Use of chlorophyll fluorescence in metal-stress research: a case study with the green microalga Scenedesmus

Metal toxicity on the photosystem II (PS II) photochemistry of the green microalga Scenedesmus obliquus was investigated in vivo using a pulse-amplitude-modulated fluorometer. The results demonstrated that the test metals (copper, chromium, nickel, cadmium, and zinc) inhibited PS II photochemistry substantially, which was clearly evident for F0, Fv/Fm, qN, and qP following 12 h of metal exposure, whereas Fm, Fv/2, and F0/Fv depicted significant alteration after only 1 h of treatment. Thus, metals are not only found to affect the initial and maximal fluorescence, maximum quantum yield, photochemical and nonphotochemical quenching, and plastoquionone pool but the water-splitting apparatus was also significantly altered under metal stress. Among all these fluorescence characteristics, the F0/Fv registered the highest sensitivity to all the five test metals, thus indicating that the water-splitting apparatus of the oxidizing side of PS II is the primary site of action of toxic metals. This study therefore, concludes that F0/Fv ratio can be used as a powerful tool in metal-stress research.