Quantitative Determination of the Unstirred Layer Permeability and Kinetic Parameters of RUBISCO inLemanea mamillosa

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Abstract
The average thickness of the unstirred layer around sections of the red freshwater macroalga Lemanea mamillosa in laboratory conditions has been quantified using benzoic acid models of the alga. Depending on the apparatus, unstirred layers for CO2 ranged from 35 to 70 μm in thickness at 15°C. For young plants, this is the main constraint on carbon fixation at low concentrations of CO2 and saturating light. The CO2-saturated activity of ribulose bisphosphate carboxylase (RUBISCO carboxylase) extracted from the alga is sufficient or in excess of that required to account for the light- and CO2-saturated rate of photosynthesis in vivo when both activities are measured at 15 °C and expressed per unit of chlorophyll a. The KM(CO2) is 4.7 mmol m−3 at 15°C and low O2, comparable with other aquatic plants when the effects of oxygen, temperature, and ionic strength are taken into account. At the beginning of the growing season, the Briggs-Maskell equation provides a good description of the C fixation-CO2 concentration relationship of the alga. The development of additional photosynthetic capacity per unit volume of plant toward the end of the growing season involves growth of the cortex of the gametophyte and of the carposporophyte. This growth also increases the pathlength of inorganic C transport in the plant and results in significant additional transport limitations on photosynthesis, apart from the unstirred layer. Plants late in the season show some evidence of HCO3- use.