This project will provide the most thorough characterization yet of DOM released from marine nitrifiers. Specifically, this work will determine the chemical composition and quantity of DOM released from nitrifiers, the mechanism by which nitrifiers release DOM, the bioavailability of nitrifier-derived DOM and which heterotrophs may respond most readily to it, and the metabolic cost of DOM release to individual nitrifying archaeal and bacterial cells. This work involves primarily laboratory culturing and incubations, but laboratory findings will be applied to the natural environment through a targeted field campaign and experiments with natural microbial communities. The resultant data will inform biogeochemical models with DOM release rates from marine nitrifiers under environmentally relevant conditions, contribute information about metabolites produced by marine nitrifiers, and identify heterotrophic taxa that depend on this energy and carbon source. This work will enhance our understanding of carbon cycling in the mesopelagic ocean, which may help to improve biogeochemical models and is also critical to understanding the potential impacts of proposed marine carbon dioxide removal activities.