The effect of pCO2 on carbon acquisition and intracellular assimilation in four marine diatoms

a r t i c l e i n f o Keywords: CCM 13 C fractionation CA C 4 photosynthesis PEPC RubisCO The effect of pCO 2 on carbon acquisition and intracellular assimilation was investigated in the three bloom-forming diatom species, Eucampia zodiacus (Ehrenberg), Skeletonema costatum (Greville) Cleve, Thalassio-nema nitzschioides (Grunow) Mereschkowsky and the non-bloom-forming Thalassiosira pseudonana (Hust.) Hasle and Heimdal. In vivo activities of carbonic anhydrase (CA), photosynthetic O 2 evolution, CO 2 and HCO 3 − uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO 2 levels of 370 and 800 μatm. To investigate whether the cells operate a C 4-like pathway, activities of ribulose-1,5-bisphosphate carboxylase (RubisCO) and phosphoenolpyruvate carboxylase (PEPC) were measured at the mentioned pCO 2 levels and a lower pCO 2 level of 50 μatm. In the bloom-forming species, extracellular CA activities strongly increased with decreasing CO 2 supply while constantly low activities were obtained for T. pseudonana. Half-saturation concentrations (K 1/2) for photosynthetic O 2 evolution decreased with decreasing CO 2 supply in the two bloom-forming species S. costatum and T. nitzschioides, but not in T. pseudonana and E. zodiacus. With the exception of S. costatum, maximum rates (V max) of photosynthesis remained constant in all investigated diatom species. Independent of the pCO 2 level, PEPC activities were significantly lower than those for RubisCO, averaging generally less than 3%. All examined diatom species operate highly efficient CCMs under ambient and high pCO 2 , but differ strongly in the degree of regulation of individual components of the CCM such as C i uptake kinetics and extracellular CA activities. The present data do not suggest C 4 metabolism in the investigated species. Diatoms are a diverse and ecologically very important group contributing up to 40% of the oceans primary production (Nelson et al., 1995). Among the large diversity in this group, bloom-forming diatoms play a major role in determining the downward transport of organic carbon from surface waters to the deep ocean (Buesseler, 1998). Numerous diatom species are known to bloom frequently along continental margins and in upwelling regions where the nutrient availability is high (Smetacek, 1999). The occurrence of high diatom abundances in nutrient-rich waters has been related to several physiological adaptations. Mostly centric diatoms have evolved a vacuole that allows accumulating nutrients in excess of its immediate growth requirements and therewith deprives competing taxa of these essential resources …