Mechanistic interpretation of carbon isotope discrimination by marine macroalgae and seagrasses

The literature, and previously unpublished data from the authors' laboratories, shows that the δ¹³C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35‰. While some marine macroalgae have δ¹³C 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 δ¹³C (lower than -30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ¹³C values (higher than -10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ¹³C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ¹³C values lower than -30‰ have pyrenoids. Previous work showed a good correlation between δ¹³C values lower than -30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ¹³C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ¹³C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ¹³C values more positive than -10‰ (i.e. more positive than the δ¹³C of CO2 in seawater) must involve HCO3^- use.