Received January 29, 2008
Accepted April 18, 2008
An external 
-Carbonic Anhydrase in a free-living marine dinoflagellate may circumvent diffusion limited carbon acquisition
Mathieu Lapointe , Tyler D.B. MacKenzie , and David Morse *
Institut de Recherche en Biologie Vegetale, Departement de Sciences Biologiques, Universite de Montreal, 4101 Sherbrooke est, Montreal, Quebec, Canada H1X 2B2
* Corresponding author; email: david.morse{at}umontreal.ca.
The oceans globally constitute an important sink for carbon dioxide due to phytoplankton photosynthesis. However, the marine environment imposes serious restraints to carbon fixation. First, the equilibrium between CO2 and bicarbonate is pH dependent, and in normal slightly alkaline seawater, [CO2] is typically low (
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, as 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, an isoform so far found only in marine algae. Immunoelectron microscopy indicates 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 dependant, suggesting preferential uptake of CO2 rather than bicarbonate, 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 this enzyme may help to increase CO2 availability at the cell surface.
