PHOTOSYNTHETIC UTILIZATION OF CO2(AQ) AND HCO3- IN THALASSIA-TESTUDINUM (HYDROCHARITACEAE)

The effects of total dissolved inorganic carbon (DIC), free carbon dioxide [CO2(aq)], and bicarbonate (HCO3-) concentrations on net photosynthetic oxygen evolution of the marine angiosperm Thalassia testudinum Banks ex Konig collected from Biscayne Bay (1988) and from Tampa Bay (1990), Florida, USA, were examined. Rates of photosynthesis declined by 85% from pH 7.25 to 8.75 in buffered seawater media with constant DIC concentration (2.20 mM), suggesting a strong influence of CO2(aq) concentration. A plateau in the pH-response curve between pH 7.75 and 8.50 indicated possible utilization of HCO3-. Responses of photosynthesis measured in buffered seawater media of varying DIC concentrations (0.75 to 13.17 mM) and pH (7.8 to 8.61) demonstrated that photosynthesis is rate-limited at ambient DIC levels. Photosynthesis increased in media with increasing HCO3- concentrations but near-constant CO2(aq) levels, confirming HCO3- assimilation. Calculated half-saturation constants (K) for CO2(aq) and HCO3- indicated a high affinity for the former [K(s)(CO2) = 3 to 18 muM] and a much lower affinity for the latter [K(s)(HCO3-) = 1.22 to 8.88 mM]. Calculated V(max) values for HCO3- were generally higher than those for CO2(aq), suggesting relatively efficient HCO3- utilization, despite the apparent low affinity for this carbon form.