Uptake and Accumulation of Inorganic Carbon by a Freshwater Diatom

Colman, B. and Rotatore, C. 1988. Uptake and accumulation ofinorganic carbon by a freshwater diatom.—J. exp Bot 39:1025–1032. The mechanism of uptake of inorganic carbon and its accumulationhas been studied in the freshwater diatom Navicula pelliculosa.No external carbonic anhydrase could be detected, although itwas detected in cell extracts. The rate of photosynthetic O₂evolution, in media in the range pH 7.5–8.5, exceededthe calculated rate of CO₂ supply 2- to 5-fold, indicating thatHCO^–₃ was taken up by the cells. At an external pH of7.5, the internal pH, measured by ¹⁴C-dimethyloxazolidine-2,4-dione distribution between the cells and the medium, was pH7.6 in the light and pH 7.4 in the dark. Accumulation of inorganiccarbon was determined by the silicone oil centrifugation methodand inorganic carbon pools of 23.5 mol m^–3 were found,a concentration 21.6-fold that in the external medium. The resultsindicate an active accumulation of inorganic carbon againstpH and concentration gradients in this diatom, probably by activeHCO^–₃ uptake. Key words: Bicarbonate transport, carbon dioxide, carbonic anhydrase, CO₂ affinity, CO₂ concentrating mechanism, internal pH, Navicula pelliculosa