External HCO3- dehydration maintained by acid zones in the plasma membrane is an important component of the photosynthetic carbon uptake in Ruppia cirrhosa

Ruppia cirrhosa, a temperate seagrass growing in brackish water, featured a high capacity for HCO3- utilisation, which could operate over a wide pH range (from 7.5 up to 9.5) with maintained efficiency. Tris buffer inhibited this means of HCO3- utilisation in a competitive manner, while addition of acetazolamide, an inhibitor of extracellular carbonic anhydrase activity, caused a 40 - 50% inhibition. A mechanism involving periplasmic carbonic anhydrase-catalysed HCO3- dehydration in acid zones, followed by a ( probably diffusive) transport of the formed CO2 across the plasma membrane was thus, at least partly, responsible for the HCO3- utilisation. This mechanism, which comprises a CO2-concentrating mechanism (CCM) associated with the plasma membrane, is thus shown for the first time in an aquatic angiosperm. Additional mechanisms involved in the Tris-sensitive HCO3- utilisation could be direct HCO3- uptake (e.g., in an H+/ HCO3- symport) or ( more likely) non-catalysed HCO3- dehydration in the acid zones. Based on these results, and on earlier investigations on Zostera marina, a generalmodel for analysis of HCO3- utilisation mechanisms of seagrasses is suggested. In this model, three 'systems' for HCO3- utilisation are defined which are characterised ( and can to some extent be quantified) by their capability to operate at high pH in combination with their response to acetazolamide and Tris. Some consequences of the fact that HCO3- utilisation and osmoregulation probably depend on the same energy source (ATP via H+-ATPase in the plasma membrane) are discussed.