Carbonic anhydrase in deep-sea chemoautotrophic symbioses

Carbonic anhydrase (CA) facilitates the rapid interconversion of carbon dioxide and bicarbonate. It is ubiquitous among organisms, and participates in numerous cellular processes. In several photoautotrophic marine organisms, CA facilitates uptake of inorganic carbon by catalyzing the reversible dehydration of bicarbonate to carbon dioxide, which diffuses more rapidly across cell membranes. In algal/invertebrate symbioses, this mechanism is correlated with significantly elevated CA activity in tissues where zooxanthellae are housed. Here, we determine whether elevated CA activity is also characteristic of tissues involved in inorganic carbon uptake by chemosynthetic invertebrates. Measurements of CA activity in several chemosynthetic clam and vestimentiferan species indicate that CA facilitates inorganic carbon uptake, with high activities present in clam gill tissue, and vestimentiferan plume and trophosome tissues. However levels of CA activity among species varied widely, from very low in mytilids to extraordinarily high in some vesicomyids and vestimentiferans. High CA activity is generally correlated with other adaptations for control of internal chemical environments where symbionts are housed, such as the presence of serum-borne sulfide-binding moieties. These findings suggest that a CA-based mechanism of carbon uptake may be one of a suite of adaptations to provide symbionts with essential metabolites.