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

Christopher E Cornwall; Christopher D Hepburn; Daniel Pritchard; Kim I Currie; Christina M McGraw; Keith A Hunter; Catriona L Hurd

doi:10.1111/j.1529-8817.2011.01085.x

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

J Phycol. 2012 Feb;48(1):137-44. doi: 10.1111/j.1529-8817.2011.01085.x. Epub 2011 Dec 2.

Authors

Christopher E Cornwall¹, Christopher D Hepburn¹, Daniel Pritchard¹, Kim I Currie¹, Christina M McGraw¹, Keith A Hunter¹, Catriona L Hurd¹

Affiliation

¹ Department of Botany, University of Otago, PO Box 56, Dunedin 9054, New ZealandDepartment of Marine Sciences, University of Otago, PO Box 56, Dunedin 9054, New ZealandDepartment of Botany, University of Otago, PO Box 56, Dunedin 9054, New ZealandNational Institute for Water and Atmospheric Research Ltd., Centre of Excellence for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, PO Box 56, Dunedin 9054, New ZealandDepartment of Chemistry, University of Otago, PO Box 56, Dunedin 9054, New ZealandDepartment of Botany, University of Otago, PO Box 56, Dunedin 9054, New Zealand.

PMID: 27009658
DOI: 10.1111/j.1529-8817.2011.01085.x

Abstract

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 .

Keywords: HCl; bicarbonate; carbon acquisition; carbon dioxide; climate change; dissolved inorganic carbon; macroalgae; ocean acidification; pH drift; photosynthesis.