2.1 Microbial-Induced Calcium Carbonate Precipitation
(MICCP)
Calcium carbonate is the prominent global reservoir of carbon. The two carbonates,
limestone and dolomite alone, account nearly 42% of the global carbon reserve
[53]. It is believed that a significant amount of the calcium carbonate available in
earth’s surface is of biogenic origin [54]. Stromatolites and whiting events can be
good examples of massive MICCP phenomena [55, 56] that contribute to the
biogenic surface calcium carbonate deposit. As shown in Table 1, MICCP is driven
by microbial activities such as denitrification [65], photosynthesis [66], ammonification [67], ureolysis [63], sulfate reduction [68], and methane oxidation processes
[60]. In addition to these metabolic activities, microbial surfaces including cell walls
and extracellular polymeric substances serve as calcium carbonate nucleation and
crystal growth sites.
Microbial-mediated CaCO 3 formation is a type of biomineralization process
which is dependent on environmental conditions [69]. There is no specialized
means or specific molecular activities involved in production of CaCO 3 [70]. In
fact, calcium carbonate mineralization is a straightforward simple chemical process
which is governed primarily by (1) calcium concentration, (2) pH, (3) concentration
of dissolved inorganic carbon, and (4) availability of nucleation sites [71]. In
addition to these precipitation factors, factors such as salinity and composition of
the culture medium are also reported to influence calcium carbonate precipitation
[72–74]. Microorganisms contribute to carbonate precipitation by changing almost
any of these four precipitation factors. Often, the microbes alter one or more of the
factors at a time to induce the precipitation process [71]. It is believed that the major
role of microbes is their ability to create conducive pH through their metabolic
activities [71].
An interesting study made on molecular level with Bacillus subtilis that precipitates calcite revealed the presence of a gene cluster involved in calcite precipitation [70, 75]. This cluster contains five genes named lcfA, ysiA, ysiB, etfB, and
etfA. Mutational studies on these genes resulted in five constructs, FBC1–FBC5.
Except FBC1 (which contains a mutated lcfA), the mutants were unable to form
calcite crystals. This shows that at least these four genes are somehow relevant for
the precipitation of calcite. Closer analysis of the etfA gene indicated that its product
resembles an a-subunit of prokaryotic heterodimeric flavoproteins which is involved
in electron transport during fatty acid metabolism [70]. Further studies on this
mutant demonstrated that inactivation of etfA led to a decrease in the pH of the
precipitation medium, which is the reason why this mutant does not precipitate
calcite. This agrees with the observation that the same mutant can produce calcite in
buffered medium [76]. Some heterotrophic as well as autotrophic pathways are
known to raise the pH of an environment [77], and this includes the metabolic
processes indicated in Table 1. One of the heterotrophic pathways that raise pH is the
dissimilatory sulfate reduction process of the sulfur cycle accomplished under
anoxic conditions by sulfate-reducing bacteria. The other heterotrophic pathway
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
301
(MICCP)
Calcium carbonate is the prominent global reservoir of carbon. The two carbonates,
limestone and dolomite alone, account nearly 42% of the global carbon reserve
[53]. It is believed that a significant amount of the calcium carbonate available in
earth’s surface is of biogenic origin [54]. Stromatolites and whiting events can be
good examples of massive MICCP phenomena [55, 56] that contribute to the
biogenic surface calcium carbonate deposit. As shown in Table 1, MICCP is driven
by microbial activities such as denitrification [65], photosynthesis [66], ammonification [67], ureolysis [63], sulfate reduction [68], and methane oxidation processes
[60]. In addition to these metabolic activities, microbial surfaces including cell walls
and extracellular polymeric substances serve as calcium carbonate nucleation and
crystal growth sites.
Microbial-mediated CaCO 3 formation is a type of biomineralization process
which is dependent on environmental conditions [69]. There is no specialized
means or specific molecular activities involved in production of CaCO 3 [70]. In
fact, calcium carbonate mineralization is a straightforward simple chemical process
which is governed primarily by (1) calcium concentration, (2) pH, (3) concentration
of dissolved inorganic carbon, and (4) availability of nucleation sites [71]. In
addition to these precipitation factors, factors such as salinity and composition of
the culture medium are also reported to influence calcium carbonate precipitation
[72–74]. Microorganisms contribute to carbonate precipitation by changing almost
any of these four precipitation factors. Often, the microbes alter one or more of the
factors at a time to induce the precipitation process [71]. It is believed that the major
role of microbes is their ability to create conducive pH through their metabolic
activities [71].
An interesting study made on molecular level with Bacillus subtilis that precipitates calcite revealed the presence of a gene cluster involved in calcite precipitation [70, 75]. This cluster contains five genes named lcfA, ysiA, ysiB, etfB, and
etfA. Mutational studies on these genes resulted in five constructs, FBC1–FBC5.
Except FBC1 (which contains a mutated lcfA), the mutants were unable to form
calcite crystals. This shows that at least these four genes are somehow relevant for
the precipitation of calcite. Closer analysis of the etfA gene indicated that its product
resembles an a-subunit of prokaryotic heterodimeric flavoproteins which is involved
in electron transport during fatty acid metabolism [70]. Further studies on this
mutant demonstrated that inactivation of etfA led to a decrease in the pH of the
precipitation medium, which is the reason why this mutant does not precipitate
calcite. This agrees with the observation that the same mutant can produce calcite in
buffered medium [76]. Some heterotrophic as well as autotrophic pathways are
known to raise the pH of an environment [77], and this includes the metabolic
processes indicated in Table 1. One of the heterotrophic pathways that raise pH is the
dissimilatory sulfate reduction process of the sulfur cycle accomplished under
anoxic conditions by sulfate-reducing bacteria. The other heterotrophic pathway
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
301