Active uptake of CO2 by the diatom Navicula pelliculosa
1992
Rotatore, C. | Colman, B.
Mass spectrometry has been used to investigate the transport of CO2 in the freshwater diatom Navicula pelliculosa. The time-course of CO2 formation in the dark after addition of 100 mmol m-3 dissolved inorganic carbon (DIC) to cell suspensions showed that no external carbonic anhydrase (CA) was present in these cells. Upon illumination, cells pre-incubated at pH 7.5 with 100 mmol m-3 DIC, removed almost all free CO2 from the medium at an initial rate of 28.5 micromoles CO2 mg-1 Chl h-1. Equilibrium between HCO3(-1) and CO2 in the medium occurred rapidly upon addition of bovine CA, showing that CO2 depletion resulted from a selective uptake of CO2 rather than an uptake of all inorganic carbon species. However, photosynthetic O2 evolution rate remained constant after CO2 had been depleted from the medium indicating that photosynthesis is sustained primarily by active HCO3(-1) uptake. Treatment of cells with 2-iodoacetamide (8.3 mol m-3) completely inhibited CO2 fixation but had little effect on CO2 transport since initial rates of CO2 depletion were about 81% that of untreated cells. Transfer of iodoacetamide-treated cells to the dark caused a rapid increase in the CO2 concentration in the medium largely due to the efflux of the unfixed intracellular DIC pool which was found to be about 194 times the concentration of that in the external medium. These results indicate that Navicula pelliculosa actively takes up molecular CO2 against a concentration gradient by a process distinct from HCO3(-1) transport.
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