Abstract
Although seagrasses and marine macroalgae (macro‐autotrophs) play critical ecological roles in reef, lagoon, coastal and open‐water ecosystems, their response to ocean acidification ( OA ) and climate change is not well understood. In this review, we examine marine macro‐autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [ DIC ], carbon dioxide [ CO 2 ], and lower carbonate [ CO 3 2− ] and pH . We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [ CO 2 ]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro‐autotroph photosynthesis is overwhelmingly C 3 (≥ 85%) with most species capable of utilizing HCO 3 − ; however, most are not saturated at current ocean [ DIC ]. These results, and the presence of CO 2 ‐only users, lead us to conclude that photosynthetic and growth rates of marine macro‐autotrophs are likely to increase under elevated [ CO 2 ] similar to terrestrial C 3 species. In the tropics, many species live close to their thermal limits and will have to up‐regulate stress‐response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [ CO 2 ] effects on thermal acclimation are unknown. Fundamental linkages between elevated [ CO 2 ] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [ CO 2 ] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H + and DIC . These fluxes control micro‐environments that promote calcification over dissolution and may be more important than CaCO 3 mineralogy in predicting macroalgal responses to OA . Calcareous macroalgae are highly vulnerable to OA , and it is likely that fleshy macroalgae will dominate in a higher CO 2 ocean; therefore, it is critical to elucidate the research gaps identified in this review.