environmental “engine” that sustains carbonate precipitation in lithifying microbial
mats. In addition, the mere presence of SRB cells, even metabolically inactive, may
favor CCP by providing heterogeneous nucleation sites. The role of SRB external
polymers (EPS) in affecting precipitation and crystal growth is discussed in
Sect. 5.4.
In complex natural environments, the different pathways from microbial
communities above discussed can also combine to induce precipitation. A specific
case of dolomite precipitation is described by Wright (1999): dead photosynthetic
cyanobacteria, together with other organic matter, were degraded by SRB, which
resulted in the production and release of DIC and ammonia. This was responsible
for an increase in pH and carbonate levels, which eventually lead to precipitation.
Some authors consider metabolic activities of heterotrophic bacteria to be the
most relevant mechanism in CCP. In heterotrophic bacterial communities, pathways
of carbonate precipitation always appear to be a response to organic matter enrichment (Castanier et al. 1999).
According to Hammes and Verstraete (2002), the precipitation mechanisms
mediated by metabolic pathways focus primarily on chemical changes induced in
the macroenvironment surrounding the bacteria and are general in nature. This
accounts for the common occurrence of BCCP and thereby, to some extent, neglects
the relevance of the precipitation event in regard to the precipitating organism and
its microenvironment. The latter aspect is discussed in Sect. 5.3.2.1.
5.3.2 Role of Calcium Carbonate Precipitation in Bacterial
Metabolism
Although a lot of knowledge has been acquired on how metabolic pathways can
influence calcium carbonate precipitation (Sect. 5.3.1), little is known on the
possible role of calcium precipitation in bacterial metabolism. Among prokaryotes,
there are no organisms with a specific function to precipitate CC, except for several
cyanobacteria (Zavarzin 2002). Although there is no apparent known role for CC in
bacterial metabolism (i.e., no calcite structures with a specific function seem to be
built by prokaryotes), few exceptions of CCP established role are found in stromatolite formation (Erlich 1996; Barton et al. 2001; Sect. 5.2). Certain organisms
precipitate calcite during their growth (Barton et al. 2001) and this suggests a
defined role for CCP. Unique among bacteria, Achromatium okaliferum contains
internal calcite inclusions during growth (Head et al. 1996).
This topic leads to a more general question on role of BCCP on bacterial physiology at the different scales of single bacterial cell, cell population, and microbial
communities in a complex microenvironment. In this view, the question “how
do bacteria precipitate CC” is strictly linked to “why do bacteria precipitate CC.”
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mats. In addition, the mere presence of SRB cells, even metabolically inactive, may
favor CCP by providing heterogeneous nucleation sites. The role of SRB external
polymers (EPS) in affecting precipitation and crystal growth is discussed in
Sect. 5.4.
In complex natural environments, the different pathways from microbial
communities above discussed can also combine to induce precipitation. A specific
case of dolomite precipitation is described by Wright (1999): dead photosynthetic
cyanobacteria, together with other organic matter, were degraded by SRB, which
resulted in the production and release of DIC and ammonia. This was responsible
for an increase in pH and carbonate levels, which eventually lead to precipitation.
Some authors consider metabolic activities of heterotrophic bacteria to be the
most relevant mechanism in CCP. In heterotrophic bacterial communities, pathways
of carbonate precipitation always appear to be a response to organic matter enrichment (Castanier et al. 1999).
According to Hammes and Verstraete (2002), the precipitation mechanisms
mediated by metabolic pathways focus primarily on chemical changes induced in
the macroenvironment surrounding the bacteria and are general in nature. This
accounts for the common occurrence of BCCP and thereby, to some extent, neglects
the relevance of the precipitation event in regard to the precipitating organism and
its microenvironment. The latter aspect is discussed in Sect. 5.3.2.1.
5.3.2 Role of Calcium Carbonate Precipitation in Bacterial
Metabolism
Although a lot of knowledge has been acquired on how metabolic pathways can
influence calcium carbonate precipitation (Sect. 5.3.1), little is known on the
possible role of calcium precipitation in bacterial metabolism. Among prokaryotes,
there are no organisms with a specific function to precipitate CC, except for several
cyanobacteria (Zavarzin 2002). Although there is no apparent known role for CC in
bacterial metabolism (i.e., no calcite structures with a specific function seem to be
built by prokaryotes), few exceptions of CCP established role are found in stromatolite formation (Erlich 1996; Barton et al. 2001; Sect. 5.2). Certain organisms
precipitate calcite during their growth (Barton et al. 2001) and this suggests a
defined role for CCP. Unique among bacteria, Achromatium okaliferum contains
internal calcite inclusions during growth (Head et al. 1996).
This topic leads to a more general question on role of BCCP on bacterial physiology at the different scales of single bacterial cell, cell population, and microbial
communities in a complex microenvironment. In this view, the question “how
do bacteria precipitate CC” is strictly linked to “why do bacteria precipitate CC.”
126
B. Perito and G. Mastromei