Low and high doses of UV-B differentially modulate chlorpyrifos-induced alterations in nitrogen metabolism of cyanobacteria

• UV-BL and UV-BH modulated the CP induced responses in two cyanobacteria.
• UV-BL enhances protective mechanisms whether singly or in combination with CP.
• CPL is initially inhibitory but later becomes stimulatory.
• High dose of CP remains inhibitory.
• UV-BH is inhibitory and inhibition becomes severer when CPH combines it.

The present study assessed the comparative responses on the specific growth rate, nitrogen metabolism and enzymes associated with nitrogen metabolism in two nitrogen fixing cyanobacteria—Nostoc muscorum and Phormidium foveolarum exposed to two UV-B doses (low; UV-BL: 0.5472 kJ m−2 and high; UV-BH: 5.472 kJ m−2) and two doses of the insecticide chlorpyrifos (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate; low i.e. CPL, 1 µg ml−1 and high i.e. CPH, 2 µg ml−1) singly and in combination. The specific growth rate, NO3− and NO2− uptake, nitrate assimilating enzymes – nitrate reductase and nitrite reductase and ammonium assimilating enzymes – glutamine synthetase and glutamate synthase were severely affected when treated either with CPH or/and UV-BH while glutamate dehydrogenase exhibited a stimulatory response. CPL also reduced all the measured parameters (except GDH activity) after 24 h, however, a stimulatory effect was observed after 72 h due to an increase in nitrogen metabolism (and other antioxidant) enzymes during this period. UV-BL did not cause significant alteration in the studied parameters while in combination with CP doses, it either alleviated the inhibitory effects or further enhanced the CPL induced activities of these enzymes (except GDH). Overall results indicate the resistant nature of P. foveolarum against the inhibitory doses of UV-B and chlorpyrifos in comparison to N. muscorum.

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