Uptake of inorganic carbon by Cladophora glomerata (Chlorophyta) from the Baltic Sea

Carbon uptake in the green macroalga Cladophora glomerata (L.) Kutz. from the brackish Baltic Sea was studied by recording changes in pH, alkalinity, and inorganic carbon concentration of the seawater medium during photosynthesis. The use of specific inhibitors identified three uptake mechanisms: 1) dehydration of HCO3- into CO2 by periplasmic carbonic anhydrase, followed by diffusion of CO2 into the cell; 2) direct uptake of HCO3- via a 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate-sensitive mechanism; and 3) uptake of inorganic carbon by the involvement of a vanadate-sensitive P-type H+-ATPase (proton pump). A decrease in the alkalinity of the seawater medium during carbon uptake, except when treated with vanadate, indicated a net uptake of the ionic species contributing to alkalinity (i.e. HCO3-, CO32-, and OH-) from the medium, where OH (-) influx is equivalent to H (+) efflux. This would suggest that the proton pump is involved in HCO3- transport. We also show that the proton pump can be induced by carbon limitation. The inducibility of carbon uptake in C. glomerata may partly explain why this species is so successful in the upper littoral zone of the Baltic Sea. Usually, carbon limitation is not a problem in the upper littoral of the sea. However, it may occur frequently within dense Cladophora belts with high photosynthetic rates that create high pH and low carbon concentrations in the alga's microenvironment.