Inhibition by glucose of chlorophyll a and phycocyanobilin biosynthesis in the unicellular red alga Galdieria partita at the stage of coproporphyrinogen III formation

Abstract

The repression effect of d-glucose on the pigment apparatus of photosynthesis in the unicellular acidophilic red alga Galdieria partita capable of heterotrophic growth was studied. The rate of cell growth under heterotrophic conditions was higher by more than one order of magnitude than that in the autotrophic ones, and the size of those cells was larger. The addition of 1% d-glucose to the growth medium caused an increase in the number of mitochondrion profiles in the cell and a decline of thylakoid number in the chloroplast of G. partita. The loss of thylakoid membranes is accompanied by a reduction of the contents of chlorophyll a. C-phycocyanin and allophycocyanin in the heterotrophic cells. Simultaneously, switching G. partita from autotrophic growth to heterotrophic nutrition led to the excretion of coproporphyrin(ogen) III into the growth medium. Thus, the presence of d-glucose caused a reduction of pigment contents in the cell, due to the inhibition of chlorophyll a and phycocyanobilin biosynthesis at the stage of the transformation of their universal precursor, coproporphyrinogen III, to protoporphyrinogen IX. In the mixotrophic culture of G. partita light induces both the biosynthesis of photosynthetic pigments and the excretion of coproporphyrin(ogen) III. It is supposed that light affects an earlier stage in the biosynthetic pathway of chlorophyll a and phycocyanobilin than does d-glucose.