Seasonal variability in environmental parameters influence bacterial communities in mangrove sediments along an estuarine gradient

Abstract

Mangrove ecosystems are repositories of divergent microbial communities which respond differently to environmental characteristics. Hence, it is important to understand the community structure with respect to pertinent controlling environmental factors. Seasonal down-core variations in bacterial communities were investigated at 3 mangrove sites at upstream, mid-stream and mouth region along the economically-important Mandovi estuary, west coast of India. Genomic DNA extracted from 45 sediment samples were analyzed by 454 pyrosequencing. A total of 15008 operational taxonomic units (OTUs) were assigned to 54 phyla and 127 classes re-affirming mangroves to be biodiversity hotspots. Members of class Alphaproteobacteria constituted up to 35% of the tag sequences and showed marked seasonal variability at all locations (Kruskal-Wallis test, p < 0.05). Occurrence of other abundant phyla like Bacteroidetes, Firmicutes, Actinobacteria, Chloroflexi and Acidobacteria at the study sites indicated their participation in biogeochemical cycling. Seasonal fluctuations in the environmental variables influenced the bacterial communities. Statistical analysis revealed that during the monsoon, bacterial communities proximal to the mouth region were largely influenced by pore water NH4+ concentrations due to its accumulation in the sub-surface sediments. At the upstream site, elevated organic carbon and NO2- content during the post-monsoon caused variations in the bacterial community.