We investigated the consequences of growth at Limiting photon flux density (PFD) for the uptake of inorganic carbon by 2 species of red seaweed which are able to use H C 0 3 for photosynthesis [Palmaria palmata (Huds.) Larnour and Laurencia pinnatifida (L.) Kuntze] and one which is dependent on diffusive uptake of bulk CO, [Lomentaria articulata (Huds.) Lyngb.] We attempted to induce energetic limitation of plants and/or reduce their demand for inorganic carbon by growing them at a low PFD and comparing their carbon use characteristics with those of plants grown at high PFD. There were significant increases in the carbon isotope discrimination (more negative delta values relative to the PDB standard) when the 3 species were grown at 25 relative to 75 pm01 photons m-2 S-'. In the 2 species which are able to use bicarbonate, the decreased SI3c was associated with lower initial slopes of inorganic carbon uptake curves. Taken together, the changes in 6I3C and dissolved organic carbon uptake indicated that light-limited P. palmata and L. pinnatifjda were not using low concentrations of inorganic carbon as efficiently as were plants grown at high PFD. This hypothesis was further supported by evidence that light limitation also caused a reduction in C:N ratio and in the % dry matter made up of carbon in those species. In contrast, growth of L. articulata, which is dependent on diffusive uptake of CO, for photosynthesis, was inhibited by the higher PFD and there were no significant effects of growth PFD on inorganic carbon uptake efficiency, C:N ratio or % C in this species. KEY WORDS: Laurencja pinnatifida-Lomentaria articulata. Palmaria palmata. CO2-HCO,-. Light limitation. Photosynthesis. Stable carbon isotope ratios 0 Inter-Research 1994 Resale of full article not permitted