Discrimination between12C and13C by marine plants.

The natural abundance13C/12C ratios (as δ13C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO2 and HCO 3- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ13C values for species able or unable to use HCO 3- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO 3- had δ13C 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 δ13C 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 δ13C of the algae, the δ13C of CO2 in seawater, and the known13C/12C discrimination of CO2 diffusion and RUBISCO carboxylation suggest that only 15-21% of the limitation to photosynthesisin 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 onin 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 aDelesseria sanguinea frondin situ. For HCO 3- -using marine macroalgae the range of δ13C values measured can be accommodated by a CO2 efflux from algal cells which range from 0.306 of the gross HCO 3- influx forEnteromorpha intestinalis (δ13C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ13C=-22.55‰). The relatively high computed CO2 efflux for those HCO 3- -users with the more negative δ13C 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 δ13C values are also interpreted in terms of water movement regimes and obtaining CO2 from the atmosphere. Published δ13C values for freshwater macrophytes were compared with the ability of the species to use CO2 and HCO 3- and again there was an apparent separation in δ13C values for these two groups. δ13C 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 HCO 3.