Geology Reference
In-Depth Information
and are at present biotic fundamental components
of natural biogeochemical cycles (Newman &
Banfield 2002). In this context bacterially mediated
biomineralization is crucial for the complex
interactions of biological, chemical and physical
processes. Discovery of the biosphere in deep sub-
seafloor has shown that microbial communities are
active in sediments of several million years of age
and up to several hundred metres in depth. It has
also been confirmed that the deep biosphere is sig-
nificantly involved in the global cycling of elements
and represents a major reservoir of organic carbon
(e.g. Pedersen 2000). On the other hand, the techno-
logical and environmental applications of bacterial
mineralization are now being demonstrated to be
far reaching (McIntosh & Groat 1997). For instance,
the use of biogenic minerals for the sequestration of
radionuclides and heavy metals in contaminated
sediments and groundwater offers considerable pot-
ential for environmental cleanup (Bhagat et al. 2004;
Haferburg et al. 2008). Long-lived radionuclides
neptunium and plutonium have been removed from
contaminated solutions using uranyl phosphate-
coated bacteria, with likely exchange of neptunyl
and plutonyl ions for uranyl ions within the crystal
lattice (Macaskie & Baskanova 1998). The effici-
ency of these microbiological-based strategies is
enhanced by the high capacity of biogenic minerals
to remove heavy metal if compared with those
produced abiotically. For instance, biogenic Mn
oxides have been shown to display a larger metal
binding capacity than well-crystallized synthetic
Mn oxides (Nelson et al. 1999). Bacterial mineraliz-
ation has also been recently proposed as a method
for the conservation of ornamental stone (Castanier
et al. 2000; Rodriguez-Navarro et al. 2003;
Webster & May 2006; Gonz´lez-Mu ˜oz 2008).
Due to the ubiquity and importance of bacteria-
mineral interactions, the number of publications
on this topic has grown steadily over the last
decades. Here, we will therefore focus on studies
of biomineralization by a microorganism, Myxo-
coccus. We have chosen to study these bacteria
because they display an unusual capacity for produ-
cing mineral precipitates of varying compositions
and morphologies. The following section presents
an overview of the recent history of bacterial miner-
alization and briefly describes which minerals are
most commonly precipitated in the presence of
bacteria. Current models on bacterial precipitation
of minerals are also described. The third section
presents a description of myxobacteria. Myxo-
coccus-induced precipitation of a number of phos-
phates, carbonates, sulphates, chlorides, oxalates,
and silicates is presented and discussed in the
fourth section. Finally, implications of bacterial
mineralization and perspectives for future research
are outlined.
The significance of bacterial
mineralization
Microorganisms have been recognized to influence
the formation of a wide variety of minerals such
as carbonates, oxides, sulphides, phosphates, sul-
phates, nitrates, halides and silicates (Ferris et al.
1986; Thompson & Ferris 1990; Schultze-Lam
et al. 1996; Fortin et al. 1997; P
´
sfai et al. 1998;
Castanier et al. 1999; Labrenz et al. 2000).
Although it is now beyond doubt that bacterial
activity and mineral formation are closely related
in a range of environments, the complex interplay
between biotic and abiotic processes in mineral
precipitation is only beginning to be appreciated
(Siering 1998). Current knowledge on how bacteria
induce or mediate biomineralization is far from
complete and the underlying causes of bacterial
mineralization are a matter of controversy. For
instance, some researchers believe that there is a
genetic control on bacterial mineral precipitation
(Barabesi et al. 2007), while others claim that bac-
teria induce/mediate mineral precipitation simply
as a result of their metabolic activity without any
kind of direct genetic control (Knorre & Krumbein
2000; Rodriguez-Navarro et al. 2007). As we will
see below, there is growing evidence showing that
the second line of thought could be correct.
The implications of microbial mineralization
are far reaching. Microbial mineralization may
help disclose how and when living organisms first
appeared on Earth and how they evolved. This is
particularly relevant for our growing understanding
of the interplay between geochemical and biological
processes. In addition to the ability of microbes
to precipitate minerals within, on or around cells,
they dramatically affect the speciation and distri-
bution of ions through redox reactions, through
release of organic and inorganic by-products and
by directly or indirectly changing the rates or
mechanisms of mineral weathering (Banfield et al.
1998). As a result, microbes are co-responsible for
the co-evolution of Earth surface and near surface
environments, including Earth climate and the influ-
ence of this climate on the development of higher
life forms (Schwartzman & Volk 1989).
The science of microbiologically controlled or
mediated geological processes, known as geomicro-
biology, has undergone substantial growth in recent
years (Ehrlich 2002). Early reviews on microbial
mineralization can be found in Ehrlich (2002),
Banfield & Nealson (1997) and McIntosh & Groat
(1997). Despite the amount of work done on the
precipitation of a single mineral by a number of
microbes, the production of many mineral phases
by a single microorganism has not yet been fully
explored. The latter is quite relevant if we are to
understand the ultimate mechanisms responsible
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