Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 236

pH-dependent speciation, and its fast biological and chemical oxidation. Only the
protonated compound hydrogen sulfide (H 2 S) is volatile, whereas sulfide (S 2 )
dominates under alkaline conditions, and at neutral pH, most of inorganic sulfide is
present as bisulfide anion (systematically named sulfanide, HS ).
1.1 Overview of Bacteria and Archaea Associated
with Hydrogen Sulfide Metabolism
1.1.1 Sulfate-Reducing Bacteria and Archaea
Dissimilatory sulfate reduction (also known as anaerobic sulfate respiration) is an
essential step in the global sulfur cycle and is exclusively mediated by the sulfate-
reducing prokaryotes (SRP), a physiologically and phylogenetically versatile group
of microorganisms [ 2 , 3 ]. SRP are of major functional and numerical importance in
many ecosystems and they can grow under different physico-chemical conditions.
SRP are found in almost all ordinary environments on this planet: they are present
in geothermal areas and hot springs, soils, fresh, marine, brackish, and artesian
waters, estuarine muds, cyanobacterial microbial mats, oil and natural gas wells,
anaerobic sludge, digestive tracts of humans and animals [ 3 - 7 ]. Dissimilatory
sulfate reduction has evolved approximately 3.47 billion years ago [ 8 ] and
sulfate-reducing bacteria (SRB) should be considered as ancestral microorganisms,
which have contributed to the primordial biogeochemical cycle for sulfur as soon as
life emerged on the planet [ 9 ]. SRB contribute to the complete oxidation of organic
matter and participate through metal reduction and sulfide production to the overall
biogeochemistry of these extreme environments [ 2 , 10 ].
As of 2012, 65 genera containing 250 species of SRP have been isolated and
characterized [ 2 ]. They belong to five phyla within the Bacteria [the Deltaproteo-
bacteria (the most frequently represented lineage among SRB), the spore-forming
Desulfovirgula , Desulfotomaculum , Desulfurispora , Desulfosporomusa , and
Desulfosporosinus species within the phylum Firmicutes , the Thermodesulfovibrio
species within the phylum Nitrospirae and two phyla represented by Thermode-
sulfobium narugense and Thermodesulfobacterium/Thermodesulfatator species],
and two divisions within the Archaea [the euryarchaeotal genus Archaeoglobus
and the three crenarchaeotal genera Vulcanisaeta , Thermocladium , and Caldivirga ,
affiliated with the Thermoproteales ] (Table 1 )[ 3 , 6 , 7 , 11 - 13 ].
As of June 2013, a total of 101 genomes of SRP were available at the Integrated
Microbial Genomes website including 36 Desulfovibrio species, 10 Desulfoto-
maculum species, 8 Desulfobulbus species, 5 Desulfosporosinus species and
4 Archaeoglobus species.
SRP may have a heterotrophic, autotrophic, lithoautotrophic, or respiration-type of
life under anaerobiosis and their possible microaerophilic nature has also been
reported [ 2 , 3 , 10 , 14 ]. More than one hundred compounds including H 2 ,sugars,
amino acids, mono- and dicarboxylic acids, alcohols, and aromatic compounds are