Mechanistic interpretation of carbon isotope discrimination by marine macroalgae and seagrasses.

The literature, and previously unpublished data from the authors' laboratories, shows that the δC of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35‰. While some marine macroalgae have δC values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between -30 and -35‰). The organisms with very negative δC (lower than -30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δC values (higher than -10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δC value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δC values lower than -30‰ have pyrenoids. Previous work showed a good correlation between δC values lower than -30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δC values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δC values if diffusive conductance was relatively low. The photosynthesis of organisms with δC values more positive than -10‰ (i.e. more positive than the δC of CO2 in seawater) must involve HCO3 use.