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

• Published in: Scientific Reports
• Main Authors: Regina C. Zweng, Marguerite S. Koch, George Bowes
• Format: Article in Journal/Newspaper
• Language: English
• Published: Nature Portfolio 2018
• Subjects: Medicine; R; Science; Q; Ocean acidification
• Online Access: https://doi.org/10.1038/s41598-018-27333-0; https://doaj.org/article/3cd3fc1524654eea960cb9c430cc121f

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 μmol photon m−2 s−1), pH levels (7.5–8.5) and CO2 concentrations (3–43 μmol kg−1). Species-specific CCM strategies were assessed using inhibitors and δ13C 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.