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

Abstract

The aim of this study was to determine how Chondrus crispus, a marine red macroalga, acquires the inorganic carbon (Ci) it utilizes for photosynthetic carbon fixation. Analyses of Ci uptake were done using silicone oil centrifugation (using.cntdot.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 accumualte Ci internally above the concentration attainable by diffusion; (b) the initial Ci fixation rate of C. crispus fragments saturates at approximately 3 to 4 millimolar Ci; (c) CA is involved in carbon uptake; its involvement is greatest at high HCO3- and low CO2 concentration, suggesting its participation in the dehydration of HCO3- to CO2; (d) C. crispus has an intermediate Ci compensation point; and (e) no evidence of any active or facilitated mechanism for the transport of HCO3- was detected. These data support the view that photosynthetic Ci uptake does not involve active transport. Rather, CO2, derived from HCO3- catalyzed by external CA, passively diffuses across the plasma membrane f C. crispus. Intracellular CA also enhances the fixation of carbon in C. crispus.