occur in which microbial cells (inactive living or dead) behave as solid-phase
sorbents of dissolved metals, and heterogeneous nucleation templates for
authigenic mineral deposition (Beveridge 1989). Functional groups on the
B. subtilis cell wall were considered able to bind dissolved Ca
2+ in a calcite
dissolution study with dead cells (Friis et al. 2003). B. subtilis dead cells, as well
as a cell fraction comprising the cell wall, were demonstrated able to induce calcite
formation in a laboratory test, acting as heterogeneous crystallization nuclei, with
an applicative potential in the reinforcement of monumental calcareous stones
(Barabesi et al. 2003). Precipitation occurring onto surface of inactive or dead
cells as well as onto isolated cell fractions has to be considered as microbiologically
influenced mineralization, according to Dupraz et al. (2009; Sect. 5.2.1).
Other bacterial cell surface components are known to favor precipitation and
refer, in almost all cases, to extracellular polymeric substances or EPS.
EPS is a broad term that groups a large variety of organic polymers secreted
by microbial cells in the environment. Polysaccharides are the major component
of most EPS, commonly in combination with polypeptides, nucleic acids,
phospholipids, and other polymeric compounds (Decho 2010). According to
Costerton et al. (1995), the organic EPS matrix can be considered as an extension
of the microbial cell. Microbial EPS can bind and accumulate ions. The interactions
between metal ions and EPS are mediated by several functional groups (such as
carboxylic acids or amino groups) present in the EPS matrix. Deprotonation of
functional groups takes place when the pH increases, providing a negative charge to
the polymer. In addition to sugar monomers, the EPS matrix may include
noncarbohydrate acidic moieties such as sulfate or phosphate, which also contribute
to the overall negative charge of the EPS (Braissant et al. 2007).
The role of EPS in CC nucleation and growth is well documented in laboratory
experiments on bacterial cultures or isolated EPS (Braissant et al. 2007; Ercole et al.
2007; Tourney and Ngwenya 2009) as well as in natural environments such as
microbial mats (Dupraz et al. 2009)
Bacterial EPS have been shown to be involved in the process of biocalcification
by entrapping ions and serving as a nucleation site as well as by the action of specific
proteins that influence precipitation and CaCO 3 polymorphism (Braissant et al.
2007; Kawaguchi and Decho 2002; Ercole et al. 2007; Tourney and Ngwenya 2009).
The S layer of the cyanobacterium Synechoccus acts as a template for fine-grain
calcite formation by providing discrete, regularly arranged nucleation sites for the
critical initial events in the mineralization process (Schultze-Lam et al. 1992).
Pentecost and Bauld (1988) proposed that calcite deposition by cyanobacteria
is initiated at sheath polymeric sites, on heteronuclei bound to sheath surfaces,
or upon associated bacterial surfaces. The presence of cyanobacterial EPS also
increases the viscosity of the medium, acting as a diffusion barrier, impacting
calcium ions mobility and the kinetics of precipitation, and consequently the
mineralogy of CC.
The role of EPS produced by SRB in CaCO 3 precipitation has been well
documented in modern stromatolites and lithifying microbial mats, where SRB
134
B. Perito and G. Mastromei
sorbents of dissolved metals, and heterogeneous nucleation templates for
authigenic mineral deposition (Beveridge 1989). Functional groups on the
B. subtilis cell wall were considered able to bind dissolved Ca
2+ in a calcite
dissolution study with dead cells (Friis et al. 2003). B. subtilis dead cells, as well
as a cell fraction comprising the cell wall, were demonstrated able to induce calcite
formation in a laboratory test, acting as heterogeneous crystallization nuclei, with
an applicative potential in the reinforcement of monumental calcareous stones
(Barabesi et al. 2003). Precipitation occurring onto surface of inactive or dead
cells as well as onto isolated cell fractions has to be considered as microbiologically
influenced mineralization, according to Dupraz et al. (2009; Sect. 5.2.1).
Other bacterial cell surface components are known to favor precipitation and
refer, in almost all cases, to extracellular polymeric substances or EPS.
EPS is a broad term that groups a large variety of organic polymers secreted
by microbial cells in the environment. Polysaccharides are the major component
of most EPS, commonly in combination with polypeptides, nucleic acids,
phospholipids, and other polymeric compounds (Decho 2010). According to
Costerton et al. (1995), the organic EPS matrix can be considered as an extension
of the microbial cell. Microbial EPS can bind and accumulate ions. The interactions
between metal ions and EPS are mediated by several functional groups (such as
carboxylic acids or amino groups) present in the EPS matrix. Deprotonation of
functional groups takes place when the pH increases, providing a negative charge to
the polymer. In addition to sugar monomers, the EPS matrix may include
noncarbohydrate acidic moieties such as sulfate or phosphate, which also contribute
to the overall negative charge of the EPS (Braissant et al. 2007).
The role of EPS in CC nucleation and growth is well documented in laboratory
experiments on bacterial cultures or isolated EPS (Braissant et al. 2007; Ercole et al.
2007; Tourney and Ngwenya 2009) as well as in natural environments such as
microbial mats (Dupraz et al. 2009)
Bacterial EPS have been shown to be involved in the process of biocalcification
by entrapping ions and serving as a nucleation site as well as by the action of specific
proteins that influence precipitation and CaCO 3 polymorphism (Braissant et al.
2007; Kawaguchi and Decho 2002; Ercole et al. 2007; Tourney and Ngwenya 2009).
The S layer of the cyanobacterium Synechoccus acts as a template for fine-grain
calcite formation by providing discrete, regularly arranged nucleation sites for the
critical initial events in the mineralization process (Schultze-Lam et al. 1992).
Pentecost and Bauld (1988) proposed that calcite deposition by cyanobacteria
is initiated at sheath polymeric sites, on heteronuclei bound to sheath surfaces,
or upon associated bacterial surfaces. The presence of cyanobacterial EPS also
increases the viscosity of the medium, acting as a diffusion barrier, impacting
calcium ions mobility and the kinetics of precipitation, and consequently the
mineralogy of CC.
The role of EPS produced by SRB in CaCO 3 precipitation has been well
documented in modern stromatolites and lithifying microbial mats, where SRB
134
B. Perito and G. Mastromei