The effect of pCO 2 on carbon acquisition and intracellular assimilation in four marine diatoms

Abstract

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.) 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.