C4-Metabolism in Marine Brown Macrophytic Algae

Abstract

Abstract Photosynthesis, D ark Fixation, β-Carboxylation, C 4-M etabolism , Brown Algae Marine brown macroalgae including Macrocystis integrifolia, Nereocystis luetkeana, Lessoniop-sis 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 β-carboxylation of phosphoenolpyruvate supplem enting photosynthetic CO2 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 approxim ately equal in the light and in the dark. Distribution of 14C between C 1 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 β-carboxylation of phosphoenolpyruvate is therefore assumed to be quantitatively and qualitatively integrated into photosynthetic CO2 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 term ed as C4 metabolism.