Carbon Metabolism in Seagrasses: I. THE UTILIZATION OF EXOGENOUS INORGANIC CARBON SPECIES IN PHOTOSYNTHESIS

Abstract

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.