Calgary (09/25/04)- Researchers are unraveling the details of just
how certain types of bacteria are able to use iron in their metabolic processes,
and cause corrosion in oil pipelines. All this thanks to the recent sequencing
of the genome for Desulfovibrio vulgaris. This bacterium is a type
of anaerobic prokaryote known as sulfate-reducing bacteria (SRB), which changes
sulfate
to sulfide as part of its normal
metabolism and respiration.
SRBs
are found throughout the environment in soil, marshy lands and marine sediment.
They play a vital role in the global cycling and transport of sulfur and other
elements. Some varieties of SRB create a slimy biofilm on the inside of natural
gas and oil pipelines, creating a narrowing of the pipe as well as corrosion.
The corrosion, caused by the bacteria utilizing the metal for its sulfide
production, creates damage that is both extensive and costly in the oil and
natural gas industries.
The biofilms form because the bacteria are essentially fishing for food,
according to Gerrit Voordouw, PhD, professor of microbiology at the University
of Calgary. He points out that the inside of a pipeline has few nutrients
for bacteria, but they have adapted to become more efficient at harvesting
the sulfates they need.
"When an environment is nutrient poor and is flowing, it's a good idea to
attach to a surface. That way, you are exposed to more goodies than when
you flow with the flow," he said. He likened it to the way some bacteria
attach themselves to rocks in Alpine streams where almost pure water offers
little food. "If the bacterium was suspended in the stream, it would just
float with the water and not get any nutrients. But if it attaches to a rock,
forms a biofilm and the water flows past, it can sample a high amount of
water," he said.
What SRBs need from their environment are sulfates, which they chemically
turn into, or "reduce" into sulfide as part of their metabolic
and anaerobic processes. They also take up hydrogen atoms (some of which
are available from
the pipe itself) which are used on the reducing process.
In order to reduce sulfate into sulfide, the bacteria must facilitate a
chemical reaction that adds electrons to the sulfate. To do this, electrons
are taken from hydrogen or other donors such as iron in the pipe.
"If you put a piece of metallic iron into a culture of these bacteria
in a petri dish, it corrodes very rapidly," Dr. Voordouw said. The removal
of electrons weakens the metal, and the steps the bacteria took to accomplish
this were a mystery until now. Now that the genome has been sequenced (there
are more than 3.5 million base pairs), researchers can unravel the actual
mechanisms of how the bacteria can perform chemical reduction of metals
-- and weaken pipes. The things being learned about the D. vulgaris genome
are
applicable to other SRBs.
The genome sequencing project was spearheaded by researchers at The Institute
for Genomic Research (TIGR) in Rockville, Maryland, and Dr. Voordouw was
one of two Canadian members on the team. Researchers found there is a specific
group of proteins that allow the organism to use iron as a source of electrons
for its metabolic processes. These proteins are a specialized group of c-type
cytochromes which facilitate the transfer of electrons (from the metal to
sulfate) and cause the chemical reduction of metals.
Identifying the proteins is important because there are a numerous practical
applications for the information, in addition to finding ways to protect
pipelines and equipment. For one, the ability of SRBs to reduce metals has
implications
for cleaning-up land contaminated with toxic metals such as chromium and
uranium.
"These elements have a water soluble form that can migrate easily throughout
the environment. They also have an insoluble form that does not migrate very
well," he said. The water-soluble form of the toxic metals gets into
the water table and the contamination can spread for a long distance, while
insoluble
forms
are
not
transported through the environment as easily.
The insoluble forms of metal ions have been chemically reduced, that is
have had electrons added to them. These additional electrons change the toxic
metals' chemical
features making them less reactive. The key for the field of bioremediation
is that SRBs can chemically reduce metal contaminants.
"By reducing these metal ions (such as chromium) into their more insoluble
forms, SRBs can contribute to a decrease in the spread of these metal ions
in the environment," he said. SRBs won't get rid of metal contaminants, but
they do alter some of their chemical characteristics for the better.
end
* Image is of pipe corroded by SRB. Non-corroded is smooth.
Courtesy of Dr. Voordouw.
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