The Role of Carbon Concentrating Mechanisms in the Varied Response of the Green Macroalga, Ulva Lactuca, To Ocean Acidification

The majority of fleshy macroalgae, across all groups, contain carbon-concentrating mechanisms (CCMs) that facilitate use of dissolved inorganic carbon. Rising atmospheric CO₂ concentrations and the resulting shift in seawater carbonate chemistry (ocean acidification, OA) may have varying effects on algae depending on their mode of inorganic carbon concentration, if any, and its regulation. Energetic costs of carbon uptake mechanisms may change with rising pCO₂ and that may explain the variability in effects of OA on growth and photosynthesis. We investigated the relative activity of CCMs in the green alga Ulva lactuca across a range of atmospheric CO₂ concentrations (385-1200 μatm) spanning recent concentrations to the high end predictions for 2100 (IPCC). U. lactuca is a fast growing species with a well characterized, multistep CCM that uses HCO₃^- to saturate photosynthesis with inorganic carbon. To test for responses to OA, we cultured the alga for 21 days at various pCO₂ concentrations. A pH drift experiment was then performed to determine the relative activity of the CCMs if present. The CCM of Ulva showed a potentially bimodal response with sustained activity of bicarbonate anion exchange proteins in some individuals with a declining efficiency of external carbonic anhydrase, as pCO₂ increased. The growth rate was enhanced by intermediate pCO₂ relative to recent and very high pCO₂. The results indicate that the CCM of Ulva is regulated in response to ocean acidification, and there is substantial variation between individual responses. This concurs with the finding that growth increases in Ulva as pCO₂ increases, but indicates an upper threshold of this benefit and potentially high variation within populations.