Bacterioplankton abundance and community structure during post-monsoon in mangrove dominated estuaries of the Indian Sundarbans; An insight to biogeochemical processes

Abstract

Bacterioplankton abundances in three major estuaries of the Indian Sundarbans (Saptamukhi, Thakuran and Matla) were determined by flow cytometry and fluorescence microscopy in a post-monsoonal study followed by microbial community composition (16S rRNA clone library), during the period 2012–2016. All three estuaries were oxygenated and meso - to polyhaline. They had similar levels of dissolved inorganic carbon and nutrients and were net heterotrophic, as evidenced by net negative community productivity. Mean post-monsoon bacterioplankton abundances (x 109 cells L−1) in these estuaries were: Saptamukhi (1.30 ± 0.78 × 109 cells L−1); Matla (1.38 ± 1.34 × 109 cells L−1); Thakuran, (0.92 ± 0.46 × 109 cells L1). Bacterioplankton abundance was associated with water temperature coupled with dissolved inorganic nutrients and carbonate parameters. Highest abundances were at the most upstream sites in the Saptamukhi and Matla estuaries, while a mid-estuarine peak was evident in Thakuran. In the Saptamukhi and Thakuran the bacterioplankton was dominated by Proteobacteria (mostly Deltaproteobacteria), with a contribution from Bacteroidetes in the Saptamukhi and from Cyanobacteria and Actinobacteria in the Thakuran. Despite the oxygenated nature of these estuaries, clones resembling the sulfate reducer Desulfovibrio sp. were detected, and this could be a consequence of anoxic pore water exchange across the sediment-water interface. Sulfate reduction may thus be an important pathway for carbon metabolism in these ecosystems.