Carbon fixation and carbonic anhydrase activity in Haslea ostrearia (Bacillariophyceae) in relation to growth irradiance

The metabolic pathway of primary carbon fixation was studied in a peculiar pennate marine diatom, Haslea ostrearia (Bory) Simonsen, which synthesizes and accumulates a blue pigment known as "marennine". Cells were cultured in a semi-continuous mode under saturating [350 mu mol(photon) m(-2) s(-1)] or non-saturating [25 mu mol(photon) m(-2) s(-1)] irradiance producing "blue" (BC) and "green" (GC) cells, characterized by high and low marennine accumulation, respectively. Growth, pigment contents (chlorophyll a and marennine), C-14 accumulation in the metabolites, and the carbonic anhydrase (CA) activity of the cells were determined during the exponential growth phase. Growth rate and marennine content were closely linked to irradiance during growth: higher irradiance increased both growth rate and marennine content. On the other hand, the Chl a concentration was lower under saturating irradiance. The distribution between the Calvin-Benson (C-3) and beta-carboxylation (C-4) pathways was very different depending on the irradiance during growth. Metabolites of the C-3 cycle contained about 70 % of the total fixed radioactivity after 60 s of incorporation into cells cultured under the non-saturating irradiance (GC), but only 47 % under saturating irradiance (BC). At the same time, carbon fixation by beta-carboxylation was 24 % in GC versus about 41 % in BC, becoming equal to that in the C-3 fixation pathway in the latter. Internal CA activity remained constant, but the periplasmic CA activity was higher under low than high irradiance.