Collaborative Research: Evaluating the contribution of small eukaryotes to nitrate-based new production in the North Pacific Subtropical Gyre

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The subtropical oceans in the middle latitudes are “ocean deserts,” where there are relatively little nutrients in the surface waters. Nitrogen is an important nutrient that is particularly scarce. Nitrogen in the ocean occurs as nitrate or ammonium. As a result of limited nutrients in the subtropical gyres, marine algae or phytoplankton there grow slowly. In deeper waters below the surface, there are more nutrients. However, it is not well understood how phytoplankton living in surface waters in summer can access the nutrients in the deeper waters. This award will investigate how phytoplankton access subsurface nitrogen at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). Phytoplankton will be sorted by size and pigment composition. The researchers will make measurements of the stable nitrogen isotope ratios of sorted populations and determine where phytoplankton are getting the needed nutrients, either nitrate from the subsurface or ammonium from the surface waters. The researchers will use metatranscriptomic analysis to investigate the physiological pathways that different phytoplankton populations use to take up the needed nitrogen. The metatranscriptomic analysis involves analyzing the RNA sequences transcribed by plankton, comparing the sequence to documented gene sequences and mapped to documented physiological pathways. The researchers will assess which taxonomic groups are using nitrate versus recycled sources of nitrogen. They will test the hypothesis that eukaryotic phytoplankton primarily use nitrate whereas prokaryotic phytoplankton mostly use recycled sources of nitrogen. With these measurements, the researchers will help the society anticipate climate impacts on the productivity of subtropical gyres. This project will support two graduate students and a post-doctoral researcher in laboratory and ship-based research. Principal Investigator (PI) Granger in Connecticut will engage high school students and sponsor graduate students from underrepresented groups via AGU’s Bridge Partner Program. PI Marchetti in North Carolina will sponsor the internship of a high school student from a local minority county. PI White in Hawaii will engage with Hawaiian high school students though the Indigenous Partnership for Ocean Monitoring program to mentor and teach essential research skills. The researchers of this project will investigate the drivers of productivity at station ALOHA in NPSG. They will exploit a high-sensitivity method for natural abundance N isotope analysis (the ‘persulphate-denitrifier’ method) to evaluate the extent to which taxonomically distinct components of the plankton sorted by flow cytometry rely on nitrate vs. reduced N sources in surface waters. We will also query physiological nitrogen pathways of the plankton with metatranscriptomic analysis to infer which clades are reliant on nitrate. We will quantify the fraction of primary production fueled by nitrate to characterize seasonal trends and explore potential mechanisms of nitrate supply to the euphotic zone and its mixed layer. The work proposed here will resolve the relative contribution of nitrate mixed from the ocean interior to surface production at station ALOHA, to better define seasonal dynamics of the biological pump in the NPSG. We will identify plankton groups that contribute dominantly to nitrate-based production and resolve eco-physiological strategies that confer fitness in a severely N-limited environment. Seasonal trends in the contributions of nitrate to total nitrogen production will be evaluated considering incident hydrography to infer potential mechanisms of nitrate supply.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.