Interactive effect of melatonin and gamma aminobutyric acid in Nostoc muscorum and Anabaena sp. and their mechanisms of cadmium toxicity alleviation

Cyanobacteria most often beneficially associated with paddy crops and support the productivity. However, their growth is negatively affected by abiotic stress. In the present study, impact of cadmium (Cd) toxicity was explored in cyanobacteria Nostoc muscorum and Anabaena sp., in the presence of melatonin (MT) and gamma-aminobutyric acid (GABA). In addition to control, cultures treated with Cd, Cd+MT, Cd+GABA and Cd+GABA+CPA were assessed for analysing the MT and GABA interaction. The outcomes illustrated that Cd at 5 µM disturbed light-harvesting pigments and photosynthesis (whole cell oxygen evolution and PS II photochemistry), thereby diminished the growth by 22% and 25% in N. muscorum and Anabaena sp., respectively. Excessive accumulation of Cd increased oxidative biomarkers: superoxide radical (SOR; O 2 .- ), hydrogen peroxide (H 2 O 2 ) and malondialdehyde (MDA) contents. Under this condition, increased oxidative damage occurred despite of accelerated activity of antioxidant enzymes: superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and glutathione- S -transferase (GST). The cultures treated with Cd+MT and Cd+ GABA combinations exhibited decreased Cd accumulation and oxidative biomarkers, while significant rise in antioxidant enzyme activity was noticed as compare to respective values recorded in Cd alone treated samples. Thus, exogenous MT and GABA supported the growth of both the cyanobacteria even under Cd treatment by improving light-harvesting pigments and photosynthetic activity, and the effect was more pronounced with GABA. The endogenous MT inhibitor CPA (4-chloro DL-phenylalanine) suggested the interactive role of MT and GABA in Cd toxicity alleviation, hence, GABA appeared to be prime player in Cd toxicity alleviation.