Seasonal variations in viral distribution, dynamics, and viral-mediated host mortality in the Arabian Sea

Abstract

Viruses are key players in the marine ecosystem. It is critical to study specific viral processes and their inter-relationship with various biotic and abiotic variables to quantify their impact on the marine environment. This study investigates the influence of seasonality on viral distribution and their mediated processes in the coastal region of the southeastern Arabian Sea (India) for two consecutive years (2014 and 2015). Water samples were collected from four sampling stations, S1, S2, S3 and S4, in the southeastern Arabian Sea on a monthly basis. Samples were analyzed for the variations in viral abundance, viral production, and viral-induced host mortality covering three seasons namely, pre-monsoon, monsoon and post-monsoon. Viruses and bacteria were enumerated by epifluorescence microscopy following staining with SYBR Green I and viral production was estimated using viral dilution method. Seasonal variations in viral-mediated mortality and microzooplankton grazing mortality of phytoplankton hosts were estimated by modified dilution method. The results revealed a pronounced seasonal pattern of viral abundance (0.04–4.03 × 107 viruses mL−1) and viral production (0.71 × 1010–4.94 × 1010 L−1d−1). The observed high viral-induced mortality of prokaryotes (25.85%) and phytoplankton (7.98–28.9%) during the pre-monsoon season was eventually linked to high viral abundance and production rates. Viral-mediated processes were essentially linked to host density and water temperature. Transmission electron microscopy analysis revealed that myoviruses and non-tailed viruses were dominant in the study region with rod-shaped bacteria being more susceptible to viral infection. The results indicated the prevalence of seasonally induced active viral processes in this tropical marine ecosystem which may significantly contribute to rapid recycling of nutrients in this region.