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.
Understanding phytoplankton photosynthetic energy allocation strategies is needed to meaningfully connect environmental factors and expressions of cell growth obtained either from field and remote sensing measurements or genome-based sequencing efforts. Results from this grant show that unifying properties of energy use govern photosynthetic energy allocation into major metabolic pathways such as respiration, lipid biosynthesis, and carbon storage. We found that these cell behaviors are shared across broad phylogenetic groups including a green alga, a diatom, and a prasinophyte. However, a very different energy allocation strategy was observed in a motile prasinophyte. This new strategy may be a consequence of a physiological trait (e.g., motility, in this case) that evolved to meet the demands of this species' environmental niche and changed the distribution of photosynthetic energy among the cell's metabolic pathways. This research identified three different photosynthetic energy allocation strategies that are associated with environmental factors and physiological traits. These patterns will provide the foundations for a physiological framework for interpreting variability in measurements of photosynthetic activity and give strong mechanistic bases and physiological constraints for new models cell growth. Five undergraduate students, including one with physical disabilities and one female Hispanic student received training in culturing techniques and measurements of photosynthesis. One graduate student was trained through this grant, and she will complete her MS in 2015 with at least one first-author publication expected. A post-doctoral researcher participated in training and mentoring of students and has expanded this research project to include molecular analyses. Multiple presentations at university, national, and international meetings were made. One high school student was an intern through a pre-college program. She presented her findings at a meeting of her peers at the University of Portland. This project also initiated an international collaboration through a Czech-USA cooperative. This collaboration included training of two Czech graduate students and one post-doctoral researcher in different measurements of photosynthetic activity. Three peer reviewed journal articles have already been published, and two additional articles are forthcoming.
