“…The photosynthetic apparatus may also be affected at various levels of PSII, oxygen-evolving complex, PSI, plastocyanin and Ferredoxin/Ferredoxin NADP + oxidoreductase (Clijsters and Van Assche, 1985;. All these effects are usually pronounced even at low concentrations of Hg and short exposure times, as several authors have reported for cyanobacteria (Lu et al, 2000), algae (Rai et al, 1991), higher plant seedlings (Prasad and Prasad, 1987a;Munzuroğlu and Geckil, 2002) and submersed aquatic plants (Gupta and Chandra, 1998;Ali et al, 2000), which are less tolerant to the toxic effects of Hg than plants growing on the surface like the water hyacinth (Küpper et al, 1996). Küpper et al (1996) reported that heavy metals can easily substitute the Mg central atom of chlorophylls, and that Hg-substituted chlorophylls are unstable and unable to emit fluorescence, measured as F s , the steady-state fluorescence.…”