Some characteristics of photosynthetic inorganic carbon uptake of a marine macrophytic red alga

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Abstract
Some characteristics of photosynthetic inorganic carbon uptake byPalmaria palmata, a marine red macroalga, have been measured under physiological conditions in artificial seawater. The apparent affinity of thallus for CO2[K1/2(CO2)] at pH 8.0 and 15°C was 21.4±3.0mmol m−3CO2under air, and 25.7±70mmol m−3CO2under N2. The corresponding values ofVmaxwere 2.98 ± 0.42 and 3.65±0.87 mmol O2evolved g Chr−1s−l. The apparent Km(CO2) of isolated ribulose bisphosphate carboxylase was determined at pH 8.0 and 30 °C to be 30.2 mmol m−3CO2, and the corresponding value of Vmaxwas 19.67 μniol CO2g protein−1s−1. The CO2compensation points of the thallus were measured in artificial seawater at pH 8.0 under air and N2, using a gas‐chromatographic method. The values were relatively low, rising from 10 cm3m−3at 15°C, to 35 cm3m−3at 25°C, but were not affected by the O2concentration. The lack of an effect of O2on photosynthesis and on compensation point indicates that there is little photorespiratory CO2loss in this macroalga. The high affinity of the thallus for CO2, and the low CO2compensation concentrations, are consistent with the occurrence of bicarbonate uptake in this alga.