Photosynthetic induction of dual phosphate uptake kinetics inPorphyra umbilicalis

The rates of phosphate uptake and photosynthesis were simultaneously determined in order to investigate the relationship between phosphate use and photosynthesis.Porphyra umbilicalis(L.) Kützing exhibited two different phosphate uptake kinetics. The first one followed a saturation model and was observed in light (maximum phosphate uptake rate. Vmax= 94 ± 30 nmol P1m−2s−1: semisaturation constant. S15= 4.0 ± 3.4 μM: phosphate compensation point. PCP = 0.3 ± 0.4 μMγ: the seeraid one was linear and worked at high external phosphate concentrations in the dark. Inhibition of photosynthesis by removing the inorganic carbon from the medium produced the same effect aa darkness on phosphate uptake. Successive bicarbonate additions produced increments of photosynthesis rate and the recovering of the phosphate uptake pattern observed in light. The results showed thatPorphyra umbilicalis, at the typical phosphate concentrations in its natural habitat, takes up phosphate in the light through the operation of a photosynthetically controlled active system.