Predicting Effects of Ocean Acidification and Warming on Algae Lacking Carbon Concentrating Mechanisms

Seaweeds that lack carbon-concentrating mechanisms are potentially inorganic carbon-limited under current air equilibrium conditions. To estimate effects of increased atmospheric carbon dioxide concentration and ocean acidification on photosynthetic rates, we modeled rates of photosynthesis in response to pCO(2), temperature, and their interaction under limiting and saturating photon flux densities. We synthesized the available data for photosynthetic responses of red seaweeds lacking carbon-concentrating mechanisms to light and temperature. The model was parameterized with published data and known carbonate system dynamics. The model predicts that direction and magnitude of response to pCO(2) and temperature, depend on photon flux density. At sub-saturating light intensities, photosynthetic rates are predicted to be low and respond positively to increasing pCO(2), and negatively to increasing temperature. Consequently, pCO(2) and temperature are predicted to interact antagonistically to influence photosynthetic rates at low PFD. The model predicts that pCO(2) will have a much larger effect than temperature at sub-saturating light intensities. However, photosynthetic rates under low light will not increase proportionately as pCO(2) in seawater continues to rise. In the range of light saturation (I-k), both CO2 and temperature have positive effects on photosynthetic rate and correspondingly strong predicted synergistic effects. At saturating light intensities, the response of photosynthetic rates to increasing pCO(2) approaches linearity, but the model also predicts increased importance of thermal over pCO(2) effects, with effects acting additively. Increasing boundary layer thickness decreased the effect of added pCO(2) and, for very thick boundary layers, overwhelmed the effect of temperature on photosynthetic rates. The maximum photosynthetic rates of strictly CO2-using algae are low, so even large percentage increases in rates with climate change will not contribute much to changing primary production in the habitats where they commonly live.