Discrimination betweenC andC by marine plants.

The natural abundanceC/C ratios (as δC) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO and HCO for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δC values for species able or unable to use HCO . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO had δC values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO had δC 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 CO to complicate the analysis. For these species, calculations based on the measured δC of the algae, the δC of CO in seawater, and the knownC/C discrimination of CO diffusion and RUBISCO carboxylation suggest that only 15-21% of the limitation to photosynthesisin situ results from CO 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 -using marine macroalgae the range of δC values measured can be accommodated by a CO efflux from algal cells which range from 0.306 of the gross HCO influx forEnteromorpha intestinalis (δC=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δC=-22.55‰). The relatively high computed CO efflux for those HCO -users with the more negative δC 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 δC values are also interpreted in terms of water movement regimes and obtaining CO from the atmosphere. Published δC values for freshwater macrophytes were compared with the ability of the species to use CO and HCO and again there was an apparent separation in δC values for these two groups. δC values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO or HCO .