Geochemical constraints on the rates and fates of nitrogen fixation in the southwest Pacific

Texas A&M University, College Station TX

Investigators

Abstract

The base of the marine food web consists of phytoplankton, or microscopic plants that need to be fertilized by the elements nitrogen (N), phosphorus (P), and iron (Fe), to grow and sustain global fisheries. The ultimate source of N to the ocean, and thus to marine phytoplankton, is provided by marine microbes that carry out a process known as N fixation. While N is an essential component of phytoplankton biomass, we do not have good estimates of the locations, times, or magnitudes when N fixation occurs in the ocean. Without knowing where and how much N phytoplankton obtain from N fixation we cannot accurately predict the capacity of the ocean to support fish stocks and their ability to withstand environmental changes. The southwest Pacific Ocean has some of the highest rates of N fixation in the global ocean, but we don’t have a complete understanding of the fate of newly fixed N, or how it may change depending on which microbes carry out N fixation. This project seeks to track the fate of N from N fixation using geochemical tools in a collaboration with French scientists. French colleagues will collect samples to be sent to Texas A&M University for analysis. The French scientists will also share data that will be used to interpret the geochemical measurements made as part of this study. Together, this information will be used to understand the fate of newly fixed N. Specifically, the study will examine whether newly fixed N is retained in the surface ocean in a form that phytoplankton can use, or whether it sinks to the deep ocean to be recycled by microbes, and whether the fate changes depending on which microbes carry out N fixation. A graduate student, undergraduate student, and technician will be supported by this award and trained in cutting-edge biogeochemical techniques. The proposed work will analyze the dissolved organic nutrient concentration and the nitrogen and oxygen isotopic composition of dissolved nitrogen species in samples currently being collected by French colleagues in the tropical southwest Pacific. These data will be used to evaluate the rates and fates of biologically-mediated di-nitrogen (N2) fixation and specifically, the contribution of N2 fixation to export production using seasonal “delta 15N budgets”. Additionally, this project will evaluate whether newly fixed nitrogen (N) is accumulating in the dissolved organic nitrogen (DON) pool. Variation in the partitioning of newly fixed N between the sinking flux and the surface DON pool will be compared with diazotroph community composition evaluated by French colleagues. Finally, changes in the stoichiometry of dissolved organic nutrients (i.e., DON:DOP concentration ratios) will be used to further track their production and consumption in these regions where inorganic nutrients are scarce, and especially the importance of DOP for supporting N2 fixation. The results will be evaluated in the context of rich complementary data, including sediment trap and water column geochemical and molecular biological as well as physical oceanographic data, collected by French collaborators as part of their “HOPE” project. This collaboration represents significant cost savings to NSF as ship time, sample collection, and all complementary analyses will be funded by the French collaboration. 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.