624
K. M. G. Mostofa et al.
(Mostofa et al. 2009, 2011). After formation of the strong π–electron bonding
system in the metal-protein (or organism) complex, the remaining outer-shell
electrons (e.g. 4s 2 for As) are loosely bound and can easily move (see chapter
“Complexation of Dissolved Organic Matter With Trace Metal Ions in Natural
Waters” for detailed explanation). After complex formation between metals and
proteins (or amino acids) in PSII, the normal cells metabolism can be disrupted by
electrons in the outer shell of the metal ion itself, or via HO
• formation in Fenton
or Fenton-like or other unknown processes, finally leading to cell death.
Interestingly, As-protein complexes may be accumulated in the human skin and,
when the skin is exposed to natural sunlight (mostly UV-light), irradiation induces
the formation of HO
• or other reactive oxygen species (e.g. O 2
•− and H 2 O 2 ).
These species can cause damage to DNA and finally induce cancer in the human
body. Coherently, it has been suggested that DNA damage induced by methylated
trivalent arsenicals is mediated by reactive oxygen species (Nesnow et al. 2002).
Furthermore, arsenite can play a role in the enhancement of UV-induced skin cancers (Rossman et al. 2004). The carcinogenic effects may be connected with accumulation of As 3+ or Sb 3+ and other toxic metals. As 3+ or Sb 3+ have two electrons
in the outer shell, while their inner shells are entirely filled with electrons. This situation makes the outer-shell electrons of metal-protein complexes highly mobile.
As and cigarette smoke are synergistic, producing an elevated risk of bladder
and lung cancer in smokers (Hopenhayn-Rich et al. 1998; Steinmaus et al. 2003;
Chen et al. 2004). Smoking could help promoting the excitation of electrons from
the outer shells of As in As-protein complexes, which can finally induce formation of HO • that damages DNA. While eating soil is quite unusual for humans, in
some rural Bangladesh villages it is customary for pregnant women to eat shikal
(it sounds like “chain” in English), which consists of small (2 in. × 1 in. × 1/2 in.)
pellets made out of flooded soil (information source is personal experience of one
of the authors). While the origin of this tradition is quite uncertain, it is noteworthy
that it was observed in one of the world’s regions where human exposure to As
is the highest. Interestingly, intake of black soil (dark brown soil) with high contents of humic acid could reduce As levels in the human body, because complexes
between As and humic acids are much stronger than As-protein or As-fulvic acids
ones (see chapter “Complexation of Dissolved Organic Matter With Trace Metal
Ions in Natural Waters” for detailed discussion).
Coming back to photosynthetic microorganisms, the decrease in photosynthetic
efficiency can be cause by complex formation between metals and the functional
groups of PSII. Two possible mechanisms can be operational. First, the formed
metal-cells or metal-proteins complexes in the PSII of aquatic microorganisms can
produce electrons (e − ) photolytically upon exposure to sunlight, which can finally
lead to H 2 O 2 generation. (see chapter “Photoinduced and Microbial Generation of
Hydrogen Peroxide and Organic Peroxides in Natural Waters” for detailed mechanism) (Komissarov 2003; Mostofa and Sakugawa 2009; Mostofa et al. 2011;
Fujiwara et al. 1993). Coherently, it has been observed that methyl viologen acts
as an inducer of photo-oxidative stress (Donahue et al. 1997; Mano et al. 2001)
and can induce the photoreduction of dioxygen (O 2 ) by accepting electrons from
K. M. G. Mostofa et al.
(Mostofa et al. 2009, 2011). After formation of the strong π–electron bonding
system in the metal-protein (or organism) complex, the remaining outer-shell
electrons (e.g. 4s 2 for As) are loosely bound and can easily move (see chapter
“Complexation of Dissolved Organic Matter With Trace Metal Ions in Natural
Waters” for detailed explanation). After complex formation between metals and
proteins (or amino acids) in PSII, the normal cells metabolism can be disrupted by
electrons in the outer shell of the metal ion itself, or via HO
• formation in Fenton
or Fenton-like or other unknown processes, finally leading to cell death.
Interestingly, As-protein complexes may be accumulated in the human skin and,
when the skin is exposed to natural sunlight (mostly UV-light), irradiation induces
the formation of HO
• or other reactive oxygen species (e.g. O 2
•− and H 2 O 2 ).
These species can cause damage to DNA and finally induce cancer in the human
body. Coherently, it has been suggested that DNA damage induced by methylated
trivalent arsenicals is mediated by reactive oxygen species (Nesnow et al. 2002).
Furthermore, arsenite can play a role in the enhancement of UV-induced skin cancers (Rossman et al. 2004). The carcinogenic effects may be connected with accumulation of As 3+ or Sb 3+ and other toxic metals. As 3+ or Sb 3+ have two electrons
in the outer shell, while their inner shells are entirely filled with electrons. This situation makes the outer-shell electrons of metal-protein complexes highly mobile.
As and cigarette smoke are synergistic, producing an elevated risk of bladder
and lung cancer in smokers (Hopenhayn-Rich et al. 1998; Steinmaus et al. 2003;
Chen et al. 2004). Smoking could help promoting the excitation of electrons from
the outer shells of As in As-protein complexes, which can finally induce formation of HO • that damages DNA. While eating soil is quite unusual for humans, in
some rural Bangladesh villages it is customary for pregnant women to eat shikal
(it sounds like “chain” in English), which consists of small (2 in. × 1 in. × 1/2 in.)
pellets made out of flooded soil (information source is personal experience of one
of the authors). While the origin of this tradition is quite uncertain, it is noteworthy
that it was observed in one of the world’s regions where human exposure to As
is the highest. Interestingly, intake of black soil (dark brown soil) with high contents of humic acid could reduce As levels in the human body, because complexes
between As and humic acids are much stronger than As-protein or As-fulvic acids
ones (see chapter “Complexation of Dissolved Organic Matter With Trace Metal
Ions in Natural Waters” for detailed discussion).
Coming back to photosynthetic microorganisms, the decrease in photosynthetic
efficiency can be cause by complex formation between metals and the functional
groups of PSII. Two possible mechanisms can be operational. First, the formed
metal-cells or metal-proteins complexes in the PSII of aquatic microorganisms can
produce electrons (e − ) photolytically upon exposure to sunlight, which can finally
lead to H 2 O 2 generation. (see chapter “Photoinduced and Microbial Generation of
Hydrogen Peroxide and Organic Peroxides in Natural Waters” for detailed mechanism) (Komissarov 2003; Mostofa and Sakugawa 2009; Mostofa et al. 2011;
Fujiwara et al. 1993). Coherently, it has been observed that methyl viologen acts
as an inducer of photo-oxidative stress (Donahue et al. 1997; Mano et al. 2001)
and can induce the photoreduction of dioxygen (O 2 ) by accepting electrons from