Currently, with the intensification of human activities, the total amount of municipal wastewater has been increasing year by year1. In the era of climate change2, 3 and energy crisis4, 5, the widely used conventional activated sludge (CAS) process is under pressure due to high energy consumption and carbon emissions6. As an energy-saving and environmentally-sustainable process, the microalgal-bacterial granular sludge (MBGS) can directly utilize solar energy to generate oxygen7, 8, which is further used assimilate wastewater organics and nutrients into microalgae biomass, while helping to mitigate carbon emissions9. In fact, the MBGS process can even absorb carbon dioxide of industrial origin10 and methane11, which is more conducive to achieving the carbon neutrality of wastewater treatment 12, 13, 14.
Understanding of the removal pathways of nutrients and organics by MBGS are of vital significance, which, nevertheless, has not been fully determined, especially for the complex organics. For nitrogen metabolism, most of the NH4+-N can be converted into glutamine or glutamate for microbial assimilation in non-aerated MBGS system9, 15, and NO3−-N and NO2−-N can be converted to NH4+-N before they are metabolized16, while the organic nitrogen (e.g. urea, amino acids, etc.) can be hydrolyzed to NH4+-N through intracellular urea amidolyase17. For phosphate metabolism, phosphorus could be removed by MBGS through microbial assimilation and poly-phosphate accumulation9, and PO43−-P can be transported into cells through the oxidative phosphorylation and removed through poly-phosphate accumulation under the action of poly-phosphate kinase15. However, it should be noted that for organics metabolism, limited knowledge is currently available since the laboratory-scale studies usually adopted the simple carbon source, e.g., acetate and glucose15, 18, while the carbon source in real municipal wastewater is mainly composed of complex organics, e.g., starch19, but its degradation mechanisms remained unclear. In addition, the algae-bacteria symbiosis ubiquitously exist in natural aquatic ecosystems20. Considering the complex organics discharged into the natural water due to human activity21, 22, it is also necessary to explore the degradation mechanisms for organic matter by microalgal-bacterial symbiosis. Meanwhile, the carbon metabolism is closely related to greenhouse gases emissions23 and the organics removal is usually intimately related to nutrients removal24, there is an urgent need to decipher the complex organics removal mechanisms of microalgal-bacterial symbiosis, providing more information of real wastewater complex organics removal pathways.
On the other hand, to date, the detailed cooperative interaction between algae and bacteria has not been depicted, and the relationships between microbial community and functional genes of MBGS in complex organics metabolism has also been unclear, which hinders its further application. It has been proven that the key role of the algae-bacteria interactions is reflected by the exchange of CO2 and O2, which is a necessary prerequisite for the efficient removal of organics by MBGS9. However, the previous studies focused mainly on the microbial community components or the bacterial coherence, while lacking the algae-bacteria interactions in microalgal-bacterial symbiosis25, 26.
To discover the organics removal pathways and algae-bacteria interactions of microalgal-bacterial symbiosis in treating real municipal wastewater, this study investigated the performance and upregulated functional genes of MBGS process on real municipal wastewater under diel fluctuation. Meanwhile, the evolution of microbial community and relationships between algae and bacteria of MBGS were determined. Moreover, the relationships between microbial community and functional genes were elucidated. This study provided more information on the complex organics removal mechanisms under the algae-bacteria interactions of MBGS process for treating complex organic wastewater, which is expected to add basic knowledge for the engineering applications of MBGS process. More importantly, this study can help to understand the carbon cycling in natural aquatic systems mediated by microalgal-bacterial symbiosis.