Microbial diversity in a coastal environment with co-existing upwelling and mud-banks along the south west coast of India

Upwelling and mud banks are two prominent oceanographic features in the coastal waters along the south west coast of India during the southwest monsoon (MON) season. The present study investigates the microbial diversity in the coastal environments of Alappuzha, India, where upwelling and mud banks co-exist. Water samples were collected from three stations, M1, M2, and M3, on a weekly basis to estimate the physico-chemical parameters and microbial abundance (MA). Presence of cold waters (<26 degrees C) with high nitrate (6-8 mu M) and low dissolved oxygen (5 mu M) in the sub surface waters during monsoon (M) confirmed the presence of upwelling at all the three stations. Simultaneously, presence of unusually calm waters was seen at M2 alone during M indicating the formation of mud banks. The microbial diversity was determined from three stations, with distinct oceanographic conditions (M1: coastal reference station with only upwelling, M2: mud banks+upwelling, and M3: offshore reference station with only upwelling). The water samples were collected during two seasons, pre-monsoon (April) and M (July) and analysed using 16S rRNA-based Illumina high-throughput metagenomic sequencing. Proteobacteria was the most dominant phyla, followed by Bacteroidetes, Firmicutes, Cyanobacteria, Actinobacteria, and Verrucomicrobia in order, with variations in their relative abundance spatially and seasonally. Though the MA increased during M at all the stations, the relative abundance of most of the bacterial phyla except Proteobacteria decreased during M season. Interestingly, most of the sequences at M2 during mud banks were unclassified at the class level indicating the presence of unique microbial populations in this station. Prediction of metabolic activity revealed ammonia oxidation, nitrite reduction, sulphate reduction, xylan degradation, dehalogenation, chitin degradation, etc. as important functions. The metabolic activity throws light on the role of microbes in this environment thereby providing a system-scale perspective of microbial community interactions.