The role of irradiance and C-use strategies in tropical macroalgae photosynthetic response to ocean acidification.

Fleshy macroalgae may increase photosynthesis with greater CO availability under ocean acidification (OA) and outcompete calcifying macroalgae important for tropical reef accretion. Macroalgae use energy-dependent carbon concentrating mechanisms (CCMs) to take up HCO, the dominant inorganic carbon for marine photosynthesis, but carbon-use strategies may depend on the pCO, pH and irradiance. We examined photosynthesis in eight tropical macroalgae across a range of irradiances (0-1200 μmol photon m s), pH levels (7.5-8.5) and CO concentrations (3-43 μmol kg). Species-specific CCM strategies were assessed using inhibitors and δC isotope signatures. Our results indicate that the log of irradiance is a predictor of the photosynthetic response to elevated pCO (R > 0.95). All species utilized HCO, exhibited diverse C-use pathways and demonstrated facultative HCO use. All fleshy species had positive photosynthetic responses to OA, in contrast to a split amongst calcifiers. We suggest that shifts in photosynthetically-driven tropical macroalgal changes due to OA will most likely occur in moderate to high-irradiance environments when CCMs are ineffective at meeting the C-demands of photosynthesis. Further, facultative use of HCO allows greater access to CO for photosynthesis under OA conditions, particularly amongst fleshy macroalgae, which could contribute to enhance fleshy species dominance over calcifiers.