Dimethoate delimits both the donor and acceptor side functions of photosystem II of the soil cyanobacterium Anabaena sp. PCC 7119.

The non-target toxicity of organophosphorus (OP) insecticides has affected the distribution and metabolic performances of native microbial communities resulting in loss of soil productivity. Dimethoate (DM) and other OP insecticides are known to decrease physiological performances and change the biochemical composition of cyanobacteria but detailed evaluation of the impact of DM on the photosystem II (PS II), specifically on the major light harvesting and water splitting processes, is lacking. We observed that DM (at 50-200 μM) reduced growth, pigment contents, and photosynthesis, but enhanced respiration, lipid peroxidation and accumulation of fluorescing chlorophyll catabolites of Anabaena sp. PCC 7119. OJIP fluorescence transients showed an increase in OJ fluorescence with proportionate enhancement at J inflection. Donor limitation of PS II was observed with fluorescence rise at K and L inflections, which could be compared with that of ammonia. The increase in the magnitude of parameters of PS II acceptor side functions, VJ, φD0, Abs/RC and D0/RC, indicated reduced PS II-PS I electron transport. This was supported by decrease in proportion of PS I reduction, which could be increased with addition of ascorbate. Increase in PS I reduction under ascorbate rich medium confirmed the donor limitation of PS II.