[54,76,82] and the iron-sulfur center FB[83]. Electron paramagnetic resonance (EPR) experiments also
indicated that the reaction center of photosystem I is oxidized by mercury in the dark [80].
Photosystem II is also affected on both donor and acceptor sides by mercury [19]. The inhibition on
the acceptor side was proposed between the quinone acceptors QAand QB[84]. Most reports concentrated
on the inhibition on the donor side of the photosystem [54,85,86]. Accordingly, mercury was shown to
decrease variable fluorescence in cyanobacteria and isolated photosynthetic membranes [19,86–88]. The
inhibition on the donor side was studied in more detail using photosystem II submembrane fractions. It
was shown that the inhibition could be reversed by chloride ions that act as a cofactor for the oxygen-
evolving complex [88] and that mercury selectively removed the 33-kDa extrinsic polypeptide associated
with the oxygen-evolving complex [89]. Mercury also releases the Mn ions from the manganese cluster
of the water-oxidizing complex [80]. Those results associate an inhibitory active site of mercury more
closely with the oxygen-evolving complex. In an earlier report, mercury was shown to be an electron ac-
ceptor for photosystem II [90]. Those studies were performed in the presence of relatively high chloride
concentrations (30 mM NaCl and 2.5 mM MgCl 2 ) that prevented the inhibitory action at the oxygen-
evolving complex. Mercury was also shown to form metal-protein complexes in isolated photosystem II
submembrane fractions involving SH, CBO, and CMN amino acids groups [65]; this type of interaction
may be involved in the inhibitory process.
Studies have shown that the mercury-induced decline of variable fluorescence is enhanced under il-
lumination [91]. Inhibition by mercury was associated with an increased nonphotochemical component
of fluorescence quenching [15]. In the flagellate green alga Haematococcus lacustris, the rise of non-
photochemical quenching was comparable to the rise caused by photoinhibition [91]. Mercury was thus
proposed to increase the pH-independent rise of photoinhibitory quenching of chlorophyll fluorescence.
The alga could recover from mercury-enhanced photoinhibition only after a few days of illumination. The
recovery from the inhibition by mercury was not possible in the dark or in the presence of a chloroplast
protein synthesis inhibitor, indicating a possible connection of the inhibitory mode of action of mercury
with the turnover mechanism of the D1 polypeptide in the reaction center of photosystem II [92].
An action of mercury on the light-harvesting complex of photosystem II in the cyanobacteria Spir-
ulina platensiswas also suggested from the decline of the initial fluorescence Fo as mercury concentra-
tion was raised above 3 M [87]. A further increase in concentration above 18 M caused a strong
quenching of chlorophyll fluorescence. Fluorescence is also quenched in isolated chloroplasts [80]. Di-
rect interaction was also supported by Fourier transform infrared spectroscopy showing the formation of
organometallic complexes between mercury and photosystem II light-harvesting complexes isolated from
spinach, although those results were obtained at relatively high mercury concentrations [93]. Absorption
and fluorescence spectroscopy experiments in Synechococcushave also shown significant changes in the
chlorophyll spectral properties indicating a modification in the chlorophyll-protein complexes [94]. Mer-
cury also affected the spectral characteristics of the phycobilisomes in Spirulina platensis[95].
E. Lead
Lead is considered another important phytotoxic pollutant that is not essential for growth. In several stud-
ies in intact plants or leaves, lead was found to affect only minimally photosynthetic electron transport
[96–98]. However, some other reports indicated that the accumulation of lead in several plant species re-
duces the rate of photosynthetic reactions [99–101]. A reduction in the photosynthetic pigment composi-
tion has been observed [51,102,103]. Lead was also reported to disturb the granal structure of the chloro-
plasts [104]. A study of Parys and coauthors [105] indicated that the photochemical efficiency of
photosystem II is reduced by about 10% after a 2-h exposure of detached pea leaves to lead. However, the
primary inhibitory site is supposed to be at the level of the Calvin cycle enzymes [106]. It must be em-
phasized that lead is not very well translocated in plants and that its deleterious effects on photosynthesis
are seen only after prolonged exposure.
An early study of Miles et al. [107] indicated that lead inhibited photosystem II electron transport
without any effect on photosystem I. However, Wong and Govindjee [108] found that this metal directly
inhibited the reaction center of photosystem I in isolated maize chloroplasts. Lead is now considered to
influence both photosystems, although photosystem II is more sensitive [51]. This metal, assayed either
with intact detached leaves or with isolated photosynthetic membranes, produces a decline of variable
chlorophyll fluorescence indicating an inhibition on the donor side of photosystem II [19,105,109]. Ac-
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