Towards an understanding of ecosystem effects of viruses in the Eastern Tropical North Pacific Oxygen Minimum Zone

Oxygen minimum zones (OMZs) constitute ~7% of oceanic volume and the metabolic activity of their unique microbial communities is known to have significant impacts on global biogeochemical cycles. However, viral diversity and viral influence on microbial processes in these regions are much less studied. Here we describe viral community diversity and activity at two stations in the Eastern Tropical North Pacific (ETNP) OMZ region using both quantitative microscopy and metagenomic methods. While viral abundance decreased below the oxycline, viral diversity and lytic infection frequency remained high within the OMZ. Viral community composition was also strongly related to oxygen concentration, with the most unique viral populations found within the oxycline. Very few viral-encoded auxiliary metabolic genes (AMGs) for biogeochemical cycling, including that of nitrogen and sulfur, were detected despite known prevalence of these cycles in the ETNP. However, we found several AMGs abundant in the low-oxygen portions of the water column that may contribute to host responses to oxidative stress including sporulation and iron-sulfur cluster formation. Further, while many AMG functional categories were present throughout the water column, their relative abundances changed substantially with depth, supporting the theory that viruses encode for AMGs attuned to their host needs. We hypothesize that the high microbial productivity of this region causes a shift in the viral community from primarily boosting key metabolic reactions to influencing host survivability. Together, this study shows that viruses are not only diverse and active throughout the water column in OMZ regions, but their infection of microorganisms has the potential to alter host physiological state within these biogeochemically important regions of the ocean.