The dependence of the carbon concentrating mechanism of Palmaria palmata (L.) Kuntze on the growth light level was examined 1) to determine whether or not there is a threshold photon flux density (PFD) at which the inorganic carbon uptake mechanism can operate and 2) to attempt to quantify the relative energetic costs of acclimation to the two different limiting factors, PFD and dissolved inorganic carbon (DIC) concentration. Plants were grown at six PFDs: 5, 25, 50, 75, 95, and 125 mu mol photons . m(-2) . s(-1). Growth rates increased with increasing PFD from from 5 to 50 mu mol photons . m(-2) . s(-1) and were light-saturated at 75, 95, and 125 mu mol . photons m(-2) . s(-1). Values of delta(13)C increased continuously, with increasing growth PFD and did not saturate over the range of light bevels tested. Time-resolved fluorescence characteristics indicated a progressive photoacclimation below 50 mu mol photons . m(-2) . s(-1). Analysis of chlorophyll fluorescence induction showed three levels of light use efficiency associated with growth at 5 or 25, 50, and >75 mu mol photons . m(-2) . s(-1). The light-harvesting efficiency was inversely proportional to the effectiveness of DIC acquisition in plants grown at the six PFDs. These data were interpreted to indicate that there is a physiological tradeoff between photosynthetic efficiency and bicarbonate use in this species.