Physiological and genetic responses of Chlorella sp. to nitrite accumulation in microalgal-bacterial consortium with partial nitrification treating municipal wastewater

The integration of microalgal-bacterial consortium (MBC) with partial nitrification (PN-MBC) offers a promising strategy for low-carbon wastewater treatment. However, the gradually accumulated nitrite levels challenge microalgal activities and system stability. This study demonstrated the nitrite tolerance (10-300 mg/L) of Chlorella sp., isolated from the PN-MBC system, and the underlying mechanism. Physiological assays, transcriptomic analysis, and bioinformatics revealed that nitrite significantly affected photosynthesis, DNA processing, carbon metabolism, signal transduction, and protein processing. Specifically, nitrite inhibited photosystem II by targeting the PsbO subunit, disrupting electron transport and the proton gradient, hindering carbon fixation in the Calvin cycle. It also caused DNA damage, including strand breaks, base modifications and mismatches, with upregulated DNA repair pathways and biomass growth stagnation between Days 5-7. In response, Chlorella sp. upregulated carbon metabolism and oxidative phosphorylation to enhance ATP synthesis, while exopolysaccharides were secreted for energy storage, and protein processing was downregulated to mitigate proteotoxic stress. Evolution analysis suggested that active site variations in carbon metabolism enzymes contributed to Chlorella sp.'s enhanced nitrite resilience. These findings advance current understandings of nitrite's effects on microalgae and offer insights for optimizing PN-MBC performance under high-nitrite conditions.