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.
Growth and Nutritional Composition of Eustigmatophyceae Monodus subterraneus and Nannochloropsis oceanica in Autotrophic and Mixotrophic Culture
