Effects of ocean acidification on the photosynthetic performance, carbonic anhydrase activity and growth of the giant kelp Macrocystis pyrifera

Under ocean acidification (OA), the 200 % increase in CO_2(aq) and the reduction of pH by 0.3-0.4 units are predicted to affect the carbon physiology and growth of macroalgae. Here we examined how the physiology of the giant kelp Macrocystis pyrifera is affected by elevated pCO₂/low pH. Growth and photosynthetic rates, external and internal carbonic anhydrase (CA) activity, HCO₃ ^- versus CO₂ use were determined over a 7-day incubation at ambient pCO₂ 400 µatm/pH 8.00 and a future OA treatment of pCO₂ 1200 µatm/pH 7.59. Neither the photosynthetic nor growth rates were changed by elevated CO₂ supply in the OA treatment. These results were explained by the greater use of HCO₃ ^- compared to CO₂ as an inorganic carbon (Ci) source to support photosynthesis. Macrocystis is a mixed HCO₃ ^- and CO₂ user that exhibits two effective mechanisms for HCO₃ ^- utilization; as predicted for species that possess carbon-concentrating mechanisms (CCMs), photosynthesis was not substantially affected by elevated pCO₂. The internal CA activity was also unaffected by OA, and it remained high and active throughout the experiment; this suggests that HCO₃ ^- uptake via an anion exchange protein was not affected by OA. Our results suggest that photosynthetic Ci uptake and growth of Macrocystis will not be affected by elevated pCO₂/low pH predicted for the future, but the combined effects with other environmental factors like temperature and nutrient availability could change the physiological response of Macrocystis to OA. Therefore, further studies will be important to elucidate how this species might respond to the global environmental change predicted for the ocean.