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Viral processes in the coastal and offshore regions of the Eastern Arabian Sea (west coast of India) and their contribution of viral lysis to the organic carbon and nitrogen pool was examined. Water samples were collected during the southwest monsoon season at different depths (up to 1000 m) from four transects, with each transect consisting of two sampling points (S1–S8). Abundances of viruses and prokaryotes together with viral mediated prokaryotic mortality (up to 49.7%) were significantly (p < 0.001) higher in eutrophic coastal stations, whereas high percent lysogeny (up to 93%) was observed in the offshore regions. High viral-mediated carbon (Mean ± SD = 67.47 ± 2.0 μM C L−1 d−1) and nitrogen (Mean ± SD = 13.49 ± 14.0 μM N L−1 d−1) release was evident in the surface coastal waters compared to offshore regions. The percentage contributions of carbon and nitrogen released by viral lysis to the total dissolved organic carbon and nitrogen pool were estimated to be 7.4% and 3.9%, respectively, in the coastal surface waters. Our findings suggest that the contribution of viral lysis to DOM production through viral shunt could be crucial for the cycling of major biogeochemical elements and functioning of the studied tropical ecosystem.

In spite of the fact that the interactions between environmental parameters and prokaryotic and viral abundance have been explored in various aquatic environments, only a few independent estimates of viral production and decay in the estuarine region have been explored. In this study, data were analyzed for viral and prokaryotic abundance, viral production, and viral decay in a subtropical Danshui estuary in summer 2021. Prokaryotic abundance varied from 2.4 ± 0.6 × 105 to 12 ± 2.3 × 105 cells mL−1, and viral abundance ranged from 2.3 ± 0.9 × 105 to 6.9 ± 1.3 × 105 viruses mL−1 during the study period. Viral abundance was significantly correlated with prokaryotic abundance and chlorophyll a concentration. Furthermore, studies of changes in viral to prokaryotic abundance ratio (VPR) ranged from 0.42 ± 0.11 to 2.0 ± 0.25. Viral decay values were 2.1 ± 0.5 and 2.1 ± 0.3 × 104 virus mL−1h−1, and non-significant differences were observed between the inner estuary and coastal water region. Viral decay almost balanced gross viral production in this study. The dilution experiments revealed non-significant net viral production in July; thus, a lower VPR might be explained in this estuarine environment.

Viral production (VP) and bacterial mortality by viral lysis critically influence the production and mortality of aquatic bacteria. Although bacterial production, mortality by viral lysis, and viral density have been found to exhibit diel variations, the diel change in viral production has rarely been investigated. In this study, we conducted two diel dilution incubation experiments in a semi-enclosed, nutrient-rich coastal region in northeastern Taiwan to estimate the diel viral production and the mortality by viral lysis. We also compared two methods (linear regression between viral density and time versus arithmetic mean of VP during incubation) of estimating viral production. We found that viral production estimated by linear regression and bacterial mortality by viral lysis were higher during the daytime than during the nighttime. A possible explanation for the high viral production at daytime is that the bacterial community was composed of cell types with higher burst sizes at daytime. We further argued that the classical linear regression method can be used only when viral density significantly linearly increases with time, which does not always occur in dilution incubations. This study offered observations of diel variation in viral dynamics and discussed the methods estimating viral production in a marine environment.

Climate warming can directly affect biological processes in marine environments. Here, we investigated if warming (+2 °C) can change dynamics in viral and prokaryotic populations in the cold seasons in natural seawaters. We monitored the changes in viral production and prokaryotic growth rate. The prokaryotic average gross growth rates were 0.08 and 0.34 h−1 in November and 0.06 and 0.41 h−1 in December in the in situ and warming experiments, respectively. We found that warming water temperature resulted in a significant increase in prokaryotic growth rates. In warming experiments, the overall viral production rate was about 0.77–14.4 × 105 viruses mL−1 h−1, and a rough estimate of prokaryotic mortality was about 5.6–6.8 × 104 cells mL−1 h−1. Based on our estimation, burst sizes of about 21 and 14 viruses prokaryotes−1 were measured under the experimental warming period. Moreover, the results found that an increased water temperature in the subtropical western Pacific coastal waters increases prokaryotic growth rates, enhances viral production, and changes the carbon fluxes in the trophic interactions of microbes.