Effect of arsenic on exopolysaccharide production in a diazotrophic cyanobacterium

Abstract

The Bengal Delta Plain (BDP), located across India and Bangladesh, is one of the worst arsenic (As)-affected regions, including contaminated agricultural lands. The effect of As on the growth and exopolysaccharide (EPS) production was investigated in Nostoc ellipsosporum, a diazotrophic cyanobacterium isolated from a paddy field located in BDP. The cells were exposed to 0 (control), 50, 200, and 400 μM concentrations of sodium arsenite salt and monitored for 21 days. For 50 and 200 μM arsenite treatments, cellular growth was found to be stimulated, whereas, at 400 μM treatment, a characteristic growth behavior was observed compared with growth in control culture. There was an inhibition phase for the first 9 days, followed by a steady increase of growth. During the initial 9 days, the released EPS production (normalized to cell density) in 400 μM As-treated culture was very high compared with that of the lower As treatments. Based on microscopy, it was observed that there was an aggregation of cell filaments in 400 μM treatment culture. In case of capsular EPS, no difference was observed in 50 μM treatment. However, in 200 and 400 μM treatments, capsules were disintegrated after 12 and 15 days of incubation, respectively. Energy-dispersive X-ray spectroscopy (EDS) study showed the binding potential of released and capsular EPS toward As. The adaptive capabilities of N. ellipsosporum, including behavior of multicellularity and protective role of EPS from As stress, can be useful for incorporating in cost-effective bioremediation strategies to remove As from agricultural fields.