Photosynthetic Responses of Temperate Macroalgae to Ocean Acidification

Marine macroalgae dominate intertidal and subtidal rocky coasts and reefs, where they provide food (primary production), form habitat structure, and influence local environmental factors including light, nutrients, water movement, and intertidally, temperature and moisture. It follows that macroalgal responses to climate change are thus likely to affect the structure and function of coastal communities. Macroalgae employ a variety of Carbon Concentrating Mechanisms (CCMs) that enable the use of bicarbonate as a source of carbon for photosynthesis. As macroalgae are extremely ecologically, physiologically, and phylogenetically diverse, there exists a great variety of CCM machinery, even among closely related species. These pathways may differ in energetic investment and response to small changes in the relative concentrations of inorganic carbon species in seawater, specifically carbon dioxide and bicarbonate. Here, we identify changes in CCM pathways linked to changes in phototysnthetic rates responding to laboratory acidification in the temperate red macroalgae Corallina elongata, Mastocarpus stellatus, and Osmundea pinnatifida. Further, the light intensity at which the maximal photosynthetic rate occurs shifts after laboratory acidification and exhibits species-specific response. Differential responses of common, co-occurring macroalgae have obvious implications for carbon use and cycling in the intertidal and shallow subtidal coastal zones.