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

The effect of pCO₂ 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 O₂ evolution, CO₂ and HCO₃⁻ uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO₂ levels of 370 and 800 μatm. To investigate whether the cells operate a C₄-like pathway, activities of ribulose-1,5-bisphosphate carboxylase (RubisCO) and phosphoenolpyruvate carboxylase (PEPC) were measured at the mentioned pCO₂ levels and a lower pCO₂ level of 50 μatm. 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 O₂ 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 pCO₂ 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 pCO₂, 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 C₄ metabolism in the investigated species.