The ecotoxicological effects of thiamethoxam on photosynthesis and nitrogen fixation in cyanobacteria: From inhibition to recovery

Thiamethoxam (THX) is a highly efficacious, broad-spectrum neonicotinoid insecticide that is prevalently detected in contemporary environmental settings. Although considerable research has been conducted on the adverse effects of THX on non-target organisms, there is a paucity of knowledge regarding its impact on nitrogen (N)-fixing cyanobacteria, which play a crucial role in the N-cycle. This study examined the impact of the neonicotinoid pesticide THX on N-fixing cyanobacteria Nostoc sp. PCC7120. It combined biochemical and molecular data to clarify the toxicity and recovery mechanism of Nostoc sp. PCC7120 exposed to THX. The results show that at low THX concentrations, cyanobacteria exhibited hormetic responses and then enter recovery phases, whereas at high concentrations, they showed toxicity phases before entering recovery phases. The main changes were in cell growth, antioxidant system, photosynthetic performance and N-fixation. The starch granules and vacuoles of Nostoc sp. PCC7120 became larger after THX treatment, indicating a shift in resources from growth to storage. Additionally, molecular stimulations indicated that THX could bind to molybdenum-iron cofactor sites of nitrogenase to destroy N-fixation, and also bind to Q_A and Q_B sites of photosynthetic proteins (D1 and D2) to affect photosynthesis. These findings provide a valuable reference for the ecological risks of THX and biogeochemical N-cycle.