External and Internal CO2Transport inLemanea: Interactions with the Kinetics of Ribulose Bisphosphate Carboxylase

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Abstract
The rate of photosynthesis by the freshwater alga Lemanea mamillosa is proportional to CO2 concentration, virtually to the point of saturation, and inversely proportional to the radius of the thallus. By contrast, the CO2 response curve of very thin slices of the thallus is a rectangular hyperbola with a (lower) half saturation concentration of 10 mmol m−3. For the intact plant, the kinetics of CO2 fixation are strongly masked by internal CO2 transport limitations, although the maximum rate of photosynthesis is probably determined by the rate of supply of ribulose bisphosphate (RuBP). The flow of water over the alga becomes turbulent at water velocities greater than about 90 mm s−1 and the thallus stretches significantly at higher water velocities. In its natural habitat, therefore, the external unstirred layer will be thin (< 10 μm) and the thallus will be stretched, leading to rapid external and increased internal rates of CO2 transport from the bulk solution. The estimated maximum rate of CO2 transport is commensurate with the maximum rate of photosynthesis (i.e. the rate of supply of RuBP).