Mechanism of Photosynthetic Carbon Dioxide Uptake by the Red Macroalga, Chondrus crispus

The aim of this study was to determine how Chondrus crispus, a marine red macroalga, acquires the inorganic carbon (C(i)) it utilizes for photosynthetic carbon fixation. Analyses of C(i) uptake were done using silicone oil centrifugation (using multicellular fragments of thallus), infrared gas analysis, and gas chromatography. Inhibitors of carbonic anhydrase (CA), the band 3 anion exchange protein and Na(+)/K(+) exchange were used in the study. It was found that: (a) C. crispus does not accumulate C(i) internally above the concentration attainable by diffusion; (b) the initial C(i) fixtion rate of C. crispus fragments saturates at approximately 3 to 4 millimolar C(i); (c) CA is involved in carbon uptake; its involvement is greatest at high HCO(3) (-) and low CO(2) concentration, suggesting its participation in the dehydration of HCO(3) (-) to CO(2); (d) C. crispus has an intermediate C(i) compensation point; and (e) no evidence of any active or facilitated mechanism for the transport of HCO(3) (-) was detected. These data support the view that photosynthetic C(i) uptake does not involve active transport. Rather, CO(2), derived from HCO(3) (-) catalyzed by external CA, passively diffuses across the plasma membrane of C. crispus. Intracellular CA also enhances the fixation of carbon in C. crispus.