Award: EF-1041034

Project Outcomes: Collaborative Research: Ocean Acidification Category 1: Effects of pCO2 and pH on Photosynthesis, Respiration, and Growth in Marine Phytoplankton As CO2 is released into the atmosphere through human activities a portion of this CO2 dissolves in the ocean, increasing the oceanÆs CO2 concentration and lowering its pH. The chemical changes associated with increasing CO2 concentrations are known as ocean acidification and these chemical changes are expected to effect many organisms and ecosystems in the ocean. This project investigated the physiological effects of ocean acidification on marine phytoplankton, the unicellular microbes that are responsible for nearly all photosynthesis in the ocean. Our overarching goal of this collaborative project was to identify energetic savings due to alterations in photosynthesis or respiration that allow some phytoplankton to grow at more rapid rates when oceanic CO2 concentrations increase. The specific aim of the work carried out at the University of Georgia was to investigate the role of the CO2 concentrating mechanism (CCM) in mediating responses to ocean acidification. The CCM is a system used by microalgae to increase the concentration of CO2 around the photosynthetic enzyme responsible for carbon fixation, RubisCO, so that it is working rapidly and does not inadvertently react with O2 instead. In many algae, the CCM is dramatically down-regulated at high CO2. Energetic savings from decreased activity of the CCM is the most commonly offered explanation for increases in phytoplankton growth rates at high CO2, but exactly how much energy is saved, and hence whether this is a plausible explanation, was unclear. We intensively studied the CCMs in species that represent two key groups of marine phytoplankton: Phaeodactylum tricornutum, from the diatoms, a group of phytoplankton that dominates the community in highly productive regions, and Prochlorococcus marinus, from the picocyanobacteria, a group that dominates in low productivity regions of the ocean. We showed that the CCM of the diatom is driven by active transport of carbon from the cytoplasm into the chloroplast, building up a pool of carbon in the chloroplast to elevate CO2 around RubisCO and depleting inorganic carbon in the cytoplasm, which draws CO2 into the cell. This work combined physiological characterization of the CCM, using new ways to interpret isotopic tracers, and molecular identification of genes involved in the CCM. Through our better understanding of the CCM in this diatom, we accurately estimated that the down-regulation of the CCM should lead to energetic savings that will allow the diatom to grow 5-10% faster at CO2 concentrations predicted for year 2100. However, similar characterization of the CCM in the picocyanobacterium revealed that it was not down-regulated at predicted year 2100 CO2 concentrations because the CCM must raise the CO2 concentration around RubisCO to very high concentrations in cyanobacteria. Consistent with this finding, the growth rate of Prochlorococcus marinus, did not increase at year 2100 CO2. Commonalities between CCM physiology in these two species and other model microalgae led to the proposal of a general framework to predict the energetic savings from decreased CCM activity at high CO2. The framework relates the changes RubisCO concentration and changes in the CO2 gradient between the site of elevation inside the cell and the environment to energetic savings from the CCM. This framework will help to identify instances when the CCM is a key component of the response of phytoplankton to ocean acidification and when other physiological processes must be involved. This project improved our understanding of the response of two key groups of marine phytoplankton to rising CO2 and led to the proposal of a general framework for estimating energetic savings from the CCM. It also supported the acquisition of a ...