Abstract

The eustigmatophyceans are primitive unicellular algae that represent the most basal group of ochrophytes. They are believed to be obligate photoautotrophs, occurring mainly in freshwater and soil but with some marine representatives. The freshwater eustigmatophytes Monodus subterraneus and Vischeria stellata, and the marine eustigmatophyte Nannochloropsis gaditana, have been studied by mass spectrometry with respect to their characteristics for inorganic carbon (Ci) uptake. A CO₂ concentrating mechanism was found in all three, but an external carbonic anhydrase (CA) was not detected. The acquisition of Ci from the external medium was based on the active transport of HCO₃^–, CO₂, or both. In particular, N. gaditana was able to use HCO₃^– exclusively as an exogenous carbon source for photosynthesis, with this HCO₃^– being subsequently converted to CO₂ by an intracellular CA for photosynthetic fixation. A unique characteristic of this species was its capacity to transport HCO₃^– during prolonged periods of time in the dark. In contrast, M. subterraneus utilized CO₂ alone through an active transport process, whereas V. stellataexhibited the capacity to transport both HCO₃^– and CO₂. The uptake of CO₂ also continued in the dark in V. stellatacells. Regardless of the Ci species taken up, transport was abolished by anoxia and by inhibitors of mitochondrial respiration. These results indicate that that the supply of Ci for photosynthetic CO₂ fixation is partly dependent upon mitochondrial activity in these primitive eukaryotes.