Award: OCE-1130748

Intellectual Merit: There is evidence to suggest that ocean acidification, the result of anthropogenic increases in atmospheric carbon dioxide, will alter the composition of the marine phytoplankton assemblages that are the foundation of ocean productivity. These changes in community composition, along with acidification related metabolic stress, have the potential to significantly alter the synthesis of lipids and essential fatty acids, negatively affecting planktonic food quality that support marine fisheries. Of particular concern are the effects of ocean acidification on the production of essential fatty acids; organic constituents required by all higher trophic levels, including humans, but produced only by the phytoplankton. The central question is whether ocean acidification by end-of-century and beyond time scales will be sufficient to decrease the nutritional support for higher trophic levels, leading to lower overall marine productivity, less fisheries production, and sustainability of the ocean system. There are at least three mechanisms by which ocean acidification that may affect the lipid and essential fatty acid composition of marine phytoplankton communities. First, acidification increases carbon availability for photosynthesis by increasing free CO2. This change has been shown to alter interspecies competition among phytoplankton, so the composition of future phytoplankton assemblages is anticipated to differ from present. Phytoplankton species differ in they lipid and fatty acid composition, so these community changes are likely to alter the availability of these materials. Second, decreasing ocean pH will affect the availability of the micronutrient iron; a critical constituent for all living systems. This change in availability relates to changes in the strength of complexation of iron by organic molecules in seawater; a complexation that makes iron more difficult to access by phytoplankton. Like carbon, these changes in iron availability will affect interspecies competition, and the composition and abundance of phytoplankton assemblages. The third mechanism, pH changes in nitrogen assimilation, links the first two mechanisms because changes in carbon and iron availability affect nitrogen utilization. This factor is particularly important because nitrogen comprises a substantial portion of phytoplankton biomass, and has an important influence on the production of lipids and essential fatty acids. The project goals were to investigate the mechanisms underlying these potential changes using well-controlled laboratory experiments and field experiments in eastern boundary current upwelling waters off California that are representative of the most productive fisheries regions on earth. The project studied the effects of nutrient availability (of both nitrogen and the essential trace element iron) on the cellular lipid compositions. Our data analysis currently is still underway so we are in the process of compiling our findings. A major outcome so far is the demonstration that decreasing pH in surface waters originating from the productive California coastal upwelling zone acts to increase the availability of dissolved iron for phytoplankton. This finding is important because these eastern boundary regions are the most productive areas of the oceans. The implication is that ocean acidification will not negatively affect fisheries production through decreasing iron availability. The finding also is in contrast with earlier findings that decreasing pH negatively affects iron availability in well stratified, nutrient poor oceanic waters. The differences in these findings likely is related to differences in the dominant iron-complexing ligands, which will be important for consideration in the development of models to forecast the effects of climate change in the future oceans. Broader Impacts: As a result of this award, two (2) San Francisco State University (SFSU) and two (2) Western University (WU) gra...