Active transport of CO2 by three species of marine microalgae

The occurrence of an active CO2 transport system and of carbonic anhydrase (CA) has been investigated by mass spectrometry in the marine, unicellular rhodophyte Porphyridium cruentum (S.F. Gray) Naegeli and two marine chlorop hytes Nannochloris atomus Butcher and Nannochloris maculata Butcher. Illumi nation of darkened cells incubated with 100 mu M (HCO3-)-C-13 caused a rapi d initial drop, followed by a slower decline in the extracellular COP conce ntration. Addition of bovine CA to the medium raised the CO2 concentration by restoring the HCO3--CO2 equilibrium, indicating that cells were taking u p CO2 and were maintaining the CO2 concentration in the medium below its eq uilibrium value during photosynthesis. Darkening the cell suspensions cause d a rapid increase in the extracellular CO2 concentration in all three spec ies, indicating that the cells had accumulated an internal pool of unfixed inorganic carbon. CA activity was detected by monitoring the rate of exchan ge of O-18 from (CO2)-C-13-O-18 into water. Exchange of O-18 was rapid in d arkened cell suspensions, but was not inhibited by 500 mu M acetazolamide, a membrane-impermeable inhibitor of CA, indicating that external CA activit y was not present in any of these species. In all three species, the rate o f exchange was completely inhibited by 500 mu M ethoxyzolamide, a membrane- permeable CA-inhibitor, showing that an intracellular CA was present. These results demonstrate that the three species are capable of CO2 uptake by ac tive transport for use as a carbon source for photosynthesis.