We identified the secondary metabolite of NostocXPORK14A causing these pronounced effects on SynechocystisPCC6803 cells by inhibiting photosynthesis and growth. This compound, designated as M22, has a non-peptide structure. We propose that M22 possesses a dual action mechanism: first, by photo-generation of reactive oxygen species in the presence of light, which in turn affects the photosynthetic machinery of SynechocystisPCC 6803; and second, by altering the in vivo redox status of cells, possibly through inhibition of protein kinases.