Effects of UV‐B and heavy metals on nitrogen and phosphorus metabolism in three cyanobacteria

Cyanobacteria sp. (diazotrophic and planktonic) hold a major position in ecosystem, former one due to their intrinsic capability of N 2 ‐fixation and later because of mineralization of organic matter. Unfortunately, their exposure to variety of abiotic stresses is unavoidable. Comparative analysis of interactive effect of UV‐B and heavy metals (Cd/Zn) on nitrogen and phosphorus metabolism of three cyanobacteria ( Anabaena , Microcystis , Nostoc ) revealed additive inhibition ( χ 2 significant p  < 0.05) of NH 4 + and PO 4 3− uptake whereas increase in nitrate uptake upon UV‐B + heavy metal exposure. Glutamine synthetase and Alkaline phosphatase activity was reduced after all treatments whereas Nitrate reductase activity showed slight stimulation in UV‐B and UV‐B + heavy metals treatment. Combination of UV‐B and metals seems more detrimental to the NH 4 + , PO 4 3− uptake, GS and APA activity. A significant stimulation in NO 3 − uptake and NR activity following exposure to UV‐B in all the three cyanobacteria suggests UV‐B‐induced structural change(s) in the enzyme/carriers. Metals seem to compete for the binding sites of the enzymes and carriers; as noticed for Anabaena and Microcystis showing change in K m while no change in the K m value of Nostoc suggests non‐competitive nutrient uptake. Higher accumulation and more adverse effect on Na + and K + efflux proposes Cd as more toxic compared to Zn.