Carbon metabolism in seagrasses: I. The utilization of exogenous inorganic carbon species in photosynthesis

Four species of seagrasses, Halophila stipulacea, Thalassodendron ciliatum, Halodule uninervis, and Syringodium isoetifolium, were investigated for their ability to utilize HCO-3 and CO₂ as exogenous carbon sources for photosynthesis. Rates of photosynthesis were measured as rates of O₂ evolution in a closed system in which the pH was continuously controlled. A computer program was written to calculate the concentrations of different carbon species as a function of pH and other specified experimental conditions.Bicarbonate as well as CO₂ were readily assimilated by all four seagrass species. Saturating concentrations of HCO-3, at saturating light intensities, were 0.5-1.8 mM depending on the species. Rates of photosynthesis under such conditions were 0.1-0.55 μmol O₂ min^-1 mg^-1 chlorophyll. At saturating CO₂ concentrations, i.e. 0.5-1.3 mM, rates of photosynthesis were 0.22-1.4 μmol CO₂ min^-1 mg^-1 chlorophyll. Photosynthetic rates in each species were considerably higher when CO₂ rather than HCO-3 was supplied at saturating concentrations.The concentration of HCO-3 in natural seawater was found to be saturating, and that of CO₂ insufficient for considerable photosynthetic rates in these plants under the given conditions It was thus concluded that HCO-3 is the major carbon source for photosynthesis in seagrasses.