An External δ-Carbonic Anhydrase in a Free-Living Marine Dinoflagellate May Circumvent Diffusion-Limited Carbon Acquisition  

Abstract

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 μ m). 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 δ-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.

Gene Expression in Proliferating Cells of the Dinoflagellate Alexandrium catenella (Dinophyceae)

“…Upstream of the photosynthesis process per se in the process of carbon fixation, we detected three different forms of DCA. In addition to the DCA form detected in group IV A. tamarense CCMP1598 (34) and those similar to form 2 of ␦-CA described in Lingulodinium polyedrum (28), the last form, never before reported in a dinoflagellate, was overexpressed during exponential growth (Fig. 4D).…”

Carbon Fixation in Diatoms

“…The eukaryotic algae studied so far possess α-to γ-CAs (Roberts et al 1997 ;Lane and Morel 2000 ;Lane et al 2005 ;Xu et al 2008 ). These two CA types are also present in the centric diatom T. pseudonana , while a δ-type CA was found in the haptophyte Emiliania huxleyi (Soto et al 2006 ) and the dinofl agellate Lingulodinium polyedrum (Lapointe et al 2008 ). These two "marine CAs" were identifi ed as bona fi de CAs (Xu et al 2008 ;Viparelli et al 2010 ;Alterio et al 2012 ;Lee et al 2013 ).…”

“…As aforementioned, high salinity and alkalinity of seawater tend to remove CO 2 from cell exterior which may force cells to invest energy in pumping HCO 3 − , while CO 2 uptake via external CA is energetically less expensive. In fact, in the dinofl agellates Prorocentrum micans and Lingulodinium polyedrum , external CAs have been shown to be essential (Nimer et al 1999 ;Lapointe et al 2008 ). However, it is also known that external CAs are absent in some marine microalgae (Danson et al 2004 ).…”

δ-Carbonic Anhydrases: Structure, Distribution, and Potential Roles

“…Lastly, the A. catenella sequence is grouped together with a chlorophyte and seems to be only distantly related to the other dinoflagellate d-CA sequences. Curiously, the dinoflagellate Amphidinium carterae from the gymnodiniale order does not appear to possess a d-CA (17).…”

“…In the dinoflagellate L. polyedrum, two sequences for d-CA, LpCA1 and LpCA2, were identified by colony hybridization using the TwCA1 sequence as a probe (17). These two sequences share an 80% amino acid similarity between themselves and 58-60% similarity with TwCA1.…”