Photosynthetic Carbon Metabolism of a Marine Grass 1

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The δ13C value of a tropical marine grass Thalassia testudinum is −9.04‰. This value is similar to the δ13C value of terrestrial tropical grasses. The δ13C values of the organic acid fraction, the amino acid fraction, the sugar fraction, malic acid, and glucose are: −11.2‰, −13.1‰, −10.1‰, −11.1‰, and −11.5‰, respectively. The δ13C values of malic acid and glucose of Thalassia are similar to the δ13C values of these intermediates in sorghum leaves and attest to the presence of the photosynthetic C4-dicarboxylic acid pathway in this marine grass. The inorganic HCO3− for the growth of the grass fluctuates between −6.7 to −2.7‰ during the day. If CO2 fixation in Thalassia is catalyzed by phosphoenolpyruvate carboxylase (which would result in a −3‰ fractionation between HCO3− and malic acid), the predicted δ13C value for Thalassia would be −9.7 to −5.7‰. This range is close to the observed range of −12.6 to −7.8‰ for Thalassia and agree with the operation of the C4-dicarboxylic acid pathway in this plant. The early products of the fixation of HCO3− in the leaf sections are malic acid and aspartic acid which are similar to the early products of CO2 fixation in C4 terrestrial plants.

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