Physiological responses to temperature and ocean acidification in tropical fleshy macroalgae with varying affinities for inorganic carbon

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 (delta C-13) in algae at 26, 28, or 30 degrees C under ambient or elevated pCO(2) (1000 mu atm). Under OA, delta C-13 values for the HCO3--users decreased, indicating less reliance on HCO, while (delta C-13 values for CO2-users were unaffected. Both HCO3--users decreased in growth across temperatures under ambient pCO(2), but this negative effect was alleviated by OA at 30 degrees 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.

Automated summary

Marine macroalgae have variable responses to ocean acidification and warming, with HCO3--users showing more sensitivity to future CO2 changes. The study found that environmental stressors affect carbon uptake and growth of algae, with a decoupling between photosynthesis and growth observed under interacting temperature and OA conditions.