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 CO32- 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.