Bacterial and isotopic indicators across an estuarine continuum in a cholera endemic zone. Karnaphuli: bacterial communities

The dynamic of Vibrio species and bacterial communities in relation with biogeochemical factors was studied in the Karnaphuli estuary. The endemic nature of the estuary for Vibrio cholerae was evidenced and strong seasonal differences in Vibrio distribution were detected. Higher temperatures and sewage pollution enhanced the abundance of toxigenic Vibrio cholerae. A link between non-culturable Vibrio and the particulates was inferred during the post-monsoon. Salinity and higher temperatures influenced the operational taxonomic units (OTUs) of Vibrio. The resuspension of sediments increased Vibrio abundance and organic nutrient concentrations. At the marine stations, Rhodobacterales and Oceanospirillales were probably early colonizers of allochthonous organic matter. The last orders, together with Vibrionales, most likely contributed to the degradation of organic matter with a consequent δ13C increment. Some Oceanospirillales, Tolumonas and Novosphingobium indicated likely hydrocarbon pollution. Synechococcales bacteria were the main photosynthetic microorganisms at the freshwater stations. OTUs from Bacteroidales were reliable indicators of untreated wastewater. The ammonification of particulate organic matter under hypoxic conditions nearby the sewage discharge, by Cloacibacterium and particularly by Arcobacter, contributed to the 15N depletion. Relevant genera of potential pathogens other than Vibrio were Acinetobacter, Aeromonas, Arcobacter and Bergeyella. During the post-monsoon, Geobacter influenced probably the arsenic cycle. This study evidenced clearly the eutrophic conditions of the Karnaphuli estuary and assessed comprehensively its current bacterial baseline. The current conditions together with human overpopulation and frequent natural disasters transform the region in one of the most vulnerable to climate change. Adaptive management strategies are urgently needed to enhance ecosystem health.