Synergistic microalgae–bacteria interactions and gene dynamics in an intermittently aerated vertical fluidized reactor for C and N removal

BACKGROUND The integration of microalgae and activated sludge offers a promising route for energy‐efficient wastewater treatment. This study investigated a vertical fluidized reactor intermittently aerated and operated under light/dark cycles, at a hydraulic retention time ( HRT) of 1 and 2 days, to evaluate the development and performance of a microalgae–bacteria consortium. Although Chlorella vulgaris was initially inoculated, the final community structure was dominated by Chlamydomonas eustigma , Volvox reticuliferus , and Scenedesmus sp., highlighting a dynamic ecological succession driven by operational conditions. RESULTS Under steady‐state conditions, chemical oxygen demand removal reached 99.57 ± 0.96%, while ammonium removal achieved 72.22 ± 9.48%, accompanied by notable improvements in sludge settleability. Genes encoding complete denitrification ( narGHI , nirK , norBC , nosZ ) and carbon metabolism (Entner–Doudoroff, glycolysis, tricarboxylic acid cycle) were identified. In contrast, genes for complete nitrification were absent except for nxrAB , while nitrogen fixation genes were detected, indicating probably community‐level functional resilience. CONCLUSION These results demonstrate the strong redox complementarity and metabolic cooperation within the microalgae–bacteria consortium. The system represents an energy‐efficient and sustainable alternative for simultaneous nutrient and organic matter removal, with potential for biomass valorization in advanced wastewater treatment technologies.