Is there a requirement for an external carbonic anhydrase in the extremely acid-resistant green alga Dunaliella acidophila?

The importance of carbonic anhydrase (CA) for the extremely acid-resistant green alga Dunaliella acidophila, acclimated to low or high CO2, was investigated by (1) measuring the total activity of CA in this alga as well as its intracellular distribution, (2) analysing protein by polyacrylamide gel electrophoresis and immunobiological techniques, and (3) studying the effect of impermeant and permeant inhibitors of CA on photosynthesis as well as the dependence of the substrate kinetics of photosynthesis on the CO2 supply during culture. Dunaliella parva, a salt-tolerant but neutrophilic member of the same genus, was used as reference system. Results demonstrate that D. acidophila cells contain only 5-15% of the total CA activity measured in D. parva. A similar ratio applies also to the ''external'' (= periplasmatic or plasma-membrane-bound) CA activity of D. acidophila. The majority of the CA activity in both algae was found to be localized in membranes. In both algae acclimation to low CO2 concentration caused a dramatic increase of CA activity, particularly the periplasmatic CA. D. parva expressed a plasma-membrane-localized 31 kDa protein which cross-reacts with an antibody raised against the periplasmatic CA of Chlamydomonas. A comparable reaction was not observed for D. acidophila. Acclimation to low CO2 decrease K-1/2 values of photosynthesis for CO2 in D. acidophila to values close to the corresponding K-m values of RUBISCO, whereas acclimation to high CO2 caused a significant increase in K-1/2 values of photosynthetic CO2 assimilation in vivo. Thus, even though a plasma-membrane-localized CA is present in D. acidophila and even though the quantity varies in response to the environmental CO2 concentration, its physiological importance in a growth medium of pH 1.0 is much less than in neutrophile Dunaliella species. The importance of internal CA to both D. acidophila and D. parva is proved by comparing their CA activity with the demand of photosynthesis for CO2 and by the inhibition of photosynthesis by the CA inhibitor ethoxyzolamide.