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

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, Thalassionema nitzschioides (Grunow) Mereschkowsky and the non-bloom-forming Thalassiosira pseudonana (Hust.) Haste and Heimdal. In vivo activities of carbonic anhydrase (CA), photosynthetic O-2 evolution, CO2 and HCO3- uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO2 levels of 370 and 800 mu atm. 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 mu atm. In the bloom-forming species, extracellular CA activities strongly increased with decreasing CO2 supply while constantly low activities were obtained for T. pseudonana. Half-saturation concentrations (K-1/2) for photosynthetic O-2 evolution decreased with decreasing CO2 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. (C) 2009 Elsevier B.V. All rights reserved.