Harnessing salinity induced adaptive potential of extracellular polymeric substances producing rice-field cyanobacterium Neowestiellopsis sp. VKB03: a physiochemical, morphological, and biomass characterization.

The toxicity of salinity in irrigated land has significantly affected global rice production. Cyanobacteria are a core atmospheric nitrogen-fixer in the rice-field community. However, extracellular polymeric substances (EPS) play a distinct role in the adaptive fitness of cyanobacteria under salinity stress. This study analyzed physiochemical, morphological, biomass, and polymer characteristics of rice-field cyanobacterium Neowestiellopsis sp. VKB03. Our findings showed that high salt content elevated the levels of carbohydrates, EPS, lipids, proline, stress biomarkers (H2O2 and MDA), and antioxidants (CAT and APX). Interestingly, elevated salt reduced photochemical activity (Fv/Fm and ETRmax) by inhibiting electron donor and acceptor sides, leading to PSII inactivation. Conversely, regulated and non-regulated photochemical fluorescence quenching [Y(NPQ), NPQ, and Y(NO)] increased, enhancing energy dissipation. Notably, fast kinetics showed that inactivated PSII reaction centers limited energy transfer, increased heat loss, and reduced the potential for linear electron flow (LEF). Inversely, electron transfer efficiency from transitional carriers to final PSI receivers increased, as confirmed by the post-illumination fluorescence transient (PIFT). Furthermore, microphotographs showed increased levels of sulfated and carboxylated mucopolysaccharides with high salt supplementation. Importantly, intracellular Ca2+ and Na+ levels increased, whereas K+ levels decreased to maintain cellular homeostasis. Additionally, CHNS analysis revealed increased C and H, and reduced N, thereby raising the C/N and H/C ratios. Conspicuously, FTIR peaks (1040 and 1075 cm⁻¹) and P-XRD analysis confirmed polysaccharides-related functional groups and the amorphous nature of biomass, respectively. Our findings demonstrate that rice-field Neowestiellopsis sp. adapts to elevated salt and increased carbohydrate accumulation, supporting EPS production.