Melatonin mediates gamma-aminobutyric acid induced mitigation of cadmium toxicity on growth, PSII photochemistry, and oxidative stress in Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120

Cyanobacteria, being nitrogen fixers most often beneficially associated with paddy crops and support the productivity. However, the growth of these symbionts is relatively affected by abiotic stresses. In the present study, the impact of Cd toxicity was explored in cyanobacteria Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120, in the presence of signalling molecules melatonin (MT; 90 nM) and gamma-aminobutyric acid (GABA; 40 nM). Cadmium at 5 µM disturbed light-harvesting pigments and photosynthesis (whole cell oxygen evolution and PS II photochemistry), and finally diminished the growth of cyanobacteria. Excessive intracellular accumulation of Cd raised the level of oxidative biomarkers i.e., superoxide radical, hydrogen peroxide, and malondialdehyde equivalents content despite of accelerated activity of enzymatic antioxidants i.e., superoxide dismutase, peroxidase, catalase, and glutathione- S -transferase. Furthermore, exogenous application of MT and GABA individually supported the growth by improving light-harvesting pigments and photosynthetic activity, and it was more pronounced with GABA. This could occur due to considerable decline in Cd accumulation as a result, the declining trend in the oxidative stress biomarkers was taken place by further augmentation of antioxidant activity. The endogenous MT inhibitor CPA (90 µM) suggested the interactive role of MT and GABA in Cd toxicity alleviation. Hence, the study concludes that GABA, being the prime player in Cd toxicity alleviation, appears to act downstream of MT. Further, more insight is needed by involving molecular techniques to enrich the cyanobacteria with these signalling molecules to use them as a biofertilizer for sustainable agriculture, particularly paddy, even under prevailing stress conditions.