Inorganic carbon acquisition and its energization in eustigmatophyte algae

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.