The researchers studied how different conditions affect the activity and protein loss in photosystem II (PSII) during photoinhibition. They found that PSII without manganese (Mn) showed both activity and D1 protein loss, while PSII without chloride or calcium had little D1 protein loss. Free radicals and singlet oxygen were produced during photoinhibition, with hydroxyl radicals dominating. Reconstituting the Mn cluster protected against photoinhibition, even without calcium, showing that Mn binding helps maintain the D1 protein structure. The extent of D1 protein loss during photoinhibition is not related to the water-splitting activity or the type of reactive oxygen species produced.