An investigation on involvement of the ascorbate-glutathione cycle in modulating NaCl toxicity in two cyanobacteria photoacclimatized to different photosynthetic active radiation

In the present study, comparative responses of two cyanobacteria viz. Nostoc muscorum and Phormidium foveolarum, photoacclimatized at three distinct levels (sub-optimum; 25 ± 0.5, optimum; 75 ± 2.5 and supra-optimum; 225 ± 3.5 μmol photons m− 2 s− 1) of photosynthetic active radiation (PAR), and subsequently treated with two doses (30 and 90 mM) of NaCl were determined by analyzing changes in growth pattern, oxidative stress markers, enzymes of the ascorbate-glutathione cycle i.e. ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), and associated metabolites (ascorbate and glutathione). PAR influences growth behavior of both cyanobacteria, as maximum growth observed in supra-optimum PAR acclimatized cells followed by optimum and sub-optimum PAR. Maximum superoxide radical (SOR) and hydrogen peroxide (H2O2) contents and their consequent damages to lipids in terms of MDA equivalents were observed in supra-optimum PAR acclimatized cells in individual as well as in combined treatment of NaCl. Further, NaCl-induced the least percent reduction in growth was observed in supra-optimum PAR acclimatized cells, which was in consistence with increased activities of the AsA-GSH cycle enzymes (APX, GR, MDHAR and DHAR) and its associated metabolites i.e. ascorbate (AsA) and glutathione (GSH) contents and reduced and oxidized ratios of ascorbate (AsA/GSH) and glutathione (GSH/GSSG). The results suggest that the AsA-GSH cycle might have associated with better growth in supra-optimum PAR acclimatized cells than optimum and sub-optimum PAR acclimatized cells under NaCl stress. Besides this, study also establishes fact that P. foveolarum is more tolerant than N. muscorum against these twin stresses, and may be inoculated in paddy fields under existing environmental conditions for increasing fertility of the soil.