Generation of reactive oxygen species upon strong visible light irradiation of isolated phycobilisomes from Synechocystis PCC 6803

The mechanism of photodegradation of antenna system in cyanobacteria was investigated using spin trapping ESR spectroscopy, SDS-PAGE and HPLC-MS. Exposure of isolated intact phycobilisomes to illumination with strong white light (3500 μmol m− 2 s− 1 photosynthetically active radiation) gave rise to the formation of free radicals, which subsequently led to specific protein degradation as a consequence of reactive oxygen species-induced cleavage of the polypeptide backbone. The use of specific scavengers demonstrated an initial formation of both singlet oxygen (1O2) and superoxide (O2−), most likely after direct reaction of molecular oxygen with the triplet state of phycobiliproteins, generated from intersystem crossing of the excited singlet state. In a second phase carbon-based radicals, detected through the appearance of DMPO-R adducts, were produced either via O2− or by direct 1O2 attack on amino acid moieties. Thus photo-induced degradation of intact phycobilisomes in cyanobacteria occurs through a complex process with two independent routes leading to protein damage: one involving superoxide and the other singlet oxygen. This is in contrast to the mechanism found in plants, where damage to the light-harvesting complex proteins has been shown to be mediated entirely by 1O2 generation.