Date Published: July 14, 2015
Publisher: Public Library of Science
Author(s): Janet E. Kübler, Steven R. Dudgeon, Wei-Chun Chin.
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.
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]. Changing ocean chemistry is expected to shift ratios of resources as well as environmental conditions, thereby favoring some groups of organisms at the expense of others [3–6]. 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 [7–9]. 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.
Modeled rates of photosynthesis varied in response to changing pCO2, changing temperature, and with the interaction of the two. The direction and magnitude of the predicted response to CO2 and temperature, however, changed with light intensity (Fig 1). At sub-saturating light intensities, photosynthetic rates are predicted to be low and respond positively to increasing pCO2, and negatively to increasing temperature (Fig 1A). Consequently, CO2 and temperature interacted antagonistically to influence photosynthetic rates. Standardized regression coefficients at low light intensities predict that CO2 should have a much larger effect than temperature in both linear and quadratic components (Fig 2). Photosynthetic rates should be most responsive to pCO2 under sub-saturating light among all of the combinations of light, temperature and CO2 simulated. The large (in absolute value) predicted negative quadratic effect of CO2 under low light reflects the expectation that photosynthetic rates will not increase proportionately as pCO2 in seawater continues to rise (Fig 2B). 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 (cf. CO2 in Fig 2A and 2B).
Concern about effects of climate change on marine life has spawned a rapidly increasing number of studies characterizing responses of algae to increasing CO2 and temperature in the oceans. Few of those studies include evaluation of the mechanism of inorganic carbon acquisition operating as interface between the organism and the external carbonate chemistry. Haphazard choices of fleshy algal species as experimental subjects in some cases, or the special concern about calcareous algae in other cases, and the experimental conditions to which they are subjected make an emergent synthesis difficult. Our motivation was to develop a quantitative model to predict the effects of increased CO2 on photosynthesis of strictly CO2 using (i.e., non-bicarbonate using) red macroalgae, which are estimated to comprise ~35% of all rhodophytes  and are over represented among macroalgae communities in deepwater, low transparency water and caves . A second model representing some bicarbonate-using species is in a forthcoming manuscript. We restricted our model to seawater salinities but it could be extended to include the strictly CO2 using red algae in fresh water (e.g., Lemanea spp.; [42–44, 48]). The CO2-user model is based on the known temperature and pH-dependent fractionation of carbon species in seawater , dynamics of diffusive uptake of CO2 in aquatic plants [25–27], and prior data on photosynthetic responses of CO2-using red seaweeds to temperature and light [12,15,16,18]. Such a model provides a tool to assess understanding of the relationships of photosynthesis to key environmental parameters, a valid framework upon which to base predictions of changes in primary production, distribution and abundance, and a basis from the bottom-up to inform hypotheses about community-level implications to climate change.