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Discrimination between 12C and 13C by Marine Plants

S. C. Maberly, J. A. Raven and A. M. Johnston
Oecologia
Vol. 91, No. 4 (1992), pp. 481-492
Published by: Springer in cooperation with International Association for Ecology
Stable URL: http://www.jstor.org/stable/4220100
Page Count: 12
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Discrimination between 12C and 13C by Marine Plants
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Abstract

The natural abundance 13C/12C ratios (as δ 13C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO3 - for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ 13C values for species able or unable to use HCO3 -. Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO3 - had δ 13C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ 13C values in the range -29.90‰ to -34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ 13C of the algae, the δ 13C of CO₂ in seawater, and the known 13C/12C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15-21% of the limitation to photosynthesis in situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79-85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species, Delesseria sanguinea, by incorporating data on in situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17-24 μm. This value is reasonable for a Delesseria sanguinea frond in situ. For HCO3 --using marine macroalgae the range of δ 13C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO3 - influx for Enteromorpha intestinalis (δ 13C = -8.81‰) in a rockpool to 0.787 for Chondrus crispus (δ 13C = -22.55‰). The relatively high computed CO₂ efflux for those HCO3 --users with the more negative δ 13C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ 13C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ 13C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO3 - and again there was an apparent separation in δ 13C values for these two groups. δ 13C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO3 -.

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