Viruses play an important role in the ecology and biogeochemistry of marine ecosystems. Beyond mortality and gene transfer, viruses can reprogram microbial metabolism during infection by expressing auxiliary metabolic genes (AMGs) involved in photosynthesis, central carbon metabolism, and nutrient cycling. While previous studies have focused on AMG diversity in the sunlit and dark ocean, less is known about the role of viruses in shaping metabolic networks along redox gradients associated with marine oxygen minimum zones (OMZs). Here, we analyzed relatively quantitative viral metagenomic datasets that profiled the oxygen gradient across Eastern Tropical South Pacific (ETSP) OMZ waters, assessing whether OMZ viruses might impact nitrogen (N) cycling via AMGs. Identified viral genomes encoded six N-cycle AMGs associated with denitrification, nitrification, assimilatory nitrate reduction, and nitrite transport. The majority of these AMGs (80%) were identified in T4-like Myoviridae phages, predicted to infect Cyanobacteria and Proteobacteria, or in unclassified archaeal viruses predicted to infect Thaumarchaeota. Four AMGs were exclusive to anoxic waters and had distributions that paralleled homologous microbial genes. Together, these findings suggest viruses modulate N-cycling processes within the ETSP OMZ and may contribute to nitrogen loss throughout the global oceans thus providing a baseline for their inclusion in the ecosystem and geochemical models.

Abstract: Organic carbon in seawater plays a significant role in the global carbon cycle. The concentration and composition of dissolved organic carbon reflect the activity of the biological community and chemical reactions that occur in seawater. From 2016 to 2019, we repeatedly sampled the oligotrophic northwest Sargasso Sea in the vicinity of the Bermuda Atlantic Time‐series Study site (BATS) to quantitatively follow known compounds within the pool of dissolved organic matter in the upper 1000 m of the water column. Most metabolites showed surface enrichment, and 83% of the metabolites had significantly lower concentrations with increasing depth. Dissolved metabolite concentrations most notably revealed temporal variability. Fourteen metabolites displayed seasonality that was repeated in each of the 4 yr sampled. Concentrations of vitamins, including pantothenic acid (vitamin B5) and riboflavin (vitamin B2), increased annually during winter periods when mixed layer depths were deepest. During diel sampling, light‐sensitive riboflavin decreased significantly during daylight hours. The temporal variability in metabolites at BATS was less than the spatial variability in metabolites from a previous sample set collected over a broad latitudinal range in the western Atlantic Ocean. The metabolites examined in this study are all components of central carbon metabolism. By examining these metabolites at finer resolution and in a time‐series, we begin to provide insights into the chemical compounds that may be exchanged by microorganisms in marine systems, data which are fundamental to understanding the chemical response of marine systems to future changes in climate.