Investigating the effect of arsenic on exopolysaccharide production in a diazotrophic cyanobacterium

Dutta, Soumyajit (2018) Investigating the effect of arsenic on exopolysaccharide production in a diazotrophic cyanobacterium. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Bioaccumulation of arsenic in crops and vegetables has become a major threat to public health. The Bengal Delta Plains (BDP), located across India and Bangladesh is of great concern internationally because of a vast majority of agricultural lands used in rice cultivation is severely affected with arsenic. These rice fields also harbor a rich diversity of diazotrophic cyanobacterial communities. In this study, the focus was to investigate the effect of arsenic stress on Nostoc ellipsosporium, an abundant diazotrophic cyanobacterial species, isolated from a rice field in BDP and also to understand how cell-physiology contributes to adaptive capability. The cyanobacteria cells were exposed to 50, 200 and 400 μM sodium arsenite concentrations along with a control culture with no addition of arsenite salt. At 400 μM treatment, the cellular growth was inhibited initially but after day 9, there was a marked increase in cell growth. The cell-morphology and autofluorescence patterns were also different in the treatment cultures. In 400 μM treatment, the exopolysaccharide production per day per OD value was greater than the control cells initially up to 12th day; thereafter the exopolysaccharides production rate became comparable or lesser. The bound extracellular polymeric substances (EPS) were found to be disintegrated and therefore the cyanobacterial filaments showed nude morphotype lacking the capsular coating outside filaments. These observations indicate that N. ellipsosporium show signs of adaptive response to arsenic stress and the exopolysaccharides may play protective roles under arsenic stress. Field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDS) confirmed the adsorption of arsenic by the capsular as well as the secreted EPS which essentially shows its biosorption potential towards arsenic. Overall, the information generated from these experiments gives a deeper understanding of adaptive mechanisms towards metals in diazotrophic cyanobacteria that may further help in developing efficient, eco-friendly and cost-effective bioremediation technologies for rendering arsenic free rice grains.

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