Dissertation Abstract
Nitrate assimilation by eukaryotic phytoplankton as a central characteristic of ocean productivity
Fawcett, Sarah E 2012
Department of Geosciences, Princeton University (United States), 290 pp.
Biologically available nitrogen (N) is essential for phytoplankton growth, and its availability can limit marine productivity and determine community composition. N is supplied to the upper ocean as “new” N (nitrate mixed up from depth, augmented by N2 fixation) or “recycled” N (ammonium and organic N recycled in surface waters). Annually, production fueled by new N is balanced by the sinking of organic matter from surface waters, maintaining the sequestration of CO2 in the deep ocean (the “biological pump”). The contributions of different phytoplankton groups to production and carbon export are poorly constrained; identifying the N sources supporting these groups, and understanding their dynamics and physical controls, will greatly improve those constraints.
The subtropical ocean is characterized by intense surface stratification that limits the upward supply of nitrate such that recycled N is assumed to fuel most phytoplankton growth. Here, I describe a new approach for characterizing the N sources to taxonomically-distinct components of particulate N suspended in surface waters: coupling flow cytometry with high sensitivity N isotope analyses. Applying this method to particles in the Sargasso Sea euphotic zone shows that Prochlorococcus and Synechococcus are always low in d15N, indicating reliance on recycled N throughout the year.
By contrast, in the summer, the d15N of the less abundant eukaryotic phytoplankton is consistently higher than that of the prokaryotes, reflecting consumption of subsurface nitrate despite extremely low surface concentrations. This high eukaryote d15N implies that sinking material derives largely from eukaryotic phytoplankton, and suggests that the eukaryotic contribution to carbon sequestration in the ocean interior is substantially greater than their contribution to net primary production; thus, eukaryotic phytoplankton appear to drive the subtropical ocean’s biological pump.
Eukaryote d15N declines into the fall/winter, suggesting a switch toward more complete reliance on recycled N, which appears to be driven by the density structure of the upper water column. In the summer, the very shallow low-density surface mixed layer (ML) does not contribute to stratification at the base of the euphotic zone, and subsurface nitrate can mix up into the photosynthetically-active layer. Fall ML deepening, typically taken as indicating weaker overall stratification, actually strengthens the density-driven isolation of the euphotic zone, reducing the upward supply of nitrate. With global warming, the surface stratification characteristic of the subtropics is expected to strengthen and expand. Low-latitude productivity is predicted to decline with increased stratification; however, depending on the depth structure of this increase, this work suggests that productivity throughout the euphotic zone may remain stable or even rise.
In the spring, vertical mixing erodes surface stratification, entraining nitrate into the euphotic zone to be consumed by eukaryotic phytoplankton. Annually, therefore, eukaryotes dominate both new and export production in the Sargasso Sea. Eukaryotes are also more readily removed to the deep ocean where their biomass is remineralized to nitrate. The dominant N source assimilated by eukaryotes is thus biased towards the form of N resulting from the their remineralization, raising the possibility that eukaryotic N uptake strategy is partially guided by the fate of eukaryotic biomass.
The subtropical ocean is characterized by intense surface stratification that limits the upward supply of nitrate such that recycled N is assumed to fuel most phytoplankton growth. Here, I describe a new approach for characterizing the N sources to taxonomically-distinct components of particulate N suspended in surface waters: coupling flow cytometry with high sensitivity N isotope analyses. Applying this method to particles in the Sargasso Sea euphotic zone shows that Prochlorococcus and Synechococcus are always low in d15N, indicating reliance on recycled N throughout the year.
By contrast, in the summer, the d15N of the less abundant eukaryotic phytoplankton is consistently higher than that of the prokaryotes, reflecting consumption of subsurface nitrate despite extremely low surface concentrations. This high eukaryote d15N implies that sinking material derives largely from eukaryotic phytoplankton, and suggests that the eukaryotic contribution to carbon sequestration in the ocean interior is substantially greater than their contribution to net primary production; thus, eukaryotic phytoplankton appear to drive the subtropical ocean’s biological pump.
Eukaryote d15N declines into the fall/winter, suggesting a switch toward more complete reliance on recycled N, which appears to be driven by the density structure of the upper water column. In the summer, the very shallow low-density surface mixed layer (ML) does not contribute to stratification at the base of the euphotic zone, and subsurface nitrate can mix up into the photosynthetically-active layer. Fall ML deepening, typically taken as indicating weaker overall stratification, actually strengthens the density-driven isolation of the euphotic zone, reducing the upward supply of nitrate. With global warming, the surface stratification characteristic of the subtropics is expected to strengthen and expand. Low-latitude productivity is predicted to decline with increased stratification; however, depending on the depth structure of this increase, this work suggests that productivity throughout the euphotic zone may remain stable or even rise.
In the spring, vertical mixing erodes surface stratification, entraining nitrate into the euphotic zone to be consumed by eukaryotic phytoplankton. Annually, therefore, eukaryotes dominate both new and export production in the Sargasso Sea. Eukaryotes are also more readily removed to the deep ocean where their biomass is remineralized to nitrate. The dominant N source assimilated by eukaryotes is thus biased towards the form of N resulting from the their remineralization, raising the possibility that eukaryotic N uptake strategy is partially guided by the fate of eukaryotic biomass.