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

ABSTRACT: Fleshy macroalgae may increase photosynthesis with greater CO₂ 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 HCO₃^-, the dominant inorganic carbon for marine photosynthesis, but carbon-use strategies may depend on the pCO₂, pH and irradiance. We examined photosynthesis in eight tropical macroalgae across a range of irradiances (0-1200??mol photon m^-2 s^-1), pH levels (7.5-8.5) and CO₂ concentrations (3-43??mol kg^-1). Species-specific CCM strategies were assessed using inhibitors and ?¹³C isotope signatures. Our results indicate that the log of irradiance is a predictor of the photosynthetic response to elevated pCO₂ (R²?>?0.95). All species utilized HCO₃^-, exhibited diverse C-use pathways and demonstrated facultative HCO₃^- 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 HCO₃^- allows greater access to CO₂ for photosynthesis under OA conditions, particularly amongst fleshy macroalgae, which could contribute to enhance fleshy species dominance over calcifiers.

SUBMITTER: Zweng RC 

PROVIDER: S-EPMC6013460 | biostudies-literature | 2018 Jun

REPOSITORIES: biostudies-literature