CARBON‐USE STRATEGIES IN MACROALGAE: DIFFERENTIAL RESPONSES TO LOWERED PH AND IMPLICATIONS FOR OCEAN ACIDIFICATION 1

• Published in: Journal of Phycology
• Main Authors: Cornwall, Christopher E., Hepburn, Christopher D., Pritchard, Daniel, Currie, Kim I., McGraw, Christina M., Hunter, Keith A., Hurd, Catriona L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2011
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1111/j.1529-8817.2011.01085.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1529-8817.2011.01085.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1529-8817.2011.01085.x

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO 2 . This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO 2 and HCO 3 − increase; CO 3 2− decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO 2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO 3 − ]. We measured the short‐term photosynthetic responses of five macroalgal species with various carbon‐use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO 2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO 2 , HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO 3 − increased their use of CO 2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO 2 did so exclusively in all treatments. Although CO 2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO 2 is less energetically expensive than active HCO 3 − uptake, and so HCO 3 − ‐using macroalgae may benefit in future seawater with elevated CO 2 .