Collaborative Research: Dynamics of dissolved organic phosphorus production, composition and bioavailability along a natural marine phosphate gradient

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Phytoplankton, also known as primary producers, are microscopic floating plants at the base of the marine food web. As photosynthetic organisms, phytoplankton directly take up dissolved inorganic carbon (DIC) to synthesize their tissues using energy from the sun. In addition to carbon (C), essential nutrients such as phosphorus (P) and nitrogen (N) are required by these primary producers. While dissolved inorganic phosphorus (DIP) is the preferred form of P because it is a small enough molecule that phytoplankton can directly absorb it, in the sunlit surface waters of the ocean the concentration of DIP can be drawn down to limiting levels. In this situation, the larger dissolved organic phosphorus (DOP) molecules can become available through enzyme hydrolysis reactions, which convert DOP to DIP. There was mounting evidence that DOP can play a crucial role in supporting primary production in the ocean, yet very little is known about the nature of the DOP pool. There are many analytical impediments to gaining information about the chemical structure and composition of DOP, and without knowledge of its composition there is no basis for evaluating the potential bioavailability of DOP to primary producers. The proposed work will employ a unique combination of methods to gain novel insight into the composition and bioavailability of DOP. Field and laboratory (culture) nutrient addition incubation experiments will target the drivers of DOP variability, information crucial to resolving models of primary production in the ocean. Results of this work will be transformative for understanding DOP composition, DOP variability in space and time, and microbial control on the nature of the marine DOP pool. This proposal will support a postdoctoral scholar, two undergraduate students and two undergraduate students. Recruitment of undergraduate students will focus on entraining Native Americans and Pacific Islanders, through SACNAS (Society for Advancement of Chicanos/Hispanics and Native Americans in Science) and related venues. Results will be published in peer-reviewed journals and presented at national and international meetings. We also propose to conduct outreach to middle schools, including bringing The Artistic Oceanographer Program, an interactive inquiry-based program targeting middle school age science and art standards by integrating concepts in ocean science literacy with art, to local middle schools in New York and Hawaii.The proposed work will provide foundational information on the way DOP molecular characteristics translate into P-bioavailability to marine microorganisms, and in turn how microorganisms growing under different dissolved inorganic nitrogen: dissolved inorganic phosphorus (DIN:DIP) impact the composition and bioavailability of DOP. A newly developed sequential ultrafiltration (SUF) method will be applied to samples collected along a natural P-gradient in the western North Atlantic. The SUF method quantitatively segregates and concentrates DOP into 4 molecular weight size classes, which can be subjected to bioavailability assays using phosphohydrolytic enzymes, and to liquid chromotography-mass spectrometry to provide detailed compositional information. Using these combined methods we will probe in situ DOP, the evolution of in situ DOP during shipboard incubations under different DIN:DIP, and the composition and bioavailability of DOP produced by organisms isolated from key stations along the natural phosphate gradient. Contrast of in situ patterns to those developed in controlled culture experiments using field isolates will be achieved by identifying cellular shifts in organic phosphorus biosynthesis pathways, and tracing how such shifts impact DOP composition. The combined methodological approach will provide unparalleled insight into DOP composition and bioavailability, addressing this major knowledge gap. Results of this work will be transformative for understanding DOP composition, DOP variability in space and time, and microbial control on the nature of the marine DOP pool. Such information is a prerequisite to building ecosystem models that capture the influence of P biogeochemistry on primary production and carbon cycling in aquatic systems.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.