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Microalgal-bacterial symbioses are prevalent in aquatic ecosystems and play a pivotal role in carbon sequestration, significantly contributing to global carbon cycling. The understanding of the contribution of exopolysaccharides (EPSs), a crucial carbon-based component, to the structural integrity of microalgal-bacterial symbioses remains insufficiently elucidated. To address this gap, our study aims to enhance our comprehension of the composition and primary structure of EPSs within a specific type of granular microalgal-bacterial symbiosis named microalgal-bacterial granular sludge (MBGS). Our investigation reveals that the acidic EPSs characteristic of this symbiosis have molecular weights ranging from several hundred thousand to over one million Daltons, including components like glucopyranose, galactopyranose, mannose, and rhamnose. Our elucidation of the backbone linkage of a representative exopolysaccharide revealed a →3)-ß-D-Galp-(1→4)-ß-D-Glcp-(1→ glycosidic linkage. This linear structure closely resembles bacterial xanthan, while the branched chain structure bears similarities to algal EPSs. Our findings highlight the collaborative synthesis of acidic EPSs by both microalgae and bacteria, emphasizing their joint contribution in the production of macromolecules within microalgal-bacterial symbiosis. This collaborative synthesis underscores the intricate molecular interactions contributing to the stability and function of these symbiotic relationships.

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In the pursuit of effective wastewater treatment and biomass generation, the symbiotic relationship between microalgae and bacteria emerges as a promising avenue. This analysis delves into recent advancements concerning the utilization of microalgae-bacteria consortia for wastewater treatment and biomass production. It examines multiple facets of this symbiosis, encompassing the judicious selection of suitable strains, optimal culture conditions, appropriate media, and operational parameters. Moreover, the exploration extends to contrasting closed and open bioreactor systems for fostering microalgae-bacteria consortia, elucidating the inherent merits and constraints of each methodology. Notably, the untapped potential of co-cultivation with diverse microorganisms, including yeast, fungi, and various microalgae species, to augment biomass output. In this context, artificial intelligence (AI) and machine learning (ML) stand out as transformative catalysts. By addressing intricate challenges in wastewater treatment and microalgae-bacteria symbiosis, AI and ML foster innovative technological solutions. These cutting-edge technologies play a pivotal role in optimizing wastewater treatment processes, enhancing biomass yield, and facilitating real-time monitoring. The synergistic integration of AI and ML instills a novel dimension, propelling the fields towards sustainable solutions. As AI and ML become integral tools in wastewater treatment and symbiotic microorganism cultivation, novel strategies emerge that harness their potential to overcome intricate challenges and revolutionize the domain.

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Microalgae-bacteria symbiosis system (MBS) appear to be a promising way for treating the rare earth elements (REEs) wastewater due to the natural symbiotic interactions between microalgae and bacteria. Herein, we investigated the effect of different inoculation ratios of microalgae and bacteria including 3:1 (MB_1), 1:1 (MB_2) and 1:3 (MB_3) on NH4+-N removal from REEs wastewater and analyzed the corresponding biological mechanism. The NH4+-N removal rate with MB_3 reached 17.69 ± 0.45 mg NH4+-N/L d-1, which was 2.58 times higher than that in single microalgae system. The results were further verified in continuous feeding photobioreactors and kept stable for 100 days. Metagenomic analysis revealed that the abundance of genes related to microalgae assimilation increased by 14 %-50 % in answer to photosynthesis and NH4+-N absorption, while that related to nitrification apparently dropped, indicating that MBS was a sustainable method capable of enhancing NH4+-N removal from REEs wastewater.

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The development of microalgae-bacteria symbiosis for treating wastewater is flourishing owing to its high biomass productivity and exceptional ability to purify contaminants. A nature-selected microalgae-bacteria symbiosis, mainly consisting of Dictyosphaerium and Pseudomonas, was used to treat oxytetracycline (OTC), ofloxacin (OFLX), and antibiotic-containing swine wastewater. Increased antibiotic concentration gradually reduced biomass productivity and intricately changed symbiosis composition, while 1 mg/L OTC accelerated the growth of symbiosis. The symbiosis biomass productivity reached 3.4-3.5 g/L (5.7-15.3 % protein, 18.4-39.3 % carbohydrate, and 2.1-3.9 % chlorophyll) when cultured in antibiotic-containing swine wastewater. The symbiosis displayed an excellent capacity to remove 76.3-83.4 % chemical oxygen demand, 53.5-62.4 % total ammonia nitrogen, 97.5-100.0 % total phosphorus, 96.3-100.0 % OTC, and 32.8-60.1 % OFLX in swine wastewater. The microbial community analysis revealed that the existence of OTC/OFLX increased the richness and evenness of microalgae but reduced bacteria species in microalgae-bacteria, and the toxicity of OFLX to bacteria was stronger than that of OTC.

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Much attention has been paid to developing methods capable of synchronous removal of pollutants from swine wastewater. Due to the natural symbiotic interactions between microalgae and bacteria, the microalgae-bacteria symbiosis (ABS) system has been found to have potential for treating wastewater. However, the corresponding biological mechanisms in the ABS system and the role of dynamic microbial community evolution in pollutant removal systems remain poorly understood. Therefore, we investigate the potential of an ABS system for pollutant removal applications and analyze the bacterial consortium symbiotically combined with Chlorella sp. MA1 and Coelastrella sp. KE4. The NH4+-N and PO43--P removal efficiencies were significantly increased from 12.79% to 99.52% and 35.66% to 96.06% due to biotic interactions between the microalgae and bacteria. The abundance of bacterial taxa and genes related to oxidative stress, cell growth and nitrogen transfer were found to increase in response to photosynthesis, respiration and NH4+-N uptake. Furthermore, pathogen inactivation was induced via microalgae, co-driven by microbial succession under high dissolved oxygen conditions. In this microalgae-enhanced ABS system, the interactions between microalgae and bacteria are established for pathogens elimination and nitrogen cycling, verifying that the ABS system is an effective and environmentally sustainable swine wastewater treatment method.

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Utilizing external aeration to enhance the performance of microalgae-bacteria symbiosis (ABS) system has been extensively studied. However, inappropriate aeration damaged ABS system stability. A nighttime aeration mode (NA-ABS) in different aeration intensities (20, 50, 100 mL/min) was adopted to compare to continuous aeration microalgae-bacteria symbiosis (CA-ABS) mode and no-aerated mode on pollutants removal efficiencies and system stability. Results showed that NA-ABS system performed better on total organic carbon (TOC), NH4+-N, total nitrogen (TN) and PO43- removal than CA-ABS system, especially under the aeration intensity of 20 mL/min (NAI20), with the removal efficiencies of 96.59%, 99.18%, 90.30% and 89.16%, respectively. These results were because NA-ABS system prevented CO2 stripping and provided more dissolved inorganic carbon (DIC) for the microalgae growth. Furthermore, less CO2 stripping released the competition between microalgae and autotrophic bacteria for the DIC, leading to a more stable ABS system during long-term operation. This paper suggested that NA-ABS system would provide some new insights into ABS system and be helpful for further study.

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Innovative and cost-effective technologies for advanced nutrient removal from surface water are urgently needed for improving water quality. Conventional biotechnologies, such as ecological floating beds, or constructed wetlands, are not effective in removing nutrients present at low-concentration. However, microalgae-bacteria consortium is promising for advanced nutrient removal from wastewater. Suspended algal-bacterial systems can easily wash out unless the hydraulic retention time is long, attached microalgae-bacteria consortium is more realistic. This critical review summarizes the fundamentals and status of attached microalgae-bacteria consortium for advanced nutrient removal from surface water. Key advantages are the various nutrient removal pathways, reduction of nutrients to very low concentration, and diversified photobioreactor configurations. Challenges include poor identification of functional species, poor control of the community composition, and long start-up times. Future research should focus on the selection and engineering of robust microbial species, mathematical modelling of the composition and functionality of the consortium, and novel photobioreactor configurations.

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