conditions, ensuring more reliable wastewater treatment [174]. Rong et al. [18] found that an inoculation ratio of 3 (sludge/algae = 360 mg/L: 120 mg/L) was optimal in aquaculture wastewater, achieving removal efficiencies of 96.22% for COD, 92.43% for N, 94.56% for P, and 95.06% for sulfamethoxazole (SMX).

3.2.2. Aeration Duration and Intensity

Aeration duration directly affects the DO level, which is crucial for many biochemical reactions of bacteria. Aeration also improves system mixing, enhancing the contact between microorganisms and pollutants. However, excessive CO₂ stripping from over-aeration limits algal growth, disrupts system stability, and reduces wastewater treatment efficiency, while moderate CO₂ stripping alleviates competition between nitrifying bacteria and algae, thereby enhancing both the stability and efficiency [175]. Tang et al. [176] also found that aeration rates higher than the optimal rate (0.2 L air/min) could weaken the synergistic effects and disrupt the symbiotic balance in the ABS system. In addition to efficiency, optimizing energy consumption is a critical consideration in regulating aeration, as it accounts for 60–80% of the total energy consumption in wastewater treatment plants [177]. Appropriate aeration duration maintains nutrient and gas distribution while influencing system stability, microbial community structure, and the cost-effectiveness of system operation.

3.2.3. Light Intensity and Wavelength

Light intensity and wavelength directly influence photosynthetic efficiency. Research indicates that blue light (450–495 nm) not only increases biomass productivity and lipid accumulation in Nannochloropsis but also optimizes energy use, thereby enhancing the potential of algae for biodiesel production [178]. Furthermore, different single-wavelength light qualities can lead to the gene expression differences of Spirulina platensis. Moreover, the results further showed that red light (620–750 nm) significantly increased polysaccharide content in Spirulina platensis, followed by blue (450–495 nm), green (495–570 nm), and yellow (570–590 nm) light, while the white light control group exhibited the lowest polysaccharide content. Notably, blue light had a positive effect on both the growth and polysaccharide accumulation of Spirulina platensis [179]. The effects of light intensity and wavelength will ultimately translate to the wastewater treatment process. Optimal light conditions can significantly enhance the photosynthetic efficiency of algae, leading to maximized oxygen production [180]. At a light intensity of 500 μmol/m²/s, Chlorella pyrenoidosa exhibits the highest oxygen production efficiency, reaching a rate of 136.39 μmol of oxygen per mg of chlorophyll a per hour [181]. Increasing light intensity, ranging from 50 to 300 μmol photons/m²/s, accelerates algal photosynthesis, leading to a rise in DO production of up to 9.5 mg/L. However, excessively high light intensity can cause photo-inhibition [182], greatly affecting algal growth and the long-term stability of the system [183].

3.2.4. Carbon Source and Interaction Between Algae and Bacteria

The carbon cycle in ABS systems initiates with algae converting CO₂ into organic carbon through the Calvin cycle (catalyzed by Rubisco), while bacteria decompose organic matter and regenerate CO₂ via the tricarboxylic acid (TCA) cycle, establishing a closed-loop carbon fixation and transformation process. Then, the qualitative accumulation of fat is completed as a part of high value-added products produced by the ABS system. The fixed organic carbon (from CO₂) is further channeled into lipid biosynthesis via acetyl-CoA, while residual IC (HCO₃⁻/CO₃²⁻) dynamically buffers the pH to sustain enzymatic efficiency in this process. IC typically exists in water as CO₂, H₂CO₃, HCO₃⁻, and CO₃²⁻. As the primary substrate for algal photosynthesis, algae utilize IC to synthesize organic matter, supporting biomass accumulation [184]. In an open system, continuous operation can make IC a limiting factor for algal growth. Avoiding IC limitation is crucial for optimizing biomass production and nutrient removal. Sufficient alkalinity is essential for the availability of IC in the ABS system [185]. Bacteria can enhance algal growth under carbon-limited conditions by facilitating the carbon cycle [186]. Specifically, symbiotic interactions between microalgae and nitrifying bacteria demonstrate this mechanism, wherein IC serves as a substrate for both cellular biosynthesis and energy metabolism. The concentration of IC is positively correlated with ammonium removal efficiency in the system. The carbon source equilibrium (CO₃²⁻/HCO₃⁻) shifts toward CO₃²⁻ at higher pH levels, potentially reducing organic matter oxidation rates, decreasing oxygen production, and creating hypoxic conditions for bacterial communities [181]. This will reduce the efficiency of the system to treat wastewater. Therefore, to improve the ammonium removal rate in wastewater, the IC content can be increased by optimizing the pH and enhancing the CO₂ supply [187]. Secondly, insufficient IC limits the ammonia oxidation process of ammonia-oxidizing bacteria, which in turn reduces N₂O production. This is likely due to the critical role of IC in supporting the biochemical pathways required for efficient ammonia oxidation. Li et al. [188] studied the effect of IC concentration on ammonium removal and N₂O production in an algae-nitrifying bacteria symbiotic system. Their findings showed that IC was positively correlated with both ammonium removal and N₂O production. At the optimal IC concentration of 10 mmol/L, a high ammonium removal rate and low N₂O emission rate was achieved. The interaction between IC and the ABS system requires further exploration.

Nutrient dynamics reveal complex algae–bacteria interactions—N/P deviations from the Redfield ratio trigger algal lipid (N-limited) or protein (P-limited) biosynthesis, subsequently altering bacterial EPS production. Trace elements like Fe, as essential cofactors for nitrogenase and algal enzymes, play pivotal roles, with deficiencies causing electron transport chain disruption. In the system, there is QS interaction between algae and bacteria. For example, 3OC12-HSL secreted by bacteria can up-regulate the nitrogen transport gene of algae and promote NH₄⁺ absorption [84]. Meanwhile, EPS secreted by algae can provide an adhesion matrix for bacteria and accelerate co-metabolism to degrade pollutants.

3.2.5. Application Cases and Practical Operation

The selection of reactors and carriers is crucial in practical operations. Tang et al. [189] established an algae-sludge sequencing batch reactor as an ABS prototype, operating at 24 ± 2 °C with a 24 h cycle. Algal photosynthesis enhanced bacterial activity and stimulated the secretion of bacterial EPS, which increased sludge viscosity and promoted the formation of algal biofilms. The substrate carrier plays a key role in the process of culture and biofilm formation, and different types of substrates have significant effects on the attachment and growth of microorganisms. Common matrix carriers include plastic fillers, ceramics, and activated carbon. The performance of the carrier is closely related to surface structure, hydrophilicity, and porosity. For example, plastic fillers are usually widely used because of their large surface area and customization, while ceramics can provide a richer microbial habitat and promote the degradation efficiency of pollutants because of their high porosity [190]. Chen et al. [191] implemented the system in wastewater treatment ponds, where algae absorbed nutrients and converted them into biomass, thereby reducing the nutrient load in the wastewater. The system’s efficiency was highly influenced by climatic conditions, especially the availability of sunlight, leading to performance fluctuation. Aside from climate dependence, microbial competition [192], environmental adaptability, and long-term stability are critical factors to consider [193].

Response surface methodology (RSM) is widely used to optimize multi-factor experimental conditions, enabling the exploration of factor interactions [194]. Orthogonal design is another approach that determines the best combination of multiple factors using fewer experiments [195]. Dynamic monitoring involves real-time tracking and data collection of key system variables, providing insights into system performance under varying conditions [196]. Lastly, big data technology leverages large datasets and advanced processing tools, such as machine learning, for system modeling and decision optimization [197]. Some researchers have simulated the operation of the system under different conditions by constructing mathematical models, which significantly enhance the efficiency of parameter tuning in complex systems [198].

4. Summary and Prospect

This review aims to present an overview of complex wastewater treatment by ABS systems, with particular attention on the contribution of interaction mechanisms between algae and microbes, as well as the comparative analysis of the superiority of ABS systems facing different wastewater types. Although the ABS system has made rapid progress, its operation still encounters numerous technical and operational challenges. One key challenge is controlling the conditions (e.g., light condition, carbon source amount, algae–bacteria ratio) to ensure the system operates stably [183,199]. Closed systems offer precise control over environmental factors, but they are associated with higher energy consumption and operational costs [200], and the accumulation of metabolic byproducts and fluctuations in bacterial or algal populations could also disrupt system balance and reduce efficiency [201,202]. While open systems are less resource-intensive, they are more susceptible to external pollutants and environmental changes. Pollutant removal in the ABS system is highly sensitive to extreme environmental conditions. For example, excessive increases in medium pH negatively impact algae biomass productivity by promoting ammonia volatilization and restricting carbon uptake [203]. Similarly, temperature plays a crucial role, significantly affecting bacterial growth and metabolic activity. The activity of nitrifying bacteria decreased by 50–60% in the open system at low temperatures (<10 °C) in winter, which led to the decrease in ammonia nitrogen removal efficiency. At high temperatures (<25 °C) in summer, the activity of nitrifying bacteria increased [204]. In a pilot system running for six years, key performance indicators were primarily influenced by specific outdoor conditions (irradiation, evaporation), highlighting the need to optimize these parameters in industrial applications [205]. Thus, developing robust and intelligent monitoring and control strategies to ensure consistent treatment efficiency is crucial. The integration of machine learning or artificial intelligent (AI) technologies can optimize the establishment and functional prediction of multi-species symbiotic ABS systems, offering advantages over traditional mathematical models [206].

It also poses potential risks to local ecosystems, as the interaction with native organisms and the release of treated water into the environment may affect sensitive species [207]. It is essential to address potential regulatory barriers and public perception challenges that may impede the practical application of ABS systems. Current research indicates that particular attention should be given to both the discharge standards of treated effluent and the utilization of biomass generated during wastewater treatment. Multiple studies have demonstrated that ABS-treated municipal and domestic wastewater can consistently meet the quality requirements for agricultural irrigation or secondary reuse purposes [208,209]. The ABS system demonstrates dual functionality by effectively removing pollutants while simultaneously enabling resource recovery. Research has confirmed its exceptional performance, showing that it achieved 98.56% COD removal efficiency while generating valuable biomass products. Specifically, the system produced 3.43 g/L of microalgae biomass containing 0.317 g/L of extractable lipids, with significantly enhanced accumulation of chlorophyll and carotenoids under high light intensity conditions [210]. These outputs indicate that the resulting biomass possesses excellent potential for biofuel applications.

Despite significant strides in the high-value utilization of algal biomass, challenges persist in converting algal biomass into value-added products such as biofuels, fertilizers, or pharmaceutical raw materials. Future research should focus on developing more efficient extraction technologies and optimizing transformation processes to maximize both environmental and economic benefits. Also, the optimization and further evolution of the ABS system are required to construct more effective and economical ABS systems. Research has shown that continuous light enhances biomass production and nutrient removal, while periodic light reduces membrane fouling and conserves energy [211]. Future studies should aim to find an optimal balance between photoperiod and energy consumption. Moreover, advancements in immobilization technology have demonstrated improvements in nutrient and pollutant removal. The co-immobilization of bacteria and algae has been effective in enhancing the removal of pollutants, while promoting reusability and microbial diversity. This technology is anticipated to gain widespread application across various wastewater treatment systems [212]. Bioinformatics reveals the community succession pattern and co-occurrence interaction between algae and bacteria [213]. Although keystone taxa have been largely reported, more attention should be paid to low-abundance keystone taxa to fill the current knowledge gaps. Moreover, studies have revealed that bacteria detect chemical gradients through sensor proteins, which trigger conformational changes and regulate flagellar movement in response to specific chemo-attractants or repellents [214]. A deeper understanding of this mechanism in ABS is crucial, and technologies such as single-cell sequencing [215] and CRISPR fixed-point gene editing [216] are useful.

Going forward, the discovery of novel functional species and synergistic relationships to construct ABS systems is required. Tschitschko et al. [217] investigated large-scale symbiotic systems and discovered that in marine areas with scarce cyanobacteria, newly identified Rhizobia symbiosis with diatoms may take on N fixation roles, compensating for the deficiency in cyanobacteria N fixation. This symbiotic relationship was found to be widespread across all major oligotrophic ocean regions. The ABS system can be integrated with various traditional and emerging methods. For example, appropriate magnetic field treatment can promote Chlorella growth and indirectly enhance the oxygen production efficiency of the entire system [218]. Further analysis indicated that the static magnetic field might affect cell metabolism by influencing the charges (ions and free electrons) and molecules with magnetic moments within the cell. Finally, future research should supplement standardized reports, long-term pilot-scale studies, or comprehensive multi-group data to truly capture the complexity of these symbiotic systems.