Adaptive responses and arsenic transformation potential of diazotrophic Cyanobacteria isolated from rice fields of arsenic affected Bengal Delta Plain

In the Bengal Delta Plains (BDP) of South Asia, there is an increased report of bioaccumulation of arsenic (As) in rice grains and plants which can ultimately result in health hazards in human population consuming rice as a primary staple food. Five abundant cyanobacteria were isolated from the rice fields of BDP and maintained in vitro. The characterized isolates resembled Leptolyngbya sp. (isolate LBK), Nostoc spp. (isolates NOC and NOK) and Westiellopsis spp. (isolates WEC and WEK) based on polyphasic taxonomy. All the five isolates were assessed for biotransformation potential of As vis-à-vis adaptability and survivality under different levels of arsenite compared to control set of experiments. Adaptive changes of cyanobacterial photosynthetic pigments in terms of autofluorescence emission along with nitrogenase activity and exopolysaccharide production were measured for all isolates. The inorganic As absorption in terms of bioconcentration factor (BCF) in dry biomass was found to be highest in NOC (0.201–0.220), followed by NOK (0.147–0.150), WEK (0.071–0.074), WEC (0.051) and LBK (0.014) when exposed in presence of higher (200–400 μM) to lower (100 μM) arsenite concentrations respectively for 7 days. The transformation of arsenite to relatively less toxic arsenate was detected in varying efficiency in all the studied isolates. When treated with 100–400 μM arsenite, 9.58–78.4 % arsenate was detected in growth medium whereas 33–100 % in dry biomass of cyanobacterial isolates. The cyanobacterial isolates of this study could be potentially applied to reduce bioavailability of As in rice fields of South Asia based on further field trials, thereby ultimately rendering rice grains safe for human consumption.