THE ACQUISITION OF INORGANIC CARBON BY THE SEAGRASS ZOSTERA-MARINA

In this work, it is elucidated to what degree the seagrass Zostera mar ina L. can utilise HCO3- as an external inorganic-carbon source for ph otosynthesis, and which of two possible systems for its acquisition is in effect. It was found that HCO3- was used as a major source of inor ganic carbon at the normal seawater-pH of 8.2, and that bulk CO2 contr ibuted only marginally (less than 20%) to photosynthesis at that pH. B y comparing photosynthetic rates at pH 8.2 and 9.0, it was deduced tha t CO32- could not be utilised. It was further found that HCO3- could b e acquired via extracellular dehydration to CO2, as catalysed by exter nal/surface-bound carbonic anhydrase, prior to inorganic-carbon uptake . Indications for active, ATPase-mediated, HCO3- transport was also fo und, but an inhibitor of extracellular carbonic anhydrase affected pho tosynthetic rates more than did the less specific ATPase inhibitors. T he rationale for HCO3- dehydration versus its direct uptake is discuss ed with regard to the photosynthetic performance of seagrasses, many o f which are inorganic-carbon limited in their natural habitats.