Award: OCE-1241093

Oligotrophic phytoplankton community response to changes in N substrates and the resulting impact on genetic, taxonomic and functional diversity Marine microorganisms (microbes) are a diverse group of organisms that include phytoplankton and bacteria. Phytoplankton (small, single-celled plant-like organisms) use the sun?s energy to carry out photosynthesis and are the base of the marine food web. Bacteria act as decomposers and nutrient recyclers and are primarily responsible for regenerating nutrients and carbon back into the marine system. Growth of marine microorganisms is dependent on the availability of resources such as light and nutrients (e.g. nitrogen, iron, phosphorus). Nitrogen availability is scarce through large regions of the ocean, such as the subtropical ocean gyres, and can thus limit phytoplankton growth. Phytoplankton are diverse and their requirements and capabilities to take up and utilize different nitrogen compounds, such as nitrate, ammonium, urea, or amino acids widely differs. Thus, the supply of different nitrogen compounds, and competition for it among phytoplankton, are important controls of phytoplankton growth and productivity, and ultimately the function of marine systems. Human activities and environmental changes are altering the supply and form of nitrogen compounds available to phytoplankton and thus it is important to have a baseline understanding of how different marine microorganisms grow in the presence of different nitrogen compounds. The primary focus of this project is to understand how different nitrogen forms impact the marine phytoplankton community. The results of this project will be crucial to assessing how ocean ecosystems will respond to global climate change. Our project investigated the effect of nitrate, ammonium and urea on marine microbial community composition in the waters of the North Pacific Ocean between San Diego and Hawaii. We conducted multiple experiments in bottles where seawater was enriched with different nitrogen compounds (nitrate, ammonium or urea). Our main findings were that: Across all experiments ammonium and nitrate resulted in greater increases of phytoplankton biomass than urea. There was little difference observed in the efficiency with which the phytoplankton used the sunlight for photosynthesis between the different nitrogen compounds. In the Eastern North Pacific Ocean, the Cyanobacterial genus Prochlorococcus showed the greatest growth response to urea while the genus Synechococcus had the least growth when exposed to ammonium enrichments. The bacterial community composition changed most strongly in response to ammonium. The time and magnitude of responses to different nitrogen compounds varied with location in the Pacific, likely due to initial differences in microbial community composition and their nutrient status. Deeper waters containing lots of nitrate may be injected into surface waters by internal waves that are generated near Hawaii and travel northeastward through the North Pacific. Internal waves are generated and travel in the interior of the water column (i.e. below the ocean?s surface). The input of nitrate to the surface waters by internal waves can result in annual phytoplankton blooms during the late summer and fall. The results of our project provide support for the hypothesis that changes in nitrogen supply would likely favor specific populations of phytoplankton in different oceanic regions Currently the Arrigo team has made five presentations at scientific meetings and has one paper in press and three in preparation. This work has supported training of three male undergraduate students and 1 under-represented minority female undergraduate student in laboratory and/or field procedures. Last Modified: 01/04/2017 Submitted by: Kevin R Arrigo