Extracellular Polymeric Substances Drive Symbiotic Interactions in Bacterial‒Microalgal Consortia

Diverse interactions between bacteria and microalgae: A review for enhancing harmful algal bloom mitigation and biomass processing efficiency

The interactions between bacteria and microalgae play pivotal roles in resource allocation, biomass accumulation, nutrient recycling, and species succession in aquatic systems, offering ample opportunities to solve several social problems. The escalating threat of harmful algal blooms (HABs) in the aquatic environment and the lack of cheap and eco-friendly algal-biomass processing methods have been among the main problems, demanding efficient and sustainable solutions. In light of this, the application of algicidal bacteria to control HABs and enhance algal biomass processing has been promoted in the past few decades as potentially suitable mechanisms to solve those problems. Hence, this comprehensive review aims to explore the diverse interaction modes between bacteria and microalgae, ranging from synergistic to antagonistic, and presents up-to-date information and in-depth analysis of their potential biotechnological applications, particularly in controlling HABs and enhancing microalgal biomass processing. For instance, several studies revealed that algicidal bacteria can effectively inhibit the growth of Microcystis aeruginosa, a notorious freshwater HAB species, with an antialgal efficiency of 24.87 %-98.8 %. The review begins with an overview of the mechanisms behind algae-bacteria interactions, including the environmental factors influencing these dynamics and their broader implications for aquatic ecosystems. It then provides a detailed analysis of the role of algicidal bacteria in controlling harmful algal blooms, as well as their role in bioflocculation and the pretreatment of microalgal biomass. Additionally, the review identifies and discusses the constraints and challenges in the biotechnological application of these interactions. By exploring the strategic use of algicidal bacteria, this review not only underscores their importance in maintaining aquatic environmental health but also in enhancing biomass processing efficiency. It offers valuable insights into future research avenues and the potential scalability of these applications, both in situ and at an industrial level.

Exopolysaccharides as bio-based rheology modifiers from microalgae produced on dairy industry waste: Towards a circular bioeconomy approach

The feasibility of exopolysaccharides (EPS) production from cheese whey using Chlorella vulgaris was investigated as an example of circular bioeconomy application. The effects of dairy waste utilization in EPS biosynthesis and rheological properties were evaluated, comparing with both control conditions and commercial xanthan gum (CXG). A twofold increase in yield, up to 0.32 g L-1, was observed when Chlorella vulgaris was used for EPS production from whey rather than conventional fertilizers. Additionally, the EPS produced using cheese whey exhibited superior pseudoplasticity in the 0.4-1.0 (w/v) range compared to the control. The EPS from the whey wastewater contained functional groups similar to those of CXG (82.7 %). Moreover, the solutions containing 1 % biopolymer showed rheological profiles similar to those of the 0.4 % CXG. The molecular weight averages predominantly fell within the range of 284 to 324 kDa, as deduced using diffusion NMR, an innovative and rapid determination method for estimating EPS size. The potential applications of EPS notably extend beyond the dairy industry, reaching diverse market sectors, and thereby enhancing the competitiveness of microalgal biorefineries while contributing to the achievement of Sustainable Development Goals.

Microalgae growth-promoting bacteria for cultivation strategies: Recent updates and progress

Microalgae growth-promoting bacteria (MGPB), both actinobacteria and non-actinobacteria, have received considerable attention recently because of their potential to develop microalgae-bacteria co-culture strategies for improved efficiency and sustainability of the water-energy-environment nexus. Owing to their diverse metabolic pathways and ability to adapt to diverse conditions, microalgal-MGPB co-cultures could be promising biological systems under uncertain environmental and nutrient conditions. This review proposes the recent updates and progress on MGPB for microalgae cultivation through co-culture strategies. Firstly, potential MGPB strains for microalgae cultivation are introduced. Following, microalgal-MGPB interaction mechanisms and applications of their co-cultures for biomass production and wastewater treatment are reviewed. Moreover, state-of-the-art studies on synthetic biology and metabolic network analysis, along with the challenges and prospects of opting these approaches for microalgal-MGPB co-cultures are presented. It is anticipated that these strategies may significantly improve the sustainability of microalgal-MGPB co-cultures for wastewater treatment, biomass valorization, and bioproducts synthesis in a circular bioeconomy paradigm.

Isolation of bacteria with plant growth-promoting properties from microalgae-bacterial flocs produced in high-rate oxidation ponds

Exploring plant growth-promoting (PGP) bacterial activity of microbial components aggregated by wastewater treatment can reduce dependence on fossil fuel-derived fertilisers. This study describes the isolation and identification of bacteria from microalgae-bacteria flocs (MaB-flocs) generated in high-rate algal oxidation ponds (HRAOP) of an integrated algal pond system (IAPS) remediating municipal wastewater. Amplified 16S rRNA gene sequence analysis determined the molecular identity of the individual strains. Genetic relatedness to known PGP rhizobacteria in the NCBI GenBank database was by metagenomics. Isolated strains were screened for the production of indoles (measured as indole-3-acetic acid; IAA) and an ability to mineralise NH 4 + , PO 4 3 - , and K + . Of the twelve bacterial strains isolated from HRAOP MaB-flocs, four produced indoles, nine mineralised NH 4 + , seven solubilised P, and one K. Potential of isolated strains for PGP activity according to one-way ANOVA on ranks was: ECCN 7b > ECCN 4b > ECCN 6b > ECCN 3b = ECCN 10b > ECCN 1b = ECCN 5b > ECCN 8b > ECCN 2b > ECCN 12b > ECCN 9b = ECCN 11b. Further study revealed that cell-free filtrate from indole-producing cultures of Aeromonas strain ECCN 4b, Enterobacter strain ECCN 7b, and Arthrobacter strain ECCN 6b promoted mung bean adventitious root formation suggestive of the presence of auxin-like biological activity.

Influence of growth medium on the species-specific interactions between algae and bacteria

In this study, we investigated a species-specific algal-bacterial co-culture that has recently attracted worldwide scientific attention as a novel approach to enhancing algal growth rate. We report that the type of interaction between Chlorella vulgaris and bacteria of the genus Delftia is not solely determined by species specificity. Rather, it is a dynamic process of adaptation to the surrounding conditions, where one or the other microorganism dominates (temporally) depending on the growth conditions, in particular the medium. Under laboratory conditions, we found that Delftia sp. had a negative effect on C. vulgaris growth when co-cultured in a TAP medium. However, the co-culture of algae and bacteria under BG-11 and BG-11 + acetic acid resulted in an increase in algal concentration compared to algal cultures without bacteria under the same conditions. Additional chemical analysis revealed that the presence of different carbon (the main organic carbon source-acetic acid in TAP or BG-11 + acetic acid medium and inorganic carbon source-Na2CO3 in BG-11 or BG-11 + acetic acid medium) and nitrogen (NH4Cl in TAP medium and NaNO3 in BG-11 or BG-11 + acetic acid medium) species in the growth medium was one of the main factors driving the shift in interaction type.

Linking the removal of enrofloxacin to the extracellular polymers of microalgae in water bodies: A case study focusing on the shifts in microbial communities

Microalgae can promote antibiotic removal, which has attracted growing attention. However, its synergistic removal performance with bacteria in antibiotic pollutants is still poorly understood. In this study, firstly, we selected two green algae (Dictyosphaerium sp. and Chlorella sp.) and exposed them to Enrofloxacin (ENR) to observe their extracellular polysaccharides (EPS) concentration dynamic and the removal of antibiotics. Secondly, EPS was extracted and added to in situ lake water (no algae) to investigate its combined effect with bacteria. The results indicate that both Dictyosphaerium sp. and Chlorella sp. exhibited high tolerance to ENR stress. When the biomass of microalgae was low, ENR could significantly stimulate algae to produce EPS. The removal rates of Dictyosphaerium sp. and Chlorella sp. were 15.8% and 10.5%, respectively. The addition of EPS can both alter the microbial community structure in the lake water and promote the removal of ENR. The LEfSe analysis showed that there were significant differences in the microbial marker taxa, which promoted the increase of special functional bacteria for decomposing ENR, between the EPS-added group and the control group. The EPS of Dictyosphaerium sp. increased the abundance of Moraxellaceae and Spirosomaceae, while the EPS of Chlorella sp. increased the abundance of Sphingomonadaceae and Microbacteriaceae. Under the synergistic effect, Chlorella sp. achieved a maximum removal rate of 24.2%, while Dictyosphaerium sp. achieved a maximum removal rate of 28.9%. Our study provides new insights into the removal performance and mechanism of antibiotics by freshwater microalgae in water bodies and contribute to the development of more effective water treatment strategies.

Mitigating salinity stress through interactions between microalgae and different forms (free-living & alginate gel-encapsulated) of bacteria isolated from estuarine environments

Salinity stress in estuarine environments poses a significant challenge for microalgal survival and proliferation. The interaction between microalgae and bacteria shows promise in alleviating the detrimental impacts of salinity stress on microalgae. Our study investigates this interaction by co-cultivating Chlorella sorokiniana, a freshwater microalga, with a marine growth-promoting bacterium Pseudomonas gessardii, both of which were isolated from estuary. In this study, bacteria were encapsulated using sodium alginate microspheres to establish an isolated co-culture system, preventing direct exposure between microalgae and bacteria. We evaluated microalgal responses to different salinities (5 PSU, 15 PSU) and interaction modes (free-living, gel-encapsulated), focusing on growth, photosynthesis, cellular metabolism, and extracellular polymeric substances (EPS) properties. High salinity inhibited microalgal proliferation, while gel-fixed interaction boosted Chlorella growth rate by 50.7 %. Both attached and free-living bacteria restored Chlorella's NPQ to normal levels under salt stress. Microalgae in the free-living interaction group exhibited a significantly lower respiratory rate compared to the pure algae group (-17.2 %). Increased salinity led to enhanced EPS polysaccharide secretion by microalgae, particularly in interaction groups (19.7 %). Both salt stress and interaction increased the proportion of aromatic proteins in microalgae's EPS, enhancing its stability by modulating EPS glycosidic bond C-O-C and protein vibrations. This alteration caused microalgal cells to aggregate, free-living bacteria co-culture group, and fixed co-culture group increasing by 427.5 %, 567.1 %, and 704.1 %, respectively. In gel-fixed bacteria groups, reduced neutral lipids don't accumulate starch instead, carbon redirects to cellular growth, aiding salt stress mitigation. These synergistic activities between salinity and bacterial interactions are vital in mitigating salinity stress, improving the resilience and growth of microalgae in saline conditions. Our research sheds light on the mechanisms of microalgal-bacterial interactions in coping with salt stress, offering insights into the response of estuarine microorganisms to global environmental changes and their ecological stability.

Dissolved organic carbon spurs bacterial-algal competition and phosphorus-paucity adaptation: Boosting Microcystis' phosphorus uptake capacity

Dissolved organic carbon (DOC) can alter the availability of background nutrients by affecting the proliferation of heterotrophic bacteria, which exerts a notable influence on algal growth and metabolism. However, the mechanism of how allochthonous DOC (aDOC) precipitates shifts in bacterial-algal interactions and modulates the occurrence of cyanobacteria blooms remains inadequately elucidated. Therefore, this study investigated the relationship between bacteria and algae under aDOC stimulation. We found that excess aDOC triggered the breakdown and reestablishment of the equilibrium between Microcystis and heterotrophic bacteria. The rapid proliferation of heterotrophic bacteria led to a dramatic decrease in soluble phosphorus and thereby resulted in the inhibition of the Microcystis growth. When the available DOC was depleted, the rapid death of heterotrophic bacteria released large amounts of dissolved phosphorus, which provided sufficient nutrients for the recovery of Microcystis. Notably, Microcystis rejuvenated and showed higher cell density compared to the carbon-absent group. This phenomenon can be ascribed that Microcystis regulated the compositions of extracellular polymeric substances (EPS) and the expression of relevant proteins to adapt to a nutrient-limited environment. Using time of flight secondary ion mass spectrometry (TOF-SIM) and proteomic analysis, we observed an enhancement of the signal of organic matter and metal ions associated with P complexation in EPS. Moreover, Microcystis upregulated proteins related to organic phosphorus transformation to increase the availability of phosphorus in various forms. In summary, this study emphasized the role of DOC in algal blooms, revealing the underestimated enhancement of Microcystis nutrient utilization through DOC-induced heterotrophic competition and providing valuable insights into eutrophication management and control.

Improved microalgae carbon fixation and microplastic sedimentation in the lake through in silico method

The impact of microplastics in lake water environments on microalgae carbon fixation and microplastic sedimentation has attracted global attention. The molecular dynamic simulation method was used to design microplastic additive proportioning schemes for improving microalgae carbon fixation and microplastic sedimentation. Results showed that the harm of microplastics can be effectively alleviated by adjusting the proportioning scheme of plastic additives. Besides, the decabromodiphenyl oxide (DBDPO) was identified as the main additive that affect the microalgae carbon fixation and microplastic sedimentation. Thus, a molecular modification based on CiteSpace visual analysis was firstly used and 12 DBDPO derivatives were designed. After the screening, DBDPO-2 and DBDPO-5 became the environmentally friendly DBDPO alternatives, with the highest microalgae carbon fixation and microplastic sedimentation ability enhancement of over 25 %. Compared to DBDPO, DBDPO derivatives were found easier to stimulate the adsorption and binding ability of surrounding hotspot amino acids to CO2 and ribulose-5-phosphate, increasing the solvent-accessible surface area of microplastics, thus improving the microalgae carbon fixation and microplastic sedimentation ability. This study provides theoretical support for simultaneously promoting the microalgae carbon fixation and microplastic sedimentation in the lake water environment and provides scientific basis for the protection and sustainable development of lake water ecosystem.

Deciphering the Roles of Extracellular Polymeric Substances (EPS) in Shaping Disinfection Kinetics through Permanent Removal via Genetic Disruption

Extracellular polymeric substances (EPS) ubiquitously encapsulate microbes and play crucial roles in various environmental processes. However, understanding their complex interactions with dynamic bacterial behaviors, especially during the disinfection process, remains very limited. In this work, we investigated the impact of EPS on bacterial disinfection kinetics by developing a permanent EPS removal strategy. We genetically disrupted the synthesis of exopolysaccharides, the structural components of EPS, in Pseudomonas aeruginosa, a well-known EPS-producing opportunistic pathogen found in diverse environments, creating an EPS-deficient strain. This method ensured a lasting absence of EPS while maintaining bacterial integrity and viability, allowing for real-time in situ investigations of the roles of EPS in disinfection. Our findings indicate that removing EPS from bacteria substantially lowered their susceptibility threshold to disinfectants such as ozone, chloramine B, and free chlorine. This removal also substantially accelerated disinfection kinetics, shortened the resistance time, and increased disinfection efficiency, thereby enhancing the overall bactericidal effect. The absence of EPS was found to enhance bacterial motility and increase bacterial cell vulnerability to disinfectants, resulting in greater membrane damage and intensified reactive oxygen species (ROS) production upon exposure to disinfectants. These insights highlight the central role of EPS in bacterial defenses and offer promising implications for developing more effective disinfection strategies.

Immobilized microalgae: principles, processes and its applications in wastewater treatment

Microalgae have emerged as potential candidates for biomass production and pollutant removal. However, expensive biomass harvesting, insufficient biomass productivity, and low energy intensity limit the large-scale production of microalgae. To break through these bottlenecks, a novel technology of immobilized microalgae culture coupled with wastewater treatment has received increasing attention in recent years. In this review, the characteristics of two immobilized microalgae culture technologies are first presented and then their mechanisms are discussed in terms of biofilm formation theories, including thermodynamic theory, Derjaguin-Landau-Verwei-Overbeek theory (DLVO) and its extended theory (xDLVO), as well as ionic cross-linking mechanisms in the process of microalgae encapsulated in alginate. The main factors (algal strains, carriers, and culture conditions) affecting the growth of microalgae are also discussed. It is also summarized that immobilized microalgae show considerable potential for nitrogen and phosphorus removal, heavy metal removal, pesticide and antibiotic removal in wastewater treatment. The role of bacteria in the cultivation of microalgae by immobilization techniques and their application in wastewater treatment are clarified. This is economically feasible and technically superior. The problems and challenges faced by immobilized microalgae are finally presented.

Insights into the role of extracellular polymeric substances (EPS) in the spread of antibiotic resistance genes

Antibiotic resistance genes (ARG) are prevalent in aquatic environments. Discharge from wastewater treatment plants is an important point source of ARG release into the environment. It has been reported that biological treatment processes may enhance rather than remove ARG because of their presence in sludge. Attenuation of ARG in biotechnological processes has been studied in depth, showing that many microorganisms can secrete complex extracellular polymeric substances (EPS). These EPS can serve as multifunctional elements of microbial communities, involving aspects, such as protection, structure, recognition, adhesion, and physiology. These aspects can influence the interaction between microbial cells and extracellular ARG, as well as the uptake of extracellular ARG by microbial cells, thus changing the transformative capability of extracellular ARG. However, it remains unclear whether EPS can affect horizontal ARG transfer, which is one of the main processes of ARG dissemination. In light of this knowledge gap, this review provides insight into the role of EPS in the transmission of ARGs; furthermore, the mechanism of ARG spread is analyzed, and the molecular compositions and functional properties of EPS are summarized; also, how EPS influence ARG mitigation is addressed, and factors impacting how EPS facilitate ARG during wastewater treatment are summarized. This review provides comprehensive insights into the role of EPS in controlling the transport and fate of ARG during biodegradation processes at the mechanistic level.

Bio-coatings in permeated cultivation systems: Unprecedented impacts on microalgal monoculture growth and organic matter yield

Bio-coating, a recent and promising approach in attached microalgal cultivation systems, has garnered attention due to its efficiency in enhancing immobilized algal growth, particularly in submerged cultivation systems. However, when the cells are cultured on thin solid microporous substrates that physically separate them from the nutrient medium, it remains unclear whether the applied bio-coatings still have a significant impact on algal growth or the subsequent rates of algal organic matter (AOM) release. Therefore, this current work investigated the role of bio-coatings on the microalgal monoculture growth of one freshwater species, Chlorella vulgaris ESP 31, and one marine species, Cylindrotheca fusiformis on a hydrophilic substrate, polyvinylidene fluoride membrane in a permeated cultivation system. Wide range of bio-coating sources were adapted, with the result demonstrating that bacteria-derived coating promoted algal growth by as high as 140% when compared with the control group for both species. Interestingly, two distinct adaptation mechanisms were observed between the species, with only C. fusiformis demonstrating a positive correlation between cell growth and AOM productivity, particularly in its extracellularly bound fractions. It is worth noting that despite this specific fraction exhibiting the lowest content among all; it displayed significant relevance in terms of AOM productivity. High extracellular protein-to-polysaccharide ratio (>5.7 fold) quantified on bacterial intracellular exudate-coated membranes indirectly revealed an underlying symbiotic microalgal-bacterial interaction. This is the first study showing how bio-coating influenced AOM yield without any physical interaction between microalgae and bacteria. It further confirms the practical benefits of bio-coating in attached cultivation systems.

Microalgae and nano-cellulose composite produced via a co-culturing strategy for ammonia removal from the aqueous phase

This study addresses the pressing need for sustainable bioremediation solutions to combat increasing pollution challenges in alignment with sustainability development goals. The research focuses on developing a co-culture approach involving microalgae and Komagataeibacter europaeus BCRC 14148 bacterium to create a biocomposite for efficient ammonia removal. Nanocellulose, produced by the bacterium, serves as a substrate for microalgae attachment. Optimization using specific growth media ratios resulted in biocomposite yields of 4.05 ± 0.16 g/L and 3.83 ± 0.13 g/L in HS medium with fructose and glucose, respectively. The optimal conditions include a 40:60 ratio of HS-F to TAP medium, 25 ℃ incubation, 6000 Lux light intensity, pH 5.5, and a 48-hour incubation period. When applied to wastewater treatment, the biocomposite demonstrated exceptional ammonium removal efficiency at 91.64 ± 1.27 %. This co-culture-derived biocomposite offers an eco-friendly, recyclable, and effective solution for sustainable environmental bioremediation.

A review on microalgal-bacterial co-culture: The multifaceted role of beneficial bacteria towards enhancement of microalgal metabolite production

Mass microalgal-bacterial co-cultures have come to the fore of applied physiological research, in particularly for the optimization of high-value metabolite from microalgae. These co-cultures rely on the existence of a phycosphere which harbors unique cross-kingdom associations that are a prerequisite for the cooperative interactions. However, detailed mechanisms underpinning the beneficial bacterial effects onto microalgal growth and metabolic production are rather limited at the moment. Hence, the main purpose of this review is to shed light on how bacteria fuels microalgal metabolism or vice versa during mutualistic interactions, building upon the phycosphere which is a hotspot for chemical exchange. Nutrients exchange and signal transduction between two not only increase the algal productivity, but also facilitate in the degradation of bio-products and elevate the host defense ability. Main chemical mediators such as photosynthetic oxygen, N-acyl-homoserine lactone, siderophore and vitamin B12 were identified to elucidate beneficial cascading effects from the bacteria towards microalgal metabolites. In terms of applications, the enhancement of soluble microalgal metabolites is often associated with bacteria-mediated cell autolysis while bacterial bio-flocculants can aid in microalgal biomass harvesting. In addition, this review goes in depth into the discussion on enzyme-based communication via metabolic engineering such as gene modification, cellular metabolic pathway fine-tuning, over expression of target enzymes, and diversion of flux toward key metabolites. Furthermore, possible challenges and recommendations aimed at stimulating microalgal metabolite production are outlined. As more evidence emerges regarding the multifaceted role of beneficial bacteria, it will be crucial to incorporate these findings into the development of algal biotechnology.

Strategy for the formation of microalgae-bacteria aggregates in high-rate algal ponds

This work studied the formation of aggregates used for wastewater treatment in high-rate algal ponds (HRAP). For this, the establishment of microalgae-bacteria aggregates in these systems was evaluated, considering strategies for the inoculation and start-up. Two HRAP were operated in parallel, at first in batch mode and then in continuous flow. The wastewater treatment was efficient, with removal rates around 80% for COD and N-ammoniacal. Volatile suspended solids and chlorophyll for the culture grew continuously reached a concentration of 548 ± 11 mg L^-1 and 7.8 mg L^-1, respectively. Larger photogranules were observed when the system was placed in a continuous regime. The protein fraction of extracellular polymeric substances was identified as a determinant in photogranules formation. During the continuous regime, more than 50% of the biomass was higher than 0.2 mm, flocculation efficiency of 78 ± 6%, and the volumetric sludge index of 32 ± 5 mL g^-1. The genetic sequencing showed the growth of cyanobacteria in the aggregate and the presence of microalgae from the chlorophytes and diatoms groups in the final biomass.

The interplay between microalgae and toxic metal(loid)s: mechanisms and implications in AMD phycoremediation coupled with Fe/Mn mineralization

Acid mine drainage (AMD) is low-pH with high concentration of sulfates and toxic metal(loid)s (e.g. As, Cd, Pb, Cu, Zn), thereby posing a global environmental problem. For decades, microalgae have been used to remediate metal(loid)s in AMD, as they have various adaptive mechanisms for tolerating extreme environmental stress. Their main phycoremediation mechanisms are biosorption, bioaccumulation, coupling with sulfate-reducing bacteria, alkalization, biotransformation, and Fe/Mn mineral formation. This review summarizes how microalgae cope with metal(loid) stress and their specific mechanisms of phycoremediation in AMD. Based on the universal physiological characteristics of microalgae and the properties of their secretions, several Fe/Mn mineralization mechanisms induced by photosynthesis, free radicals, microalgal-bacterial reciprocity, and algal organic matter are proposed. Notably, microalgae can also reduce Fe(III) and inhibit mineralization, which is environmentally unfavorable. Therefore, the comprehensive environmental effects of microalgal co-occurring and cyclical opposing processes must be carefully considered. Using chemical and biological perspectives, this review innovatively proposes several specific processes and mechanisms of Fe/Mn mineralization that are mediated by microalgae, providing a theoretical basis for the geochemistry of metal(loid)s and natural attenuation of pollutants in AMD.

Synergy between microalgae and microbiome in polluted waters

Microalga-microbiome interactions are central to both health and disease of aquatic environments. Despite impressive advances in deciphering how microorganisms participate in and impact aquatic ecosystems, the evolution and ecological involvement of microalgae and the microbiome in polluted waters are typically studied independently. Here, the phycosphere (i.e., the consortia of microalgae and the related microbiome) is regarded as an independent and integrated life form, and we summarize the survival strategies exhibited by this symbiont when exposed to anthropogenic pollution. We highlight the cellular strategies and discuss the modulation at the transcriptional and population levels, which reciprocally alters community structure or genome composition for medium-term acclimation or long-term adaptation. We propose a 'PollutantBiome' concept to help the understanding of microalga-microbiome interactions and development of beneficial microbial synthetic communities for pollutant remediation.

Role of rotating speed on the stability of a self-sustaining algal-bacterial photo-granules process

The effects of rotating speed (120, 130, 140 and 150 rpm) of a photo-reactor on the self-sustaining algal-bacterial photo-granules sludge (ABPG) was investigated in this study. The ABPG process maintained good granular stability at 140 rpm with an integrity coefficient of 3.0% and excellent nutrient removal. The increases of the extracellular polymeric substances (EPS) content, the portions of α-helix (32.37%) in the secondary protein structure, and the relative abundance of functional bacteria (e.g. Candidatus_Competibacter), contributed to the maintenance of granular structure stability. However, the lower rotating speed (120 and 130 rpm) resulted in slow bacterial growth and reproduction while a higher rotating speed (150 rpm) caused the disintegration of photo-granules. Overall, the results reveal the influencing mechanisms of photo-reactor rotating speed on the self-sustaining ABPG process and provide an effective approach to maintain the system stability.

Revealing the influencing mechanisms of polystyrene microplastics (MPs) on the performance and stability of the algal-bacterial granular sludge

Algal-bacterial granular sludge (ABGS) is an energy-saving and environment-friendly wastewater treatment technology; however, the effects of microplastics (MPs) on the performance and stability of the ABGS system remain unknown. Herein, the influencing mechanisms of polystyrene MPs (50 μm) on the ABGS were systematically investigated. The ABGS exhibited a high removal efficiency of MPs (over 96%) at 1 mg/L and 20 mg/L. Although the biomass content, sludge settling and particle size were not obviously affected by MPs, the COD and total phosphorus (TP) removal efficiencies were inhibited by 2.6%-4.1% and 2.9%-5.8%, respectively. Meanwhile, the structural stability of ABGS was damaged by MPs, owing to the excessive oxidative stress, low content of protein-like substance (especially tryptophan and tyrosine), and the large portion of loose protein secondary structure. Microbial community analysis revealed that the relative abundance of some functional bacteria (Candidatus_Competibacter and Rhodobacter) and algal species (Tetradesmus) were decreased under the MPs stress.

Impact of Nitrate and Ammonium Concentrations on Co-Culturing of Tetradesmus obliquus IS2 with Variovorax paradoxus IS1 as Revealed by Phenotypic Responses

Mutual interactions in co-cultures of microalgae and bacteria are well known for establishing consortia and nutrient uptake in aquatic habitats, but the phenotypic changes in terms of morphological, physiological, and biochemical attributes that drive these interactions have not been clearly understood. In this novel study, we demonstrated the phenotypic response in a co-culture involving a microalga, Tetradesmus obliquus IS2, and a bacterium, Variovorax paradoxus IS1, grown with varying concentrations of two inorganic nitrogen sources. Modified Bold's basal medium was supplemented with five ratios (%) of NO₃-N:NH₄-N (100:0, 75:25, 50:50, 25:75, and 0:100), and by maintaining N:P Redfield ratio of 16:1. The observed morphological changes in microalga included an increase in granularity and a broad range of cell sizes under the influence of increased ammonium levels. Co-culturing in presence of NO₃-N alone or combination with NH₄-N up to equimolar concentrations resulted in complete nitrogen uptake, increased growth in both the microbial strains, and enhanced accumulation of carbohydrates, proteins, and lipids. Total chlorophyll content in microalga was also significantly higher when it was grown as a co-culture with NO₃-N and NH₄-N up to a ratio of 50:50. Significant upregulation in the synthesis of amino acids and sugars and downregulation of organic acids were evident with higher ammonium uptake in the co-culture, indicating the regulation of carbon and nitrogen assimilation pathways and energy synthesis. Our data suggest that the co-culture of strains IS1 and IS2 could be exploited for effluent treatment by considering the concentrations of inorganic sources, particularly ammonium, in the wastewaters.

Phenotypic changes in microalgae at acidic pH mediate their tolerance to higher concentrations of transition metals

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Acid-tolerant microalgae were grown at pH 3.5 and 6.7 in presence of heavy metals (HMs).

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HMs-induced phenotypic changes in microalgae were evaluated by ATR-FTIR spectroscopy.

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Higher HMs bioavailability affected microalgae more at pH 6.7 than pH 3.5.

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Acclimation of microalgal strains to acidic pH significantly alleviates HMs toxicity.

Acclimatory phenotypic response is a common phenomenon in microalgae, particularly during heavy metal stress. It is not clear so far whether acclimating to one abiotic stressor can alleviate the stress imposed by another abiotic factor. The intent of the present study was to demonstrate the implication of acidic pH in effecting phenotypic changes that facilitate microalgal tolerance to biologically excess concentrations of heavy metals. Two microalgal strains, Desmodesmus sp. MAS1 and Heterochlorella sp. MAS3, were exposed to biologically excess concentrations of Cu (0.50 and 1.0 mg L^‒1), Fe (5 and 10 mg L^‒1), Mn (5 and 10 mg L^‒1) and Zn (2, 5 and 10 mg L^‒1) supplemented to the culture medium at pH 3.5 and 6.7. Chlorophyll autofluorescence and biochemical fingerprinting using FTIR-spectroscopy were used to assess the microalgal strains for phenotypic changes that mediate tolerance to metals. Both the strains responded to acidic pH by effecting differential changes in biochemicals such as carbohydrates, proteins, and lipids. Both the microalgal strains, when acclimated to low pH of 3.5, exhibited an increase in protein (< 2-fold) and lipid (> 1.5-fold). Strain MAS1 grown at pH 3.5 showed a reduction (1.5-fold) in carbohydrates while strain MAS3 exhibited a 17-fold increase in carbohydrates as compared to their growth at pH 6.7. However, lower levels of biologically excess concentrations of the selected transition metals at pH 6.7 unveiled positive or no effect on physiology and biochemistry in microalgal strains, whereas growth with higher metal concentrations at this pH resulted in decreased chlorophyll content. Although the bioavailability of free-metal ions is higher at pH 3.5, as revealed by Visual MINTEQ model, no adverse effect was observed on chlorophyll content in cells grown at pH 3.5 than at pH 6.7. Furthermore, increasing concentrations of Fe, Mn and Zn significantly upregulated the carbohydrate metabolism, but not protein and lipid synthesis, in both strains at pH 3.5 as compared to their growth at pH 6.7. Overall, the impact of pH 3.5 on growth response suggested that acclimation of microalgal strains to acidic pH alleviates metal toxicity by triggering physiological and biochemical changes in microalgae for their survival.