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Marine macroalgae have variable carbon-uptake strategies that complicate predicting responses to environmental changes. In seawater, dissolved inorganic carbon availability can affect the underlying physiological mechanisms influencing carbon uptake. We tested the interactive effects of ocean acidification (OA) and warming on two HCO3−-users (Lobophora sp. and Amansia rhodantha), a predominately CO2-user (Avrainvillea nigricans), and a sole CO2-user (Plocamium hamatum) in the Great Barrier Reef, Australia. We examined metabolic rates, growth, and carbon isotope values (δ13C) in algae at 26, 28, or 30°C under ambient or elevated pCO2 (∼1000 µatm). Under OA, δ13C values for the HCO3−-users decreased, indicating less reliance on HCO3−, while δ13C values for CO2-users were unaffected. Both HCO3−-users decreased in growth across temperatures under ambient pCO2, but this negative effect was alleviated by OA at 30°C. A. nigricans lost biomass across all treatments and P. hamatum was most sensitive, with reduced survival in all physiological responses. Metabolic rates varied greatly to interacting temperature and OA and indicated a decoupling between the relationship of photosynthesis and growth. Furthermore, our findings suggest HCO3−-users are more responsive to future CO2 changes, and highlight examining carbon physiology to infer potential responses to interacting environmental stressors.

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