Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO₂. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO₂ and HCO₃^– increase; CO₃^2– decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO₂ gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO₃^–]. 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 CO₂ gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO₂, 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 HCO₃^– increased their use of CO₂ in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO₂ did so exclusively in all treatments. Although CO₂ is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO₂ is less energetically expensive than active HCO₃^- uptake, and so HCO₃^--using macroalgae may benefit in future seawater with elevated CO₂. Key index words: bicarbonate; carbon acquisition; carbon dioxide; climate change; dissolved inorganic carbon; HCl; macroalgae; ocean acidification; pH drift; photosynthesis Abbreviations: AT, total alkalinity; CCM, carbonconcentrating mechanism; DIC, dissolved inorganic carbon; OA, ocean acidification; PFD, photon flux density; PQ, photosynthetic quotient; PS, photosynthesis

Item Type:	Refereed Article
Keywords:	ocean acidification, seaweed, carbon aquisition, pH
Research Division:	Environmental Sciences
Research Group:	Climate change impacts and adaptation
Research Field:	Ecological impacts of climate change and ecological adaptation
Objective Division:	Environmental Policy, Climate Change and Natural Hazards
Objective Group:	Adaptation to climate change
Objective Field:	Ecosystem adaptation to climate change
UTAS Author:	Cornwall, CE (Dr Chris Cornwall)
UTAS Author:	Hurd, CL (Professor Catriona Hurd)
ID Code:	91468
Year Published:	2012
Web of Science® Times Cited:	122
Deposited By:	IMAS Research and Education Centre
Deposited On:	2014-05-20
Last Modified:	2014-11-24
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