The effect of acetate metabolism on the light energy distribution between the two photosystems, on the PS II/PS I stoichiometry and on the expression of psbA and psbB and psaA genes was investigated in the green alga, Chlamydobotrys stellata during autotrophic (CO(2)), mixotrophic (CO(2) plus acetate) and photoheterotrophic (only acetate) cultivation. It was observed that acetate assimilation in the glyoxylate cycle resulted in a large drop in the ATP content and a concomitant increase in the NADPH content of the cells. The combined effect of high NADPH concentration and linear electron transport brought about an over-reduction of the inter-photosystem electron transport components. The reduced state of the inter-photosystem components initiated a state 1/state 2 transition of LHC II and a decrease in the PS II/PS I ratio. The PS II/ PS I ratio was reduced because the synthesis of PS II reaction centers was repressed and that of the PS I reaction centers was slightly enhanced by acetate cultivation. The amount of PsbA and PsbB proteins of PS II and the abundance of psbA mRNA decreased. The abundance of PS I PsaA protein and psaAmRNA were only slightly increased. All of the acetate-induced effects were reversible when the cells were transferred back to an acetate-free medium. Our observations demonstrate that the expression of the PS II psbA and psbB and PS I psaA genes is regulated by the redox state of the inter-photosystem components at the transcriptional level. Experiments carried out in the presence of DBMIB which facilitates the reduction of plastoquinone pool indicate that the expression of genes encoding the components of PS II and PS I are controlled by the redox state of a component (cytochrome b/f complex) located behind the plastoquinone pool.