Consequences of warming and acidification for the temperate articulated coralline alga, Calliarthron tuberculosum (Florideophyceae, Rhodophyta)

Global climate changes, such as warming and ocean acidification (OA), are likely to negatively impact calcifying marine taxa. Abundant and ecologically important coralline algae may be particularly susceptible to OA; however, multi‐stressor studies and those on articulated morphotypes are lacking. Here, we use field observations and laboratory experiments to elucidate the impacts of warming and acidification on growth, calcification, mineralogy, and photophysiology of the temperate articulated coralline alga, Calliarthron tuberculosum. We conducted a 4‐week fully factorial mesocosm experiment exposing individuals from a southern CA kelp forest to current and future temperature and pH/pCO(2) conditions (+2°C, −0.5 pH units). Calcification was reduced under warming (70%) and further reduced by high pCO(2) or high pCO(2) x warming (~150%). Growth (change in linear extension and surface area) was reduced by warming (40% and 50%, respectively), high pCO(2) (20% and 40%, respectively), and high pCO(2) x warming (50% and 75%, respectively). The maximum photosynthetic rate (P(max)) increased by 100% under high pCO(2) conditions, but we did not detect an effect of pCO(2) or warming on photosynthetic efficiency (α). We also did not detect the effect of warming or pCO(2) on mineralogy. However, variation in Mg incorporation in cell walls of different cell types (i.e., higher mol % Mg in cortical vs. medullary) was documented for the first time in this species. These results support findings from a growing body of literature suggesting that coralline algae are often more negatively impacted by warming than OA, with the potential for antagonistic effects when factors are combined.