Postdoctoral Fellowship: OCE-PRF: Chemical composition and bioavailability of dissolved organic matter released by marine nitrifiers

Sinking particles and dissolved organic matter produced in the sunlit surface waters of the ocean provide carbon to the deep ocean. However, carbon budgets for the deep (dark) ocean suggest that the energy required by microbes is greater than the supply of carbon from surface waters. This means that there may be additional and unexplored sources of carbon to the deep ocean. Recent work indicates that some nitrifiers, a group of marine microorganisms living in the deep ocean, may provide a source of fresh organic carbon below the sunlit ocean. However, the chemical composition of this material and its suitability as a source of food for other microbes living in the deep ocean are unknown. This project is amongst the first to study the release of dissolved organic matter from marine nitrifiers under different nutrient, temperature, and pressure conditions. The goal of this study is to help understand the role that marine nitrifiers play in ocean carbon cycling. In addition to the direct scientific impacts, this project includes mentoring opportunities for undergraduate students, educational outreach to middle and high school girls, and public outreach in collaboration with local artists and/or art students. This project aims to provide the most thorough characterization to date of dissolved organic matter (DOM) released from marine nitrifiers. Specifically, the investigators plan to 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. The science team plans to use laboratory culturing and incubation studies, but laboratory findings will also be applied to the natural environment through a targeted field campaign and experiments with natural microbial communities. Results from this study will inform biogeochemical models by providing rates of DOM release 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 is expected to 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. In addition, this project provides research experiences for undergraduates, educational activities for middle and high school girls, and a science-art collaboration that aims to communicate findings from this project to the public. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.