Glycolate metabolism in green algae

Using C-14-labelled substrates, the succession of the single steps in the glycolate metabolism was investigated in Mougeotia scalaris and Eremosphaera viridis, which, within the group of green algae, are representatives of the evolutionary lines of Charophyta and Chlorophyta, respectively. In both algae the same metabolites are formed as in higher plants, although in Eremosphaera, which in contrast to Mougeotia does not possess leaf peroxisomes, all reactions are exclusively mitochondrial. Concomitant with the oxidation of glycolate, the synthesis of ATP was demonstrated in Eremosphaera. Formation of tartronic semi-aldehyde or other products different from those in land plants could not be demonstrated in either of these algae. Excretion of glycolate by Mougeotia and Eremosphaera is enhanced by decreasing the CO2 concentration as well as by increasing the light intensity, but is completely stopped about 14 h later. Whereas increasing enzyme activities of the glycolate pathway apparently reduces glycolate excretion in Mougeotia, activation of CO2 pumps seems to be the dominant reaction to prevent glycolate excretion in Eremosphaera. Mesostigma viride is one of the phylogenetically oldest algae in the group of Charophyceae. As this alga has already been demonstrated to contain microbodies with enzymes of leaf peroxisomes, the peroxisomal glycolate pathway must have originated at a very early stage. Surprisingly, the organelles from Mesostigma contain also the glyoxysomal marker enzyme isocitrate lyase suggesting these microbodies to be prototypes from which both glyoxysomes and leaf peroxisomes evolved.