The literature, and previously unpublished data from the authors' laboratories, shows that the delta(13)C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from -3 to -35parts per thousand. While some marine macroalgae have delta(13)C values ranging over more than 10parts per thousand 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 5parts per thousand (e.g. the red alga Plocamium cartilagineum which has values between -30 and -35parts per thousand). The organisms with very negative delta(13)C (lower than -30parts per thousand) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive delta(13)C values (higher than -10parts per thousand) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The delta(13)C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with delta(13)C values lower than -30parts per thousand have pyrenoids. Previous work showed a good correlation between delta(13)C values lower than -30parts per thousand and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low delta(13)C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher delta(13)C values if diffusive conductance was relatively low. The photosynthesis of organisms with delta(13)C values more positive than -10parts per thousand (i.e. more positive than the delta(13)C of CO2 in seawater) must involve HCO3- use.