Microbial community shifts along an estuarine to open ocean continuum

Abstract The water bodies along a continuum from estuaries to open ocean display significant changes in their physical, chemical and biological characteristics. In the current study, we report the influence of such changes on the abundance and community structure of bacteria and archaea in a continuum from Cochin Estuary, in the southwest coast of India, to the open ocean of Arabian Sea. The bacterial and archaeal abundances were in the range of 10 4 to 106 and 10 3 to 105 cells ml−1 respectively in all the stations and showed a clear dependence on nutrients. The bacterial abundance decreased with depth in the open ocean, whereas the archaea showed a similar reduction at both shelf and open ocean stations. The α - Proteobacteria (35 %) and Cyanobacteria (23 %) were dominant in the estuary. More than 70 % of the community of bacteria on the shelf and in the open ocean were similar. The α - and γ -Proteobacteria contributed 37 & 27 % of the OTUs in the clone library of shelf and 20 & 29 % in the open ocean. The archaeal community structure in the open ocean and shelf were similar (90 %) composed mostly of Nitrosopumilaceae (70%–75%), while they were highly diverse in the estuary. In short, our study shows that the archaea are more diverse in the estuary compared to shelf and open ocean, while bacteria are equally diverse in all ecosystems.