Involvement of the donor tyrosine-D1 (Y d) in Photosystem II electron transport in the green alga, Chlamydobotrys stellata.

The light-induced oxidation of the accessory donor tyrosine-D (YD) has been studied by measurements of the EPR Signal IIslow at room temperature in the autotrophically and photoheterotrophically cultivated alga Chlamydobotrys stellata. After illumination and dark adaptation, YD Signal IIslow was observed only in autotrophic algae, i.e. under conditions of a linear photosynthetic electron transfer from water to NADP(+). The addition of artificial electron acceptors phenyl-p-benzoquinone (PPQ) or dichloro-p-benzoquinone (DCQ) to the autotrophic cells caused an almost negligible increase of this signal. When photosynthetic electron flow and oxygen evolution were diminished by removal of the carbon source CO2 and addition of acetate (photoheterotrophy), a pronounced YD Signal IIslow was seen only in presence of DCQ or PPQ. Several possibilities are discussed to explain the absence of YD Signal IIslow in photoheterotrophic Chl. stellata such as the existence of a cyclic PS II electron flow very effectively reducing P680 and thereby preventing the possibility of YD oxidation. Artificial electron acceptors withdraw electrons from this cycle thus keeping the primary quinone acceptor, QA, oxidized and thereby diminishing the reduction of P680 (+) by cyclic PSII. This leads to the appearance of the YD Signal IIslow also in the photoheterotrophically grown algae.