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The photosynthesis and carbonic anhydrase activity of four marine diatoms currently found in oyster-ponds near the French Atlantic coast, Haslea ostrearia, Navicula phyllepta, Entomoneis paludosa and Amphora coffeaeformis, were investigated. Photosynthetic parameters determined from photosynthesis versus irradiance curves showed that A. coffeaeformis (a benthic species) had lower maximum net photosynthesis but a higher light utilization coefficient than the other species studied. Carbonic anhydrase (CA) activity was measured using intact cells (external CA) and extracts (total CA) of these four species. In all four diatoms, the internal CA activity accounted for between 56 and 63% of the total CA activity. Two or three active forms of CA were separated by polyacrylamide gel electrophoresis. Two enzymatic bands of low molecular mass (4.5 and 6.5 kDa) were found in all four diatoms. A third enzymatic band was detected at 33 kDa in H. ostrearia and 11.7 kDa in E. paludosa. Acetazolamide, an inhibitor of external CA, reduced the net photosynthesis in H. ostrearia, E. paludosa and A. coffeaeformis by about 13%, but could not be tested on N. phyllepta as it is permeable to this substance. Direct uptake of [image omitted] was estimated using the anion-exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonate. Bicarbonate accounted for about 50% of fixed inorganic carbon in E. paludosa, and for about 60% in the other species. The higher affinity (K1/2) for dissolved inorganic carbon in H. ostrearia (24.5 µM) than in E. paludosa (38.2 µM) seemed to be related to greater external CA activity, and the presence of a more efficient anion exchange protein in the plasmalemma.