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Gammaproteobacteria [
52
]. CSB can be found wherever reduced sulfur compounds
are available (e.g., in soils sediments, at aerobic/anaerobic interfaces in water, and
at volcanic sources such as the hydrothermal vents). CSB growing at neutral to
slightly alkaline pH values are found in soils, freshwater, and marine sediments
[
52
]. The acidophilic CSB are mainly found in acid mine-drainage water.
1.2 Properties and Toxicity of Hydrogen Sulfide
The toxicological effect of H
2
S was first described in 1713 by Ramazzini and
Scheele was the first one to synthesize H
2
S gas [
1
]. H
2
S is a small gaseous molecule
freely permeable through a membrane. Like CO and NO, H
2
S fulfills all of the
criteria to define a gasotransmitter of clinical relevance [
1
,
53
]. H
2
S is produced
endogenously by 3 enzymes in mammalian cells and plays important roles in
physiological and pathophysiological conditions [
1
].
H
2
S is the only product excreted from the dissimilatory sulfur metabolism of
SRB. If sulfate is the energetically stable form of sulfur under aerobic conditions,
H
2
S is the stable form under anaerobic reduced conditions. Intermediary oxidation
states of sulfur (such as sulfite, thiosulfate, and elemental sulfur) may be formed in
natural habitats by incomplete biological or chemical oxidation of sulfide, or during
anaerobic sulfide oxidation by purple and green sulfur microorganisms.
Under ambient temperature and pressure, H
2
S (CAS registry number: 7783-06-4)
is a colorless and flammable gas heavier than air (d
1.19), slightly soluble in water,
with a molecular weight of 34.08. The smell of H
2
S is characteristic of rotten eggs or
the obnoxious odor of a blocked sewer. The melting point of H
2
Sis
ΒΌ
82.3
C, its
86
C. H
2
S is a weak acid in
aqueous solution with an acid dissociation constant (p
K
a
)of6.76at37
C. H
2
Sisa
highly lipophilic molecule and can diffuse through cell membranes without facilita-
tion of membrane channels. The half-life of H
2
S in air varies from 12 to 37 hours.
Ambient air H
2
S comes from two different sources. Inorganic H
2
S sources
include volcanic gases, sulfur deposits, petroleum refinery, natural gas, manure
pits, pulp and paper mill industry, and sulfur springs. Organic H
2
S sources include
bacteria and decomposition of organic matters such as released from sewers, water
treatment plants, or septic tanks.
Sulfide is a well-known toxin with the potential to harm organisms through, for
example, mitochondrial depolarization [
54
], decreased hemoglobin oxygen affinity
[
55
], inhibition of twenty enzymes involved in aerobic metabolism [
56
], and
reversible inhibition of cytochrome
c
oxidase [
57
]. H
2
S is as toxic as hydrogen
cyanide (HCN) because of the capacity of their corresponding anions S
2
and CN
to coordinate and precipitate metal cations [
58
]. H
2
S is the primary toxic form of
the compound because of its ability to diffuse across cellular membranes.
60.3
C, and its freezing point is
boiling point is
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