DISCRIMINATION BETWEEN C-12 AND C-13 BY MARINE PLANTS

The natural abundance C-13/C-12 ratios (as deltaC-13) 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 deltaC-13 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 deltaC-13 values in the range -8.81 parts per thousand to -22.55 parts per thousand. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO2 had deltaC-13 values in the range -29.90 parts per thousand to -34.51 parts per thousand. 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 deltaC-13 of the algae, the deltaC-13 of CO2 in seawater, and the known C-13/C-12 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 mum. This value is reasonable for a Delesseria sanguinea frond in situ. For HCO3--using marine macroalgae the range of deltaC-13 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 (deltaC-13 = -8.81 parts per thousand) in a rockpool to 0.787 for Chondrus crispus (deltaC-13= -22.55 parts per thousand). The relatively high computed CO2 efflux for those HCO3--users with the more negative deltaC-13 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 deltaC-13 values are also interpreted in terms of water movement regimes and obtaining CO2 from the atmosphere. Published deltaC-13 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 deltaC-13 values for these two groups. DeltaC-13 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-.