Thiamethoxam causes herbicide like acceptor limitation of photosystem II of the cyanobacterium Synechocystis sp. PCC 6803.

Thiamethoxam is a widely used neonicotinoid known for its effect on soil microbial community. Synechocystis sp. PCC 6803 is a soil cyanobacterium and contribute organic carbon to soil microbial community through active photosynthesis, aiding synergistic soil microbial activity. However, not much studies have been made to decipher the effect of thiamethoxam on the photosynthetic electron transport of soil microalgae and cyanobacteria. We measured the effect of thiamethoxam (0-225 μM) on growth, physiology and photochemical performance of Synechocystis sp. PCC 6803 during 24 hr of exposure with an aim to explore its effect on photoelectron transport. Thiamethoxam caused decrease in the growth and pigment content of Synechocystis with more effect on chlorophyll a and carotenoids than on phycobiliproteins. A concentration triggered increase in lipid peroxidation and photosynthetic pigment fluorescence was also observed. The OJIP fluorescence transient showed continuous rise of the OJ phase with thiamethoxam concentration. The performance indicating parameters, φP0 and φE0, decreased whereas the stress indicating parameters, VJ, M0 and φD0, increased with thiamethoxam treatment. The reaction center (RC) functions (Abs/RC, TR0/RC, and ET0/RC) indicated the increase of inactive RC with thiamethoxam treatment. The data agrees with that obtained by atrazine treatment and indicated that the insecticide imposed a PS II herbicide like effect on the acceptor side of PS II of Synechocystis. The inhibition of PS I reduction and decreased PS I functions supports our conclusion. Our observation will enable the researchers to evaluate the on-site effects of thiamethoxam on crop plants.