Photosynthetic utilization of CO2(aq) and HCO 3 - in Thalassia testudinum (Hydrocharitaceae)

The effects of total dissolved inorganic carbon (DIC), free carbon dioxide [CO_2(aq)], and bicarbonate (HCO₃^-) concentrations on net photosynthetic oxygen evolution of the marine angiosperm Thalassia testudinum Banks ex König 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 CO_2(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 CO_2(aq) levels, confirming HCO₃^-assimilation. Calculated half-saturation constants (K_s)for CO_2(aq) and HCO₃^-indicated a high affinity for the former [K_s(CO₂)=3 to 18 M] 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 CO_2(aq), suggesting relatively efficient HCO₃^-utilization, despite the apparent low affinity for this carbon form.