Chemistry Reference
In-Depth Information
reducing agents, and the type of iron source in the growth medium. In addition,
microorganisms clearly modify many of these parameters (e.g., pH, Eh) during growth.
For example, cells of
Desulfovibrio desulfuricans
produced greigite when grown in the
presence of ferrous salts but not when the iron source was goethite, FeO(OH) (Rickard
1969a).
Berner (1962, 1964, 1967, 1969) reported the chemical synthesis of a number of iron
sulfide minerals, including marcasite, mackinawite, a magnetic, cubic iron sulfide of the
spinel type (probably greigite), pyrrhotite, amorphous FeS, and even framboidal pyrite, a
globular form of pyrite that was once thought to represent fossilized bacteria (Fabricus
1961; Love and Zimmerman 1961). Rickard (1969a,b) concluded that extracellular,
biogenic iron sulfide minerals could not be distinguished from abiogenic (inorganic)
minerals. However, in many cases, the iron sulfide minerals produced by the sulfate-
reducing bacteria have not been systematically examined by high resolution electron
microscopy. In addition, in many of early studies, the role of the cell in mineralization
was not investigated.
More recent studies with sulfate-reducing bacteria show that mineralization proceeds
initially by the immobilization of amorphous FeS on the cell surface (Fig. 6) through the
ionic interaction of Fe
2+
with anionic cell surface charges and biogenic H
2
S (Fortin et al.
1994). Mineral transformations cause the production of other Fe sulfides, and eventually,
pyrite (Fortin and Beveridge 2000; Southam 2000). Despite the results of Berner (1962,
1964, 1967, 1969), the bacterially-induced transformation of FeS to pyrite appears to be
more efficient than that occurring under abiogenic conditions (Donald and Southam
1999).
Sulfide mineral oxidation
In addition to those bacteria that facilitate the mineralization of iron sulfides, there
are bacteria that can oxidize iron sulfides such as pyrite (FeS
2
) with molecular oxygen,
with release of Fe(III) and sulfate (SO
4
2−
) (Nordstrom and Southam 1997). This process
is responsible for acid mine drainage and has also been put to use in enrichment and
leaching of sulfide ores. The most studied organism is
Acidithiobacillus ferrooxidans
, an
Figure 6.
Unstained, ultrathin section transmission electron micrograph of a mineralized bacterial
microcolony from a sulfate-reducing bacterial consortium grown with lactate in the presence of Fe(II).
The cells in are encrusted with amorphous iron sulfides. Figure kindly supplied by W. Stanley and G.
Southam.
