Fractionation of the stable isotopes of inorganic carbon by seagrasses.

The delta(13)C values for seagrasses collected along the Texas Gulf Coast range from -10.9 to -11.4 per thousand. These values are similar to the delta(13)C values of terrestrial C(4) plants, but seagrasses lack bundle sheath cells which are important in determining the delta(13)C values of C(4) plants. This work attempts to explain the reason the delta(13)C values of seagrasses resemble the delta(13)C values of C(4) plants.Investigations on the photosynthetic characteristics of seagrasses show that dissolved CO(2) is the species of inorganic carbon absorbed or accumulated by Thalassia testudinum. The rate of photosynthetic CO(2) fixation varies from 9.6 to 129.0 micromoles CO(2) per milligram chlorophyll per hour in the presence of 0.042 to 1.9 millimolar dissolved CO(2) due to the high resistances of Thalassia leaves to CO(2) diffusion. Phosphoglyceric acid is the first stable product of photosynthetic CO(2) fixation in Thalassia which is a Calvin cycle plant. The light/dark ratios of (14)CO(2) release from submerged Thalassia leaf sections at 1, 21, and 100% O(2) indicate a small apparent photorespiration. Dark respiration continues in the light and is stimulated by 21 and 100% O(2). The low apparent photorespiration may be due to membrane and H(2)O resistances to CO(2) diffusion with subsequent refixation of the photorespired CO(2). The internal pool of CO(2) is not in equilibrium with the external pool of CO(2) which results in a closed system in the seagrasses.The delta(13)C value of CO(2) in sea H(2)O in isotopic equilibrium with HCO(3) (-) is -10.3 per thousand and the delta(13)C value of hexoses isolated from the leaves of Thalassia is -11.5 per thousand. In the closed system of the seagrasses there is a -1.2 per thousand fractionation of CO(2) by ribulose-1,5-bisphosphate carboxylase and the Calvin cycle. This contrasts to a fractionation of about -17 to -27 per thousand of the stable carbon isotopes of CO(2) by the Calvin cycle in the open system of terrestrial C(3) plants where the internal pool of CO(2) is in equilibrium with atmospheric CO(2).Among C(3) plants the seagrasses are very unusual in fixing CO(2) by the Calvin cycle in a closed system. This closed system metabolism is analogous to the fixation of CO(2) by the Calvin cycle in the bundle sheath cells of C(4) plants where all of the (12)CO(2) and (13)CO(2) is fixed by ribulose-1,5-bisphosphate carboxylase. Therefore, the reasons the delta(13)C values of seagrasses and C(4) plants are similar are: (a) the delta(13)C value of dissolved CO(2) in seawater resembles the delta(13)C value of the C(4) acids and the delta(13)C value of CO(2) in the bundle sheath cells, and (b) there is no fractionation of the stable carbon isotopes of CO(2) in the closed systems of the seagrasses or the bundle sheath cells of C(4) plants.