Physiological functioning of carbonic anhydrase in the hydrothermal vent tubeworm Riftia pachyptila

On the basis of our experiments, it is clear that carbonic anhydrase (CA) p lays an important role in the CO2-concentrating mechanisms in Riftia pachyp tila. Plume tissue from freshly collected animals had the highest CA activi ty, 253.7 +/- 36.0 mu mol CO2 min(-1) g(-1) wet wt, and trophosome activity averaged 109.4 +/- 17.9 mu mol CO2 min(-1) g(-1) wet wt. Exposure of livin g worms to ethoxyzolamide, a carbonic anhydrase inhibitor, resulted in a 99 % decrease in CA activity (from 103.9 +/- 38.6 to 0.7 +/- 0.2 mu mol CO2 mi n(-1) g(-1) wet wt in the plume tissue and 57.6 +/- 17.9 to 0.03 +/- 0,11 m u mol CO2 min(-1) g(-1) wet wt in the trophosome) and essentially a complet e cessation of Sigma CO2, uptake. High concentrations of CA appear to facil itate the equilibration between inorganic carbon (Ci) in the external and i nternal environments, greatly enhancing the diffusion of CO2 into the anima l. In summary, R. pachyptila demonstrates very effective acquisition of ino rganic carbon from the environment, thereby providing the symbionts with la rge amounts of CO2. This effective acquisition is made possible by three fa ctors: extremely effective pH regulation, a large external pool of CO2, and , described in this paper, high levels of carbonic anhydrase.