IMPLICATIONS OF C-13 NATURAL-ABUNDANCE MEASUREMENTS FOR PHOTOSYNTHETIC PERFORMANCE BY MARINE MACROPHYTES IN THEIR NATURAL-ENVIRONMENT

Marine macroalgae and seagrasses collected from Penguin Island, Western Australia, and red marine macroalgae from Australasia and Europe were analysed for the natural abundance of stable isotopes in their organic matter. These measurements revealed 1 species of green macroalgae and 9 species of red macroalgae with a delta(13)C below -30 parts per thousand. These new observations bring the total of reports of wild-collected marine macrophytes with delta(13)C values below -30 parts per thousand to 3 species of ulvophycean chlorophytes and 22 species of florideophycean rhodophytes. The 22 rhodophyte species are in 18 genera in 10 families of 4 orders. Marine algae with very negative delta(13)C values have been shown in other work to be unable to use HCO3- and rely on CO2 diffusion into the thallus for photosynthesis. The quantitative implications of the low C-13/C-12 ratio and the inability to use HCO3- were analysed to predict the maximum in situ rate of photosynthesis. The relatively low predicted rates agree with measured rates (by other workers) of C assimilation in photosynthesis and growth. These low potential rates of C acquisition can be related to the low mean photon flux density required for growth in 17 of the 22 species of red algae and all 3 species of green algae. Low mean photon flux densities are characteristic of habitats in the subtidal (18 species of red algae, 3 species of green algae) and shaded microhabitats in the intertidal (1 species of red algae). Even the 5 other algae, littoral and infralittoral, seem from the literature to have low photosynthetic and relative growth rates, although it is not clear whether the low metabolic rates result from dependence on CO2 diffusion or whether diffusive CO2 entry as the sole means of inorganic C supply is permitted by a low metabolic rate imposed by some other cause.