Abstract
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 pCO2, 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 pCO2 and temperature, depend on photon flux density. At sub-saturating light intensities, photosynthetic rates are predicted to be low and respond positively to increasing pCO2, and negatively to increasing temperature. Consequently, pCO2 and temperature are predicted to interact antagonistically to influence photosynthetic rates at low PFD. The model predicts that pCO2 will have a much larger effect than temperature at sub-saturating light intensities. However, photosynthetic rates under low light will not increase proportionately as pCO2 in seawater continues to rise. In the range of light saturation (Ik), 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 pCO2 approaches linearity, but the model also predicts increased importance of thermal over pCO2 effects, with effects acting additively. Increasing boundary layer thickness decreased the effect of added pCO2 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.
Highlights
Continued absorption of anthropogenic emissions of CO2 from burning fossil fuels, into seawater will inevitably lead to further declines in oceanic pH with predictable consequences for oceanic chemistry [1,2]
There are many important questions about these organisms in need of resolution to better predict the consequences of climate change to oceanic ecosystems, such as, “will, and if so, by how much will algal productivity be enhanced in acidified coastal water?”, “will productivity be determined by changing pCO2 or temperature?”, or “how will the mode of inorganic carbon acquisition interact with other determinants of productivity as pCO2 increases?”
This negative quadratic term approaches 0, and the linear coefficient increases, for CO2 as light intensity increases to saturating values indicating a reduced curvature and greater linearity of the response of photosynthetic rates to increasing pCO2
Summary
Continued absorption of anthropogenic emissions of CO2 from burning fossil fuels, into seawater will inevitably lead to further declines in oceanic pH with predictable consequences for oceanic chemistry [1,2]. Much attention has focused on calcifying organisms that are predicted to be vulnerable to ocean acidification as a result of lower saturation states of calcium carbonate species as reviewed in [7,8,9]. Non-calcifying phototrophs (e.g., macroalgae and seagrasses) are predicted to benefit in terms of growth, if not always in terms of photosynthetic rate, from ocean acidification (OA) due to the enhanced availability of dissolved CO2 in the ocean. There are many important questions about these organisms in need of resolution to better predict the consequences of climate change to oceanic ecosystems, such as, “will, and if so, by how much will algal productivity be enhanced in acidified coastal water?”, “will productivity be determined by changing pCO2 or temperature?”, or “how will the mode of inorganic carbon acquisition interact with other determinants of productivity as pCO2 increases?”
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