Carbon-use strategies in macroalgea: differential responses to lowered pH and implications for ocean acidification

• Published in: Journal of Phycology
• Main Authors: Cornwall, CE, Hepburn, CD, Pritchard, D, Currie, KI, McGraw, CM, Hunter, KA, Hurd, CL
• Format: Article in Journal/Newspaper
• Language: English
• Published: Blackwell Publishing Inc 2012
• Subjects: Environmental Sciences; Ecological Applications; Ecological Impacts of Climate Change; Ocean acidification
• Online Access: https://doi.org/10.1111/j.1529-8817.2011.01085.x; http://ecite.utas.edu.au/91468

Ocean acidification (OA) is a reduction in oceanicpH due to increased absorption of anthropogenicallyproduced CO 2 . This change alters the seawaterconcentrations of inorganic carbon species that areutilized by macroalgae for photosynthesis and calcification:CO 2 and HCO 3 increase; CO 3 2 decreases.Two common methods of experimentally reducingseawater pH differentially alter other aspects of carbonatechemistry: the addition of CO 2 gas mimicschanges predicted due to OA, while the addition ofHCl results in a comparatively lower [HCO 3 ]. Wemeasured the short-term photosynthetic responsesof five macroalgal species with various carbon-usestrategies in one of three seawater pH treatments:pH 7.5 lowered by bubbling CO 2 gas, pH 7.5 loweredby HCl, and ambient pH 7.9. There was no differencein photosynthetic rates between the CO 2 ,HCl, or pH 7.9 treatments for any of the speciesexamined. However, the ability of macroalgae toraise the pH of the surrounding seawater throughcarbon uptake was greatest in the pH 7.5 treatments.Modeling of pH change due to carbon assimilationindicated that macroalgal species that couldutilize HCO 3 increased their use of CO 2 in the pH7.5 treatments compared to pH 7.9 treatments. Speciesonly capable of using CO 2 did so exclusively inall treatments. Although CO 2 is not likely to be limitingfor photosynthesis for the macroalgal speciesexamined, the diffusive uptake of CO 2 is less energeticallyexpensive than active HCO 3 - uptake, andso HCO 3 - -using macroalgae may benefit in futureseawater with elevated CO 2 .Key index words: bicarbonate; carbon acquisition;carbon dioxide; climate change; dissolved inorganiccarbon; HCl; macroalgae; ocean acidification;pH drift; photosynthesisAbbreviations: AT, total alkalinity; CCM, carbonconcentratingmechanism; DIC, dissolvedinorganic carbon; OA, ocean acidification; PFD,photon flux density; PQ, photosynthetic quotient;PS, photosynthesis