Effect of Time Interval on Arsenic Toxicity to Paddy Field Cyanobacteria as Evident by Nitrogen Metabolism, Biochemical Constituent, and Exopolysaccharide Content.

Arsenic poisoning in aquatic ecosystem is a global concern that obstructs the productivity of agricultural lands (paddy fields) by targeting the growth of cyanobacteria. The cyanobacteria also tolerate and accumulate elevated concentration of arsenic (As) inside the cell and excrete out from cells in less toxic forms after the successive time interval. Thus to validate this, the study was carried out at two different time intervals, i.e., 48 h and 96 h. Two redox forms of As arsenate (AsV) and arsenite (AsIII) at different concentrations (50, 100, and 150 mM AsV; 50, 100, and 150 μM AsIII) caused substantial reduction in growth, pigments (Chl a/Car and phycobiliproteins: phycocyanin, allophycocyanin, and phycoerythrin), inorganic nitrogen ( nitrate (NO3-) and nitrite (NO2-)) uptake, activity of enzymes (NR, NiR, GS, and GOGAT) of nitrogen metabolism, biochemical constituents (protein, carbohydrate, and exopolysaccharide (EPS) contents of Nostoc muscorum, and Anabaena sp. PCC7120. The tested doses of AsV and AsIII after 48 h of exposure exhibited adverse impact on these parameters, but after 96 h with lower doses of AsV (50 mM and 100 mM) and AsIII (50 μM and 100 μM), significant recovery was recorded. Contrary to this, at higher dose of AsV (150 mM) and AsIII (150 μM), the adverse impact was further aggravated with increasing time exposure. Contrary to the activity of NR, NiR, GS, and GOGAT, GDH activity (alternative NH3+ assimilating enzyme) was found to increase, and after 96 h, the activity showed declining trend but still higher than the control. The biochemical constituent EPS (first protective barrier) under scanning electron microscope showed more accumulation of dry adsorbent in the case of AsIII stress hence displayed more toxic nature of AsIII than AsV. The study concludes that with increasing time exposure, the recovery in growth and related parameters mainly at lower doses of AsV and AsIII points toward adaptability of cyanobacteria which was more pronounced in Nostoc muscorum.