Carbon acquisition mechanisms in Chara tomentosa

Abstract

Carbon uptake mechanisms of the stonewort Chara tomentosa from the brackish Baltic Sea were studied by recording changes in pH, alkalinity and inorganic carbon concentrations of the seawater medium during photosynthesis in a closed system. The use of inhibitors identified three mechanisms: (1) a vanadate-sensitive P-type H +-ATPase (proton pump) was involved in carbon uptake. This was previously shown for perfused cells of Chara corallina, but not for living cells. (2) Periplasmic carbonic anhydrase that catalyses the dehydration of HCO 3 − into CO 2 outside the cell membrane was highly active during carbon uptake, also at high pH (>9). (3) At high pH, there was direct uptake of HCO 3 − with the help of an anion exchange protein, which previously has not been shown in Chara. We also document here the occurrence of charasomes in the cell membrane of C. tomentosa, always with mitochondria located in their direct vicinity. The simultaneous high periplasmic carbonic anhydrase and proton pump activities and the occurrence of charasomes suggest proton-pump driven H + extrusion and membrane transport of CO 2 derived from HCO 3 − as the major form of DIC acquisition in this alga. Probably, this occurs in acidic bands in C. tomentosa in which we found a banding pattern of CaCO 3 incrustations (alkaline bands). The results were compared with a similar study on the green alga Cladophora glomerata from the same area, which had very low carbonic anhydrase activity (almost negligible), no structures isolating the periplasm from the bulk seawater medium analogous to charasomes and no CaCO 3 incrustation.