Application of herbicides for the control of weeds in agri-ecosystem poses a great threat to natural inhabiting beneficial microbes. Toxicological impact of herbicides on the survivability of cyanobacteria has recently gained much attention. Thus, the present study was undertaken to understand the mode of action of pendimethalin on assimilation of carbon and nitrogen in diazotrophic cyanobacterium Desmonostoc muscorum PUPCCC 405.10. The lethal dose of pendimethalin for the test organism was 20 mg/l. Pendimethalin (412 mg/l) exhibits negative effect on the growth, photosynthetic pigments (8%85%), photosynthesis (29%58%), photochemical activities (16%64%), and respiration (25%62%). Nitrogen assimilation parameters such as nitrate (39% 65%), nitrite (37%83%), and ammonium (11%37%) uptake and its enzymes such as nitrate reductase (27% 40%), nitrite reductase (3%18%), and glutamine synthetase (15%39%) were also negatively affected by herbicide. Kinetic studies further revealed that the affinity of nitrate and nitrite uptake was unaffected by the herbicide, but the affinity of ammonium uptake was affected. Pendimethalin interacted non-competitively with activities of nitrogen assimilation enzymes.
Key words: Cyanobacteria, Desmonostoc, Pendimethalin, Carbon metabolism, Nitrogen assimilation
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