Strategies for carbon acquisition in the red marine macroalga Coccotylus truncatus from the Baltic Sea
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Marine macroalgae need carbon-concentrating mechanisms because they have only limited access to CO2 in their natural environment. Previous studies have shown that one important strategy common to many algae is the activity of periplasmic carbonic anhydrases that catalyse the dehydration of HCO3– into CO2. The latter can then cross the plasma membrane by passive diffusion. We hypothesised that an active (energy-consuming) mechanism might also be involved in the membrane transport of CO2, as is the case in a number of microalgae. Coccotylus truncatus was chosen as a model organism for this study because it belongs to a group of algae that usually lack direct HCO3– uptake: sublittoral red algae. The method used to study carbon uptake was pH drift of the seawater medium surrounding the algae in a closed vessel, with and without the addition of specific inhibitors or proton buffers. Measured parameters included pH, total inorganic carbon and alkalinity of the seawater medium. Our results suggest that, in C. truncatus, periplasmic carbonic anhydrase as well as H+ extrusion, probably driven by a vanadate-sensitive P-type H+-ATPase (proton pump), are involved in CO2 uptake. No direct uptake of HCO3– was discovered. This paper also presents data on the buffer capacity of several proton buffers and the carbon-uptake inhibitors acetazolamide, 4,4′-diisothiocyano-stilbene-2,2′-disulfonate (DIDS) and orthovanadate in Baltic Sea water with a salinity of 6.5 psu.
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