We hypothesise that viral activity regulates the elemental composition of the large standing pool of marine dissolved organic matter (DOM) which is of similar magnitude to the standing stock of carbon in the terrestrial biosphere. We present a simple model of carbon, nitrogen and phosphorus flux from surface particulates through a deep ocean ecosystem composed of bacteria and viruses. High viral nucleic acid content causes efficient P assimilation, but shunts much of the C and N from sinking particulates into the dissolved pool. Our model thus predicts DOM in the ocean is enriched in C due to viral lysis, may account for up to 80% of C enrichment and enhancing ocean carbon storage by approximately 560 Pg C, similar to the total standing stock of carbon in atmospheric CO2.