Photochemical activity and components of membrane preparations from blue-green algae. I. Coexistence of two photosystems in relation to chlorophyll a and removal of phycocyanin

Nostoc muscorum (Strain 7119) cells were disrupted and the accessory pigment phycocyanin was removed from membrane fragments by digitonin treatment. The phycocyanin-depleted membrane fragments retained both Photosystem I and Photosystem II activity, as evidenced by high rates of NADP+ photoreduction either by water or by reduced 2,6-dichlorophenolindophenol, indicating that phycocyanin is not an essential component for electron transport activity. No separation of the two photosystems was effected by the digitonin treatment. Even drastic digitonin treatments failed to diminish significantly the remarkably stable electron transport from water to NADP+. Action spectra and relative quantum efficiency measurements demonstrated the existence of both Photosystem I and Photosystem II in membrane fragments which contained chlorophyll a as the only significant light-absorbing pigment.