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Articles

Fractionation of the Stable Isotopes of Inorganic Carbon by Seagrasses ¹

Department of Plant Sciences, Texas A&M University, College Station, Texas 77843

The ¹³C values for seagrasses collected along the Texas Gulf Coast range from –10.9 to –11.4. These values are similar to the ¹³C values of terrestrial C₄ plants, but seagrasses lack bundle sheath cells which are important in determining the ¹³C values of C₄ plants. This work attempts to explain the reason the ¹³C values of seagrasses resemble the 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 to 129.0 micromoles CO₂ per milligram chlorophyll per hour in the presence of 0.042 to 1.9 millimolar dissolved CO₂ due to the high resistances of Thalassia leaves to CO₂ diffusion. Phosphoglyceric acid is the first 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 resistances 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 ¹³C value of CO₂ in sea H₂O in isotopic equilibrium with HCO₃^– is –10.3 and the ¹³C value of hexoses isolated from the leaves of Thalassia is –11.5. In the closed system of the seagrasses there is a –1.2 fractionation of CO₂ by ribulose-1,5-bisphosphate carboxylase and the Calvin cycle. This contrasts to a fractionation of about –17 to –27 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. Therefore, the reasons the ¹³C values of seagrasses and C₄ plants are similar are: (a) the ¹³C value of dissolved CO₂ in seawater resembles the ¹³C value of the C₄ acids and the ¹³C value of CO₂ in the bundle sheath cells, and (b) 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.

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