The acquisition of inorganic carbon by the seagrass Zostera marina

In this work, it is elucidated to what degree the seagrass Zostera marina L. can utilise HCO₃/^as an external inorganic-carbon source for photosynthesis, and which of two possible systems for its acquisition is in effect. It was found that HCO³/^- was used as a major source of inorganic carbon at the normal seawater-pH of 8.2, and that bulk CO₂ contributed only marginally (less than 20%) to photosynthesis at that pH. By comparing photosynthetic rates at pH 8.2 and 9.0, it was deduced that CO₃/^2- could not be utilised. It was further found that HCO₃/^- could be acquired via extracellular dehydration to CO₂, as catalysed by external/surface-bound carbonic anhydrase, prior to inorganic-carbon uptake. Indications for active, ATPase-mediated, HCO₃/^- transport was also found, but an inhibitor of extracellular carbonic anhydrase affected photosynthetic rates more than did the less specific ATPase inhibitors. The rationale for HCO₃/^- dehydration versus its direct uptake is discussed with regard to the photosynthetic performance of seagrasses, many of which are inorganic-carbon limited in their natural habitats.