are closely associated with calcifying aragonitic micritic layers and high-Mg calcite
micritic layers (Braissant et al. 2007).
According to Braissant et al. (2007), three main processes characteristic of EPS
matrices have been shown to control the precipitation of carbonate minerals: (1)
if calcium concentration exceeds the EPS-binding capacity under suitable pH
conditions (i.e., pH >8.4), precipitation will occur inside the EPS matrix due to
local super saturation; (2) self (re-)arrangement of acidic functional groups in the
EPS matrix may create a template that favors the nucleation of carbonate minerals;
(3) degradation of EPS by heterotrophic bacteria will contribute to the release of
calcium, increasing the SI and enhancing carbonate precipitation.
Bontognali et al. (2008) have investigated mineralization within EPS in laboratory culture experiments carried out with Desulfovibrio brasiliensis, a SRB known
to mediate dolomite formation under anoxic conditions. By combining CLSM and
cryo-SEM imaging techniques, they showed that carbonate crystals nucleate
as nanoforms within the EPS produced by the SRB, and not on bacterial cells, as
previously proposed. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules
displaying a grainy texture. In terms of microbial ecology, mineralization through
excretions of EPS appears consistent with an expected self-preservation behavior of
bacteria which remain prevalently mobile and are not entombed within the mineral.
The authors suppose that mineralization through excretion of EPS could be a
widespread process occurring both today and in the geological past.
According to Decho (2010), a common location for biopolymers, such as EPS,
mediating biologically influenced precipitation (2.1), is the microbial “biofilm”.
Biofilms occur ubiquitously in a wide range of environments. Emerging evidence
now suggests that the organic EPS matrix, which is an integral part of the microbial
biofilms, plays a twofold role, either inhibiting or promoting carbonate formation,
depending on the specific intrinsic (i.e., physicochemical) characteristics. The
biofilm is also a microenvironment where precipitation can be facilitated or
inhibited over a microspatial scale (i.e., mm to mm) and exhibit some spatial
organization. Microorganisms, however, can accomplish a level of environmental
control within the confines of the biofilm and EPS, e.g., in lithifying mats of marine
stromatolites, may represent a primitive regulation of precipitation. The biofilm and
its associated EPS matrix, therefore, serve as a useful starting point to investigate
how organic molecules influence the precipitation process. According to Decho
(2010), research on biopolymer-mediated precipitation is in its academic infancy,
but it is poised to rapidly gain attention.
5.5 Conclusions
BCCP is a widespread process which bacteria carry out both in microbiologically
induced (active) and/or in microbiologically influenced (passive) way (Dupraz
et al. 2009).
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
135
micritic layers (Braissant et al. 2007).
According to Braissant et al. (2007), three main processes characteristic of EPS
matrices have been shown to control the precipitation of carbonate minerals: (1)
if calcium concentration exceeds the EPS-binding capacity under suitable pH
conditions (i.e., pH >8.4), precipitation will occur inside the EPS matrix due to
local super saturation; (2) self (re-)arrangement of acidic functional groups in the
EPS matrix may create a template that favors the nucleation of carbonate minerals;
(3) degradation of EPS by heterotrophic bacteria will contribute to the release of
calcium, increasing the SI and enhancing carbonate precipitation.
Bontognali et al. (2008) have investigated mineralization within EPS in laboratory culture experiments carried out with Desulfovibrio brasiliensis, a SRB known
to mediate dolomite formation under anoxic conditions. By combining CLSM and
cryo-SEM imaging techniques, they showed that carbonate crystals nucleate
as nanoforms within the EPS produced by the SRB, and not on bacterial cells, as
previously proposed. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules
displaying a grainy texture. In terms of microbial ecology, mineralization through
excretions of EPS appears consistent with an expected self-preservation behavior of
bacteria which remain prevalently mobile and are not entombed within the mineral.
The authors suppose that mineralization through excretion of EPS could be a
widespread process occurring both today and in the geological past.
According to Decho (2010), a common location for biopolymers, such as EPS,
mediating biologically influenced precipitation (2.1), is the microbial “biofilm”.
Biofilms occur ubiquitously in a wide range of environments. Emerging evidence
now suggests that the organic EPS matrix, which is an integral part of the microbial
biofilms, plays a twofold role, either inhibiting or promoting carbonate formation,
depending on the specific intrinsic (i.e., physicochemical) characteristics. The
biofilm is also a microenvironment where precipitation can be facilitated or
inhibited over a microspatial scale (i.e., mm to mm) and exhibit some spatial
organization. Microorganisms, however, can accomplish a level of environmental
control within the confines of the biofilm and EPS, e.g., in lithifying mats of marine
stromatolites, may represent a primitive regulation of precipitation. The biofilm and
its associated EPS matrix, therefore, serve as a useful starting point to investigate
how organic molecules influence the precipitation process. According to Decho
(2010), research on biopolymer-mediated precipitation is in its academic infancy,
but it is poised to rapidly gain attention.
5.5 Conclusions
BCCP is a widespread process which bacteria carry out both in microbiologically
induced (active) and/or in microbiologically influenced (passive) way (Dupraz
et al. 2009).
5 Molecular Basis of Bacterial Calcium Carbonate Precipitation
135