Fractionation of the Stable Isotopes of Inorganic Carbon by Seagrasses

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Abstract
The .delta.13C values for seagrasses [Thalassia testudinum, Diplanthera wrightii and Halophila engelmannii] collected along the Texas Gulf Coast [USA] range from -10.9 to -11.4.permill.. These values are similar to the .delta.13C values of terrestrail C4 plants, but seagrasses lack bundle sheath cells which are important in determining the .delta.13C values of C4 plants. This work attempts to explain the reason the .delta.13C values of seagrasses resemble the .delta.13C values of C4 plants. Investigations on the photosynthetic characteristics of seagrasses show that dissolved CO2 is the species of inorganic carbon absorbed or accumulated by Thalassia testudinum. The rate of photosynthetic CO2 fixation varies from 9.6-129.0 .mu.M CO2/mg chlorophyll per h in the presence of 0.042-1.9 mM dissolved CO2 due to the high resistance of Thalassia leaves to CO2 diffusion. Phosphoglyceric acid is the 1st stable product of photosynthetic CO2 fixation in Thalassia which is a Calvin cycle plant. The light/dark ratios of 14CO2 release from submerged Thalassia leaf sections at 1, 21 and 100% O2 indicate a small apparent photorespiration. Dark respiration continues in the light and is stimulated by 21 and 100% O2. The low apparent photorespiration may be due to membrane and H2O resistance to CO2 diffusion with subsequent refixation of the photorespired CO2. The internal pool of CO2 is not in equilibrium with the external pool of CO2 which results in a closed system in the seagrasses. The .delta.13C value of CO2 in sea H2O in isotopic equilibrium with HCO3- is -10.3.permill. and the .delta.13C value of hexoses isolated from the leaves of Thalassia is -11.5.permill.. In the closed system of the seagrasses there is a -1.2.permill. fractionation of CO2 by ribulose-1,5-bisphosphate carboxylase and the Calvin cycle. This contrasts to a fractionation of about -17 to -27.permill. of the stable carbon isotopes of CO2 by the Calvin cycle in the open system of terrestrial C3 plants where the internal pool of CO2 is in equilibrium with atmospheric CO2. Among C3 plants the seagrasses are very unusual in fixing CO2 by the Calvin cycle in a closed system. This closed system metabolism is analogous to the fixation of CO2 by the Calvin cycle in the bundle sheath cells of C4 plants where all of the 12CO2 and 13CO2 is fixed by ribulose-1,5-bisphosphate carboxylase. The reasons the .delta.13C values of seagrasses and C4 plants are similar are: the .delta.13C value of dissolved CO2 in seawater resembles the .delta.13C value of the C4 acids and the .delta.13C value of CO2 in the bundle sheath cells, and there is no fractionation of the stable carbon isotopes of CO2 in the closed systems of the seagrasses or the bundle sheath cells of C4 plants.