What Regulates Assimilation of Inorganic Nitrogen by Heterotrophic Bacteria vs. Phytoplankton in the Euphotic Zone

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Although it is well known that both phytoplankton and heterotrophic bacterioplankton assimilate ammonium in the euphotic zone, the controls on bacterial uptake are unclear. Little is known about factors controlling the great variation in relative ammonium uptake by bacterioplankton (10- 60% of total ammonium uptake) and nothing about nitrate uptake by bacteria. Nitrogen uptake by heterotrophic bacteria can greatly impact our understanding of new versus regenerated production, which has emerged as an important variable in global carbon and nitrogen models. In the subarctic Pacific, for example, about 50% of the new production can be explained by changes in nitrogen use by heterotrophic bacteria. In order to understand dissolved inorganic nitrogen uptake (DIN . which includes ammonium, nitrate, and nitrite), this research project will examine uptake by both phytoplankton and bacteria in relationship to biomass, biomass production, and dissolved organic matter uptake. Improved methodologies will be utilized to examine algal and bacterial dissolved inorganic nitrogen (DIN) uptake and the researchers will carry out the project in collaboration with ongoing process oriented studies in Oregon coastal waters, particularly the California Current system off Oregon. Previous results indicate that an extensive range in nutrient concentrations, biomass, and production will be encountered. As a result of this variation, the researchers expect to observe and eventually understand changes in DIN uptake by bacteria and to provide insight into how the activity of both bacteria and phytoplankton should be incorporated into realistic models of nitrogen cycling and carbon cycling in the euphotic zone.