Abstract: | Mass spectrometry has been used to investigate the transportof CO2 in the freshwater diatom Navicula pelliculosa. The timecourseof CO2 formation in the dark after addition of 100 mmol m3dissolved inorganic carbon (DIC) to cell suspensions showedthat no external carbonic anhydrase (CA) was present in thesecells. Upon illumination, cells pre-incubated at pH 75 with100 mmol m3 DIC, removed almost all free CO2 from themedium at an initial rate of 285 µmol CO2 mg1Chl h1. Equilibrium between HCO3 and CO2 in themedium occurred rapidly upon addition of bovine CA, showingthat CO2 depletion resulted from a selective uptake of CO2 ratherthan an uptake of all inorganic carbon species. However, photosyntheticO2 evolution rate remained constant after CO2 had been depletedfrom the medium indicating that photosynthesis is sustainedprimarily by active HCO3 uptake. Treatment of cells with2-iodoacetamide (83 mol m3) completely inhibited CO2fixation but had little effect on CO2 transport since initialrates of CO2 depletion were about 81% that of untreated cells.Transfer of iodoacetamide-treated cells to the dark caused arapid increase in the CO2 concentration in the medium largelydue to the efflux of the unfixed intracellular DIC pool whichwas found to be about 194 times the concentration of that inthe external medium. These results indicate that Navicula pelliculosaactively takes up molecular CO2 against a concentration gradientby a process distinct from HCO3 transport. Key words: Dissolved inorganic carbon, carbonic anhydrase, bicarbonate transport, CO2 transport, mass spectrometry |