Urgent Necessity for Algal Bloom Mitigation and Derived Resource Recycling

1. Introduction

Water scarcity poses rigorous challenges to socio-economic development, necessitating more efficient options for water and resource management [1]. Harmful algal blooms (HABs), referring to the massive proliferation of algae, are a prominent global ecological problem in marine and fresh waters that severely threaten the aquatic biological structure and ecosystem’s functioning by causing hypoxia, nuisance odors, and water quality deterioration [2,3,4]. HAB-forming genera mainly include Microcystis, Anabaena, Aphanizomenon, Dolichospermum, Oscillatoria, and Raphidiopsis belonging to cyanobacterium, and Cladophora, Chlorella, and Ulothrix belonging to green alga, with tempo-spatial changes in their dominance due to geographic and climatic factors [5,6,7,8,9]. An outbreak of HABs is always accompanied by contamination with toxic secondary metabolites produced and released from algae, such as microcystin, cylindrospermopsin, nodularin, anatoxin, saxitoxin, and lyngbyatoxin, which are major representative toxins that induce hepatotoxicity, neurotoxicity, cytotoxicity, and/or dermatotoxicity [10,11,12]. These toxins can be bioaccumulated in aquatic animals and farm crops via aquaculture and agro-irrigation and further transferred throughout natural food webs to severely endanger wildlife and human beings through several routes, including food consumption and drinking water [12,13]. Over the past decades, HABs in freshwaters have triggered drinking water crises in China, southern Africa, and North America [14,15,16]. The ingestion of seafood contaminated with algal toxins released during HABs has led to the intoxication of humans and animals [3]. Therefore, HAB occurrence has elevated socio-economic losses and further aggravated water scarcity. Numerous studies dedicated to HAB mitigation using conventional and advanced methods have enjoyed temporary or partial success [17,18]. However, owing to ongoing climate warming and anthropogenic activities, the increasing intensity and frequency of HAB outbreaks underscore the demand for more efficient and eco-friendly integrative solutions [19,20]. Technological innovation and improvement are thus urgently required to achieve more efficient mitigation of HABs and algal toxin contamination.

Besides water scarcity, humanity also faces food, energy, and sustainability challenges in the context of the increasing global population and resource consumption per capita [21]. Novel renewable sources should be sought for energy/resource recovery. Due to high carbohydrate/lipid productivities, the majority of HAB-forming algal genera are rich in proteins, polysaccharides, lipids, phenolics, photosynthetic pigments (e.g., chlorophyll, phycocyanin, fucoxanthin), and/or nutrient elements in cells [22]. Such advantages render algal cells as promising biomass feedstock for the generation of biofuels (e.g., biodiesel), biogases, soil amendments, biofertilizers, animal feeds, food ingredients, medical care products (e.g., pharmaceuticals, cosmetics), and pigments, which make algal cells able to enhance added value [22,23,24]. It is estimated that the algal biomass mitigated from Great Lake blooms reaches thousands of tons daily, so natural HABs can supply untapped algal biomass as a renewable source of energy/resources [22]. Although the concept of adopting algae as biofuels to replace traditional fossil fuels emerged in the 1950s and related practice interests have grown in last decade, its full potential has not yet been reached [25]. For soil application, transformed algal biomass can potentially enhance soil nutrient and organic matter input, and atmospheric carbon fixation [26]. Success in the high-efficiency recycling of algal biomass depends upon comprehensive works focusing on the optimal transformation and reutilization conditions by adjusting physic-chemical and biotic factors, with essential development for bio-chemical agents. In addition, some algal genera with robust nitrogen-fixing capabilities have potential as sorption and remediation agents for wastewater treatment, where living algal cells strongly take up and accumulate nitrogen, phosphorus, and other pollutants from wastewater to jointly alleviate nutrient and contaminant over-loading during their rapid growth phase [27,28,29]. Interestingly, increasing studies have demonstrated the attractive potential of some algal secondary metabolites (e.g., cyanotoxins) in developing high-value products such as allelopathic agents, insecticides, and biomedicines, according to their specific toxicological modes [30]. For instances, microcystin has been explored in natural pesticides [31], while saxitoxin could serve as an anesthetic in combination with other drugs to improve anesthetic effects [32]; Lyngbyatoxin could be used as protein kinase activator [33], while apratoxin has exhibited antitumor effects [34]. However, their potential non-target toxicity in humans and animals could hinder their usage in commercial products, and it is essential to ensure that the anesthetic effect is reversible and does not cause permanent damage to nerve function [30]. Based on the above, it can be observed that the reasonable transformation and reutilization of algal biomass and their secondary metabolites, if their negative hazards are counteracted, are becoming the ‘win–win’ approach to simultaneously realizing HAB mitigation and resource reutilization.

Consequently, this Special Issue entitled “Technological and Mechanism Research on Algal Bloom Mitigation and Resource Recycling” aims to address the concerns and challenges in the aforementioned topics and domains. The content involves innovative techniques and/or materials for HAB mitigation and algal biomass recycling. The influences of various biotic and abiotic factors on the above processes are also included.

2. Main Contribution of This Special Issue

Based on a rigorous peer-review, five papers have ultimately been published in this Special Issue. Their contributions and implications are interpreted below:

Cladophora is algal genus that commonly emerges during HABs globally, and it can form abnormal proliferation in water bodies under eutrophication and even nutritional deficiency status to deteriorate water quality and jeopardize human health. Wang et al. (Contribution 1) summarized the influencing factors (e.g., light, temperature, water depth, water level, nutrient salts, pH, and aquatic animals) on the growth, propagation, and critical processes of Cladophora cells. From cellular functional and structural insights, this paper also analyzed the damage and destruction mechanism of Cladophora cells during prevention and control using physical, chemical, and biological measures (e.g., light-shading, ultrasonic, metallic, or oxidizing agents, herbicides, and aquatic plant allelopathy) and proposed an integrated combination measure for effectively controlling Cladophora during different growth periods, as well as the resource-reusing directions (e.g., fertilizer, wastewater treatment, pharmaceutical, biofuel, biogas, and feed) of Cladophora cells. This paper is significantly implicated in engineering application practice, further research on Cladophora mitigation in waters, and the recycling of Cladophora cells as a natural resource.

Nutrient level is considered a decisive factor for HAB outbreaks, where phosphorus is key element influencing algal growth. Guo et al. (Contribution 2) developed a modified Monod model to describe the relationship between the algal specific growth rate and phosphorus level. As the phosphorus level at the ‘zero’ growth rate is the theoretical phosphorus threshold that limits algal growth, this study proposed a phosphorus threshold for three HAB-forming algae (i.e., cyanobacterium Microcystis wesenbergii, Microcystis aeruginosa, and green alga Chlorella vulgaris) through growth tests in phosphorus-limited conditions using the modified model. This study also observed faster M aeruginosa and M. wesenbergii growth than C. vulgaris as the phosphorus level increased, which explains the reason why cyanobacterial biomass tends to be higher than that of green algae in HABs. These findings lay the theoretical foundation for diminishing phosphorus levels to prevent excessive algal proliferation, and thus provide guidance for HAB control using nutrient limitation.

Studies on algal pigments are useful for acquiring the dynamics of the HAB occurrence status in water bodies. Rodriguez-López et al. (Contribution 3) built regression models to estimate algal pigment phycocyanin during different seasons throughout the year in Lake Villarrica, Chile, based on in situ data on water quality variables. Using regression analysis for the relationships of phycocyanin with other variables, together with the importance weights of each variable, this study found that the model incorporating chlorophyll-a, temperature, and turbidity variables exhibited better statistical performance metrics and precision for phycocyanin pigment estimation, presenting a correlation coefficient R² of 0.90, with a mean squared error of 0.04 µg/L. While incorporating dissolved organic matter, the models presented a further slight improvement. Although the authors proposed that the accuracy of these models would be further improved in future by incorporating other data sources, the models presented in this study may be applicable to other aquatic ecosystems in alerting people to HAB occurrence. The above two contributions highlight the importance of improved models in monitoring and managing HABs, which is crucial in guiding HAB mitigation for natural resource conservation.

HAB decomposition modifies the microenvironment of the sediment–water interface (SWI) to alter the nitrogen (N) distribution patterns from sediment to overlying water, further affecting the algal biomass in eutrophic lakes. Yao et al. (Contribution 4) surveyed the in situ impact of HAB decomposition on labile N fraction distribution within the SWI across 18 locations of Lake Taihu, China, by employing diffusive gradients in thin films and high-resolution dialysis devices. Through an annular flume experiment, this study simulated HAB decomposition in the SWI of Lake Taihu to explore labile N transformation and transport during HAB decay. This study revealed that NH₄⁺-N that exuded from algal decomposition was converted into NO₃-N and NO₂-N via nitrification to increase the NO₃⁻-N and NO₂⁻-N concentrations in the SWI, but decreased dissolved oxygen penetration depth and pH near to the SWI, which caused denitrification processes to induce the role conversion of sediments between the “source” and “sink” of N. This study evidenced N transformation dynamics in response to HAB decomposition across the SWI of Lake Taihu, and may pose the practical implications of suitably modulating N conversion during algal decay in sediment for NH₄-N and/or NO₃-N-rich fertilizer production.

To better apply and convert algae-laden sediments, benthic microbial niches and compositions including algae should be acquired, but the related research is still insufficient. For understanding benthic microbial communities in southern Lake Taihu, China, Zhao et al. (Contribution 5) adopted 16S/18S rRNA sequencing with multivariate statistical methods to reveal the species composition differences between benthic and planktonic microorganisms. The neutral community model indicated that stochastic processes dominated planktonic communities, while deterministic processes prevailed in benthic communities. Null models showed that homogeneous selection affected benthic community assembly, while undominated processes and dispersal limitations affected planktonic communities. Network analysis confirmed more stable planktonic networks than benthic networks. Notably, dominant benthic cyanobacteria posed toxic risks, emphasizing the requirements for enhanced monitoring and eco-risk assessment. This study contrasted the microbial compositions of benthic and planktonic communities and may inspire potential insights for the management and resource utilization of benthic communities in eutrophic aquatic systems.

3. Conclusions and Future Directions

Water scarcity and energy/resource consumption represent prominent challenges to socio-economic and ecological sustainability nowadays. HAB mitigation and derived resource recycling have become increasingly attractive ongoing research hotspots, which deserve more innovative explorations. Considering the above contributions and current progress, associated research could include but would not have to be limited to the following aspects: integrative methodologic combinations with improved modeling for well monitoring, management, and mitigation, cascading optimal utilization algal resources with potential hazard avoidance, and the in-depth exploration of underlying mechanisms.

Ongoing anthropogenic activity and climate change continuously modify aquatic ecosystems, where HABs are dynamically modulated by the synergistic effects of nutrient status, light, temperature, hydrology, and abiotic/biotic interactions. Thus, mitigation strategies focusing on manipulating these dynamic factors via a combination of physic-chemical and biological methods that are more effective in HAB control and mitigation.

For recycling concerns, multiple processes could be coupled concurrently during algal biomass transformation to realize cascading energy/resource usage, besides optimizing the factors for one process. For instance, the nutrient- and/or lipid-accumulated algal biomass after wastewater treatment could be used for downstream biodiesel production. To reduce energy consumption during biodiesel production, the coupling of anaerobic digestion with biodiesel production processes is suggested, where the biogases (e.g., methane) yielded can in turn be used in other processes to decrease the external energy input. Biogas production also offers a solution for managing residual biomass after lipid extraction [35]. While converting algal biomass into carbonaceous amendment or fertilizer, the volatile matter (e.g., heat or oils) can also be retrieved as energy for other processes. Importantly, during algal recycling processes, the potential hazards of algal secondary metabolites should be strictly excluded via degradation elimination or extraction for other biotechnological usage. It is also vital to conduct a life cycle assessment during the algal recycling process to comprehensively evaluate the economic and ecological benefits and impacts [36].

The complex mechanisms of HAB mitigation and algal biomass recycling for resource reutilization are also crucial concerns and deserve comprehensive exploration from multi-level perspectives. Better uncovering the underlying mechanisms would in turn be constructive for directionally optimizing the transformation and reutilization conditions for the scientific and effective mitigation of HABs and algal biomass recycling. In-depth mechanistic explorations may involve the combined application of several advanced techniques, including multi-omics.