Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 323

environments. Biodeposition is a specific type of the biologically induced min-
eralization, referring to deposition that results from interactions between bacterial
metabolic activities and the natural environment (Weiner and Dove 2003 ; Dupraz
et al. 2009 ). Bacterial metabolic activities and cell surface structures and their
interactions with environmental physicochemical parameters are commonly rec-
ognized as the key factors in carbonate deposition (Fortin et al. 1997 ; Douglas and
Beveridge 1998 ; Rodriguez-Navarro et al. 2003 ). Essential condition of carbonate
deposition is a carbonate alkalinity and the availability of free calcium ions (the
two are combined as a saturation index, i.e., SI) and concentrations of both free
carbonate CO 3 2- and Ca 2+ ions must exceed saturation (Dupraz et al. 2009 ;
Decho 2010 ), carbonate deposition is a rather straightforward chemical process
governed by four key factors (Hammes and Verstraete 2002 ):

the calcium (Ca 2+ ) concentration, (1)

(2) the concentration of dissolved inorganic carbon (DIC),

(3) the pH, and

(4) the availability of nucleation sites.

In the deposition process, bacteria create an alkaline environment by an
increase in pH to 8.0 and higher and a DIC increase through their metabolic
activities (Castanier et al. 1999 ; Douglas and Beveridge 1998 ).
Bacterial surface cells are able to induce pH variations in the medium, which
result in variations of the pH in the surrounding microenvironment (Fortin et al.
1997 ). Accordingly, bacterial surface cells also play an important role in calcium
deposition (Fortin et al. 1997 ). Surface bacterial (macro) molecules can induce
carbonate deposition, providing a template for carbonate nucleation, both as part
of bacterial cell and as cell-free, when released in the environment; in the latter
case, primarily as polymeric substances. Bacterial surfaces, and consequently
bacterial cells, can act as important sites for the absorption of cations and con-
stitute particularly favorable templates for heterogeneous nucleation and crystal
growth (Fortin et al. 1997 ). Due to the presence of several negatively charged
groups, at neutral pH, positively charged metal ions could be bound on bacterial
surfaces, favoring heterogeneous nucleation (Fortin et al. 1997 ; Douglas and
Beveridge 1998 ). Among surface structures, bacterial cell walls have been studied
for their ability to complex metals (Jiang et al. 2004 ). Negative charges result
predominantly from deprotonation of carboxyl, phosphate, and hydroxyl func-
tional groups exposed on the outer surface of the cell wall (Fein et al. 1997 ). The
first step is a stoichiometric interaction of metal with reactive chemical groups,
which reside primarily in the peptidoglycan. After complexation, these sites
nucleate the deposition of more metal as a chemical precipitate. Surface reactivity
of bacterial cells depends on the metabolic state of the cells (Jiang et al. 2004 ). In
the absence of metabolic activity, passive interactions may occur in which
microbial cells (inactive living or dead) behave as solid-phase sorbents of dis-
solved metals, and heterogeneous nucleation templates for authigenic mineral
deposition (Beveridge 1989). B. subtilis dead cells, as well as a cell fraction
comprising the cell wall, were demonstrated able to induce calcite formation in a