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WHOLE PLANT AND ECOPHYSIOLOGY

An External -Carbonic Anhydrase in a Free-Living Marine Dinoflagellate May Circumvent Diffusion-Limited Carbon Acquisition^1,[W]

Institut de Recherche en Biologie Végétale, Département de Sciences Biologiques, Université de Montréal, Montreal, Quebec, Canada H1X 2B2

The oceans globally constitute an important sink for carbon dioxide (CO₂) due to phytoplankton photosynthesis. However, the marine environment imposes serious restraints to carbon fixation. First, the equilibrium between CO₂ and bicarbonate (HCO₃^–) is pH dependent, and, in normal, slightly alkaline seawater, [CO₂] is typically low (approximately 10 µM). Second, the rate of CO₂ diffusion in seawater is slow, so, for any cells unable to take up bicarbonate efficiently, photosynthesis could become carbon limited due to depletion of CO₂ from their immediate vicinity. This may be especially problematic for those dinoflagellates using a form II Rubisco because this form is less oxygen tolerant than the usually found form I enzyme. We have identified a carbonic anhydrase (CA) from the free-living marine dinoflagellate Lingulodinium polyedrum that appears to play a role in carbon acquisition. This CA shares 60% sequence identity with -class CAs, isoforms so far found only in marine algae. Immunoelectron microscopy indicates that this enzyme is associated exclusively with the plasma membrane. Furthermore, this enzyme appears to be exposed to the external medium as determined by whole-cell CA assays and vectorial labeling of cell surface proteins with ¹²⁵I. The fixation of ¹⁴CO₂ is strongly pH dependent, suggesting preferential uptake of CO₂ rather than HCO₃^–, and photosynthetic rates decrease in the presence of 1 mM acetazolamide, a non-membrane-permeable CA inhibitor. This constitutes the first CA identified in the dinoflagellates, and, taken together, our results suggest that this enzyme may help to increase CO₂ availability at the cell surface.

² These authors contributed equally to the article.

³ Present address: Département de Biologie Cellulaire, Université de Montréal, Montreal, Quebec, Canada H1X 2B2.

⁴ Present address: Monroe Community College, Rochester, NY 14623.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: David Morse (david.morse{at}umontreal.ca).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.117077

^* Corresponding author; e-mail david.morse{at}umontreal.ca.

Received January 29, 2008; accepted April 18, 2008; published May 8, 2008.

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