SGER: Development of an Immunoassay for Nitrogenase for Estimating Nitrogen Fixation Rates in Trichodesmium

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Trichodesmium is a filamentous nonheterocystous cyanobacterium which is widely distributed throughout tropical and subtropical seas. Trichodesmium is important in the primary productivity of these environments, but also is the major nitrogen fixing organism found in the open ocean. Recent studies of the N budgets of the central oceanic gyres has pointed towards the importance of Trichodesmium as a source of nitrogen. Trichodesmium fixes nitrogen only during the day, and also is buoyant and not evenly distributed throughout the euphoric zone. Thus, estimates of depth integrated daily nitrogen fixation rates for natural populations of Trichodesmium require multiple nitrogen fixation rate measurements at different times of day and at different depths. Although nitrogen fixation rate measurements are relatively simple, they do require shipboard incubations and specialized equipment, and need to be perfomed at sea. The proposed research exploits a recently discovered relationship between nitrogen fixation rates and abundance of a component of nitrogenase, the enzyme that catalyzes nitrogen fixation, to provide a new approach for estimating nitrogen fixation by Trichodesmium in the sea. The goal of this small grant for exploratory research is to develop an immunoassay approach for estimating the rate of nitrogen fixation by Trichodesmium. The proposal is based on preliminary results showing a strong correlation between abundance of the active form of the Fe protein of nitrogenase and nitrogen fixation rates in a culture of Trichodesmium. The proposed research is exploratory in that the mechanism of modification of the Fe protein in Trichodesmium is not yet known. However, if an antiserum can be made that distinguishes between the active and inactive forms of the Fe protein, an immunoassay can be developed for estimating nitrogen fixation activities, without the need for sample containment and incubation. This immunoassay approach will make it possible to obtain estimates of N2 fixation in Trichodesmium over large spatial and temporal scales, and is one of the first uses of an immunoassay approach for estimating rates of biogeochemical cycles in the sea. This project will provide post doctoral training in ocean sciences, as well as provide for the development of a new application of immunological approaches in the marine sciences.