Microbial distribution in Mudbank regions off Alappuzha, South-west coast of India.

The coastal waters of Kerala, in the southeastern Arabian Sea (SEAS), are unique during the Southwest monsoon season due to the concurrent occurrence of two physical processes, the upwelling and Mudbanks. However, little is known about the viral ecology and activity in a system where upwelling and mudbanks coexist, though it is generally recognized that microbial assemblages play a vital role in the food web dynamics of marine systems, particularly in upwelling. Water samples were taken from three locations, M1, M2, and M3, off Alappuzha, on the southwest coast of India to examine the viral activity and distribution. An 18-week study in 2014 and a 16-week study in 2016 on a weekly/biweekly interval were carried out from April to September, which were categorized into three seasons April to May, as pre-monsoon, June to August as monsoon, and September period as late-monsoon. The dissolved oxygen levels showed the incursion of hypoxic waters in all the stations during the peak upwelling period. Upwelling signals were prominent in all the stations, but Mudbank and upwelling co-occurred at M2 alone during monsoon. The abundance of viruses ranged from 0.86 to 15.68×106 Viral like Particles (VLPs mL-1) and prokaryotic abundance ranged from 2.73-16.26×105 cells mL-1. Viral and prokaryotic abundance was significantly higher in the monsoon compared to pre and late-monsoon. Based on Transmission electron microscopy (TEM) results, the non-tailed viruses constituted the major (43%) proportion of the total viruses during this study region. However, the viral production rates and viral-mediated bacterial mortality were high in the pre-monsoon compared to the monsoon and late-monsoon periods. There was no obvious effect of Mudbanks on viral dynamics and the observed variations in virological and hydrological features were governed mainly by coastal upwelling.