Evidence for a plasmalemma-based CO₂ concentrating mechanism in Laminaria saccharina

A kinetic analysis of the photosynthesis inhibition by buffers allowed quantification of some components of the carbon concentrating mechanism (CCM) of the brown macroalga Laminaria saccharina. The CCM was based on the presence of acid regions outside the plasma membrane that increased the CO₂ concentration available for photosynthesis by 10-20 times above that of the bulk medium at alkaline pH. Furthermore, the results suggested that the CCM is located mainly on the cell membrane and not in the chloroplast, as suggested for most macroalgae. The degree of dissipation of the acid regions by a buffer was related to the buffer anion concentration (B-), estimated from the titration of the buffer from bulk medium pH to a pH endpoint value close to the first pK a of the carbonic acid system. A kinetic model describing the relationship between inhibition of photosynthesis by a buffer and B- was developed suggesting that buffers act as competitive inhibitors with IC₅₀ (the concentration of the buffer anion which reduces the reaction velocity by half) of 5.0 mol m-³. This model can be used to estimate the inhibitory effect of any buffer on the photosynthesis of L. saccharina. Nevertheless, some buffers tested showed a lower effect than that predicted from the hyperbolic model suggesting that their strength as inhibitors depended on: (1) the pK a in relation to the first pK a of the carbonic acid system and (2) its molecular weight (i.e. its mobility).