Photosynthesis Characterization of Mutant Algae and Enhanced Carbon Fixation of Algae–Bacteria Symbiosis Treating Municipal Wastewater

Algae–bacteria symbiosis (ABS) as a sustainable wastewater treatment process has drawn mounting attention. However, nontrivial CO2 emissions were still present in municipal wastewater treatment due to the inadequate carbon fixation efficiency of microalgae under low carbon level. The obtained UV‐induced mutant Chlorella vulgaris MIHL4 performed higher carbon fixation capability (14.5%) and biomass productivity (25.3%) with improved photosynthetic fluorescence parameters and enzyme activities compared to wild‐type C. vulgaris. Transcriptome analyses showed pathways related to the carbon fixation and carbon catabolism were significantly up‐regulated in MIHL4. Compared with ABS inoculated with wild‐type C. vulgaris, CO2 emissions were significantly reduced by 32.1%–38.3% in ABS inoculated with MIHL4, where the biomass growth, metabolic activity, and sludge granulation were enhanced. Chlorella responsible for carbon fixation was the dominant population (19.3%) in ABS inoculated with MIHL4, in which the abundance of functional microbes and genes associated with photosynthesis as well as nutrient removal increased.