The short-term incorporation of NaH14CO3(10 to 60 s) into photosynthetic products has been studied in the antarctic diatomNitzschia turgiduloides. It appears that there are two pathways of inorganic carbon assimilation: the C3pathway (Calvin cycle) principally, on which are superimposed β-carboxylation reactions. Photorespiration, which contributes to decrease the net photosynthesis rate, has also been reported. With regard to these results the antarctic diatom was not different from the temperate one. However, the antarctic diatom presents some distinctive features. β-carboxylation reactions which are probably favoured in Antarctic waters because of the high nitrate or ammonium concentrations, and also because they are more economical in terms of energy, are enhanced at the expense of the Calvin cycle reactions. On the other hand, the photorespiration rate is lower than that observed in temperate species, leading to an enhancement of net photosynthesis rate; this seems to occur principally by the more advantageous tartronic semialdehyde pathway. These two features are more pronounced for cells grown under light: dark cycles, particularly a 2:2 hour regime, as compared to continuous light. By contrast to the temperate speciesSkeletonema costatum, changes in the light regime modifies not only the relative amount of inorganic carbon assimilated by the C3or the β-carboxylation pathways, but also the total amount of CO2incorporated per μg Chla. The productivity is highest in 2:2 regime, which simulates conditions of vertical mixing encountered in Antarctic Ocean more than does a 12:12 regime. This finding corroborates the view that the antarctic diatom is well adapted to its environment, although its production is not optimal compared to that of the temperate species grown under the same conditions of irradiance and temperature. Whether this is a genetic adaptation needs to be exami
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