C4-Metabolism in Marine Brown Macrophytic Algae

Photosynthesis, Dark Fixation, /?-Carboxylation, C4-Metabolism, Brown Algae Marine brown macroalgae including Macrocystis integrifolia, Nereocystis luetkeana, Lessoniopsis littoralis, Laminaria saccharina, Fucus serratus and some further representatives of the Laminariales and Fucales (Phaeophyta) have been investigated with respect to their remarkably high potential for /?-carboxylation of phosphoenolpyruvate supplementing photosynthetic C 0 2 fixation. Kinetic tracer studies indicate that 14C-labelling of C4 acids such as aspartate and malate is not restricted to dark periods, but also occurs during photosynthesis. Rates of carbon fixation into C4 compounds are approximately equal in the light and in the dark. D istribution of 14C between C1 and C4 atoms of aspartate suggests carbon flow from early occurring photosynthates such as 3-phosphoglycerate to C4 compounds including aspartate and malate. In brown macro­ algae dark carbon fixation via /?-carboxylation of phosphoenolpyruvate is therefore assumed to be quantitatively and qualitatively integrated into photosynthetic C 0 2 assimilation thus yielding appreciable 14C-labelling of C4 dicarboxylic acids. The underlying reactions and conversions are basically different from C4 photosynthesis and should preferably be termed as C4 metabolism.