The interaction of nitrite with photosynthetic electron transport under anaerobic conditions

Chlorella cells were examined in a modulated oxygen polarograph under aerobic and anaerobic conditions. At light intensities below about 600 ergs · cm^?2 · s^?1 of 650 nm light, the oxygen yield and phase lag are lower under anaerobic conditions. Addition of 25 mM sodium nitrite increases both these parameters to values close to those found in the presence of oxygen. It is proposed that nitrite is reduced by Photosystem I thus diverting electrons from the cyclic electron transport pathway. The intersystem electron transport chain becomes more oxidized and this suppresses a backflow of electrons to the oxidizing side of Photosystem II, hence increasing the oxygen yield and the phase lag. The removal of oxygen from the bathing medium also alters the response of dark adapted _Chlorella to a series of saturating light flashes. In terms of the Kok model of Photosystem II (Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457–475) there is a large increase in the parameter α. Addition of nitrite reverses this change and virtually restores the response seen in the presence of oxygen. The deactivation of the S₂ state is greatly speeded up in the absence of oxygen but the addition of nitrite again reverses this.