Microbially Induced Iron Oxidation: What, Where, How Page: 4 of 11
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The study of Geomicrobiology is not new and applies to many aspects of our
lives. Since the "study of microbial processes currently taking place in the modern
sediments of various bodies of water, in ground water circulating through sedimentary
and igneous rocks, and in weathered earth crust ... [and also] the physiology of
specific microorganisms taking part in presently occurring geochemical processes"
(Ehrlich, 1996), geomicrobiology impacts our water supply, rock formations, and the
corrosion of metals. Among the first contributors to the field of geomicrobiology was
Ehrenberg with his discovery of the association between Gallionella ferruginea and
iron (Ehrlich, 1996). Another important contribution was made by Winogradsky who
realized that Leptothrix ochracea could oxidize FeCO3 to ferric oxide (Ehrlich, 1996).
These and other studies led to the investigation of microbial iron oxidation and
precipitation (Ehrlich, 1996).
Both Gallionella and Leptothrix are neutrophilic iron-oxidizing bacteria. They
come in many varieties and can occupy many different niches of the environment.
These bacteria thrive in conditions with a pH range of 6.0-8.0 (Ehrlich, 1996; Mulder &
Deinema, 1992) although they can survive in a more acidic or basic environment.
They can also be autotrophic or mixotrophic. Among the most common types of
neutrophilic iron-oxidizing bacteria are Gallionella, Leptothrix, Sphaerotilus,
Crenothrix, and Siderococcus. They can bind and precipitate ferric iron onto their cell
surfaces or sheaths (Ehrlich, 1996).
Gallionella ferruginea is the most common species in the genus Gallionella. It
derives its energy from enzymatic iron oxidation (Ehrlich, 1996) of preformed organic
chemicals (ferrous iron) and its carbon from C02 (Boyd, 1988). This bacteria is a
gradient organism that develops at the anoxic-oxic interface where conditions are
neither strongly reducing nor highly oxidizing (Hanert, 1992). It occurs in
environments where water is moving from an anoxic into an oxic region (Emerson &
Moyer, 1997), and ferrous ion content is exposed to sufficient oxygen to satisfy growth
requirements (Kucera & Wolfe, 1957). Gallionella is usually found in iron-bearing
waters in cool climates, such as cool springs and brooks, wells, and pipe lines
(Breed et al., 1957; Skerman, 1967). Its cells are usually kidney or bean-shaped but
they may also be rod-shaped. It secretes a ribbon-like stalk which grows from the cell
in a twisted manner to give Gallionella its differentiating characteristic (Breed et al.,
1957; Emerson & Moyer, 1997; Kucera & Wolfe, 1957; Skerman, 1967). These stalks
may have a multitude of strands twisting together at nodes resulting in a "string of
pearls" like appearance.
The genus Leptothrix is characterized by trichomes of cylindrical or rod-shaped,
colorless cells within a sheath encrusted with iron or manganese oxide (Breed et al.,
1957; Skerman, 1967). This bacteria is unicellular but usually occurs in chains
(Skerman, 1967) and is mostly found in fresh water (Breed et al., 1957). Leptothrix is
often confused with the genus Sphaerotilus as they are very similar in morphology.
Sphaerotilus cells can be either free-floating or attached in chains or trichomes
(Breed et al., 1957). They are colorless rods or ellipsoids, are surrounded by a firm
sheath, and are found in stagnant or running water (Breed et al., 1957). While these
two genera are very similar, there are some differences between them. First, some
Leptothrix strains have the ability to oxidize manganese, Sphaerotilus does not
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SCHIERMEYER,ELISA M.; PROVENCIO,PAULA P. & NORTHUP,DIANA E. Microbially Induced Iron Oxidation: What, Where, How, article, August 15, 2000; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc717568/m1/4/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.