Sulfate-reducing bacteria are those bacteria that can obtain energy by oxidizing organic
compounds or molecular hydrogen while reducing sulfate (SO2−
4) to hydrogen sulfide.
In a sense, these organisms "breathe" sulfate rather than oxygen in a form of anaerobic
respiration.
Sulfate-reducing bacteria can be traced back to 3.5 billion years ago and are considered
to be among the oldest forms of microorganisms, having contributed to the sulfur cycle soon
after life emerged on Earth.
Many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell
components; this is known as assimilatory sulfate reduction.
By contrast, the sulfate-reducing bacteria considered here reduce sulfate in large amounts
to obtain energy and expel the resulting sulfide as waste; this is known as dissimilatory sulfate
reduction.
They use sulfate as the terminal electron acceptor of their electron transport chain.
Most of them are anaerobes.
Most sulfate-reducing bacteria can also reduce other oxidized inorganic sulfur compounds,
such as sulfite, thiosulfate, or elemental sulfur.
In addition, there are sulfate-reducing bacteria that can reduce fumarate, nitrate and nitrite,
iron) and some other metals, dimethyl sulfoxide and even oxygen.
Ecological importance and markers Sulfate occurs widely in seawater, sediment,
or water rich in decaying organic material.
Sulfate-reducing bacteria are common in anaerobic environments where they aid in the degradation
of organic materials.
In these anaerobic environments, fermenting bacteria extract energy from large organic
molecules; the resulting smaller compounds such as organic acids and alcohols are further
oxidized by acetogens and methanogens and the competing sulfate-reducing bacteria.
The toxic hydrogen sulfide is a waste product of sulfate-reducing bacteria; its rotten egg
odor is often a marker for the presence of sulfate-reducing bacteria in nature.
Sulfate-reducing bacteria are responsible for the sulfurous odors of salt marshes and
mud flats.
Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides.
These metal sulfides, such as ferrous sulfide, are insoluble and often black or brown, leading
to the dark color of sludge.
During the Permian–Triassic extinction event a severe anoxic event seems to have occurred
where these forms of bacteria became the dominant force in oceanic ecosystems, producing copious
amounts of hydrogen sulfide.
Uses Some sulfate-reducing bacteria can reduce
hydrocarbons such as benzene, toluene, ethylbenzene, and xylene, and have been used to clean up
contaminated soils.
Their use has also been proposed for other kinds of contaminations.
Sulfate-reducing bacteria are considered as a possible way to deal with acid mine waters
that are produced by other bacteria.
Problems caused by sulfate-reducing bacteria In engineering, sulfate-reducing bacteria
can create problems when metal structures are exposed to sulfate-containing water: Interaction
of water and metal creates a layer of molecular hydrogen on the metal surface; sulfate-reducing
bacteria then oxidize the hydrogen while creating hydrogen sulfide, which contributes to corrosion.
Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the biogenic sulfide
corrosion of concrete.
It also occurs in sour crude oil.
Some sulfate-reducing bacteria play a role in the anaerobic oxidation of methane:
CH4 + SO42– → HCO3– + HS– + H2O An important fraction of the methane formed
by methanogens below the seabed is oxidized by sulfate-reducing bacteria in the transition
zone separating the methanogenesis from the sulfate reduction activity in the sediments.
This process is also considered a major sink for sulfate in marine sediments.
In hydrofracturing fluids used to frack shale formations to recover methane, biocide compounds
are often added to water to inhibit the microbial activity of sulfate-reducing bacteria in order
to avoid anaerobic methane oxidation and to minimize potential production loss.
Biochemistry Before sulfate can be used as an electron
acceptor, it must be activated.
This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine
5'-phosphosulfate.
APS is subsequently reduced to sulfite and AMP.
Sulfite is then further reduced to sulfide, while AMP is turned into ADP using another
molecule of ATP.
The overall process, thus, involves an investment of two molecules of the energy carrier ATP,
which must to be regained from the reduction.
Phylogeny The sulfate-reducing bacteria have been treated
as a phenotypic group, together with the other sulfur-reducing bacteria, for identification
purposes.
They are found in several different phylogenetic lines.
As of 2009, 60 genera containing 220 species of sulfate-reducing bacteria were known.
Among the Deltaproteobacteria the orders of sulfate-reducing bacteria include: Desulfobacterales,
Desulfovibrionales and Syntrophobacterales.
This accounts for the largest group of sulfate-reducing bacteria, about 23 genera.
The second largest group of sulfate-reducing bacteria is found among the Firmicutes, including
the genera Desulfotomaculum, Desulfosporomusa, and Desulfosporosinus.
In the Nitrospirae division we find sulfate-reducing Thermodesulfovibrio species.
Two more groups that include thermophile sulfate-reducing bacteria are given their own phyla, the Thermodesulfobacteria
and Thermodesulfobium.
There are also three genera of Archaea known to be capable of sulfate reduction: Archaeoglobus,
Thermocladium and Caldivirga.
They are found in hydrothermal vents, oil deposits, and hot springs.
See also Microbial metabolism
Anaerobic respiration References