Award: OCE-1437458

The ocean is an important part of the earth's climate system. One aspect of this system is the biological pump—the growth, death, and eventual sinking of small algae (phytoplankton) from the surface to the deep ocean. One way of measuring the strength of the biological pump is by measuring the amount of nitrogen algae take up in the form of nitrate, and calculating how much growth, or carbon fixation, this nitrogen would support. This method is based on the hypothesis that nitrate is a "new" form of nitrogen to the surface ocean from the deep ocean, as opposed to nitrogen that is recycled internally within the upper ocean. Because what comes up must eventually go down, this "new production" paradigm is powerful because it allows an estimation of the amount of carbon sinking to the deep ocean by making a relatively simple measurement of nitrogen uptake. Our project, a collaboration between laboratories at Univ. of Southern California and UC Santa Barbara, showed that, in fact, nitrate is not always a "new" form of nitrogen. Our research measured rates of nitrification—a microbial process that produces nitrate—in the upper ocean approximately every month for two years in the coastal ocean off southern California. We found that up to 20% of the nitrate taken up by phytoplankton was not "new" but produced by nitrification. We also measured rates of primary production and nitrification supported by the organic nitrogen source, urea, two processes whose importance was not previously well understood. We found that urea usage can account for a large fraction of both the nitrogen demand of phytoplankton as well as of the nitrate produced by nitrification, which could transform our understanding of nitrogen dynamics in the upper ocean. Surprisingly, diatoms, important primary producers in coastal waters, accounted for much of the urea uptake. Microorganisms involved in nitrification are also critical to removing ammonia from aquarium and aquaculture operations. The results from this project help us to better understand their ecology and physiology. A second outcome of this award is an improved understanding of the production and transport of nitrous oxide in the ocean. Nitrous oxide is a greenhouse gas that is produced by microbes; its production in enhanced at low oxygen concentrations. Monthly measurements of nitrous oxide concentrations showed that northward flowing currents transport nitrous oxide from low-oxygen regions of the ocean off the coast of Mexico to the Southern California Bight where it can be upwelled to the surface and enter the atmosphere. These results show that regions of nitrous oxide production can be very far from where nitrous oxide ultimately reaches the atmosphere, and that studies of this gas should not be restricted to low-oxygen regions. This award trained three graduate students (one at UCSB and two at USC) and four technicians in analytical chemistry and oceanographic sampling and supported the research of an early career researcher. Several undergraduates were involved in the project in the two laboratories. Last Modified: 01/10/2020 Submitted by: Douglas G Capone