Chlorophyll a Fluorescence Response to Mercury Stress in the Freshwater Microalga Chlorella Vulgaris

The response of the freshwater microalga Chlorella vulgaris to mercuric ion ( Hg²⁺ ) stress was examined using chlorophyll a fluorescence image analysis and O-J-I-P analysis as a way to monitor the toxic effects of mercury on water ecosystems. The levels of photosynthetic pigments, such as chlorophyll a and b and carotenoids, decreased with increasing Hg²⁺ concentration. The maximum photochemical efficiency of photosystem II(Fv/Fm) changed remarkably with increasing Hg²⁺ concentration and treatment time. In particular, above 200μM\;Hg²⁺ , considerable mercury toxicity was seen within 2 h. The chlorophyll a fluorescence transient O-J-I-P was also remarkably affected by Hg²⁺ ; the fluorescence emission decreased considerably in steps J, I, and P with an increase in Hg²⁺ concentration when treated for 4 h. Subsequently, the JIP-test parameters (Fm, Fv/Fo, RC/CS, TRo/CS, ETo/CS, ΦPO , ΨO and ΦEO ) decreased with increasing Hg²⁺ concentration, while N, Sm, ABS/RC, DIo/RC and DIo/CS increased. Therefore, a useful biomarker for investigating mercury stress in water ecosystems, and the parameters Fm, ΦPO , ΨO , and RC/CS can be used to monitor the environmental stress in water ecosystems quantitatively.