EAGER: Metabolic mechanisms that align phytoplankton growth to the integrated growth environment

Oregon State University, Corvallis OR

Investigators

Abstract

Intellectual Merit: While remote sensing provides regular global views of surface phytoplankton chlorophyll levels (Chl), the more relevant property for ecosystem and biogeochemistry studies is net primary production (NPP). Field carbon assimilation measurements are commonly used to develop relationships between Chl and NPP, with the implicit assumption that the effects of light- and nutrient-stress are sufficiently well understood to allow assimilation to be equated with NPP. Recently, the PI conducted studies with Dunaliella tertiolecta demonstrating that Chl-specific NPP and GPP are invariant over a wide range of nutrient-limited growth rates, while short-term carbon assimilation rates vary dramatically (Halsey et al., 2010a). Her study employed a broad suite of measurement approaches that allowed an accounting of photosynthate utilization from the initial light harvesting reactions at PSII to the ultimate reductant sink of NPP. Results showed that, although carbon fixation consumes the vast majority of photosynthate, only a portion of the total CO2 fixed (gross carbon primary production, GPPC) is ultimately incorporated into biomass (NPP). The remaining, and very significant, portion of gross CO2 fixation was oxidized within the time span of a cell cycle. This temporary cellular carbon sink has been referred to as the 'transient carbon pool.' Nutrient-driven changes in metabolic pathways utilizing the transient carbon pool directly impact 14C-based measurements of photosynthesis, their relation to NPP, and their suitability for parameterizing global chlorophyll-based models of ocean production. The overarching objectives of this research project are to expand findings for D. tertiolecta to representative species of environmentally relevant groups, and to resolve how predominant metabolic pathways acting on the transient carbon pool are linked to cell cycle phase, diel cycle, and N-limited growth. This research project will consist of two activities. First, the experimental approach used for D. tertioleca will be repeated using five additional species. Second, key carbon metabolic pathways, that differentially influence the transient carbon pool depending on growth rate, will be linked to the cell cycle and phases of the diel cycle. The PIs will develop a process-based, physiological understanding of phytoplankton photosynthesis, its relationship to net growth, metabolic links between these attributes, and their responses to cell cycle and diel growth dynamics in varied nutrient conditions. This understanding is essential to decipher the physiological component of observed global changes in surface ocean Chl, advancing global assessments of ocean primary production, and predicting responses to climate variations. Broader Impacts: This project will give hands-on training for undergraduate interdisciplinary projects. The PIs will actively recruit undergraduate students through fellowship-supported programs such as the Howard Hughes Medical Institute fellowship program and the Subsurface Biosphere Initiative, SBI, to provide summer laboratory research experience. At the end of the two-year funding period, they will submit a more broadly encompassing proposal to the Biological Oceanography program that will include support for graduate student training. Furthermore, the PIs will develop presentations and activities for teachers that are based on research and understanding of related ocean science topics, emphasizing photophysiology.