nanostructured silicates at near-neutral pH, atmospheric pressure, and room temperature [138]. Efforts have been made to unravel natures secrete in synthesizing
complex biogenic silica using biological machineries [139]. Although Schröder
et al. [33] demonstrated the possibility of producing biosilica glass using sponge
enzymes, there has been little progress made so far. Understanding and use of
biosilicification process could be of immense potential application in concrete.
Eukaryotic organisms are not the only group known with ability of biomineralization of silica. Prokaryotes also demonstrate this process. In fact, it is not unusual
to find bacteria associated with clay-like silicate minerals in hot springs [47, 140–
143] and surface sulfide springs [41]. Clay-like bacterial surface precipitates have
also been reported even in places such as freshwater lakes and rivers with low
concentrations of dissolved silica [41]. It is believed that microbes are instrumental
in the formation of these minerals.
In microbial mats, silica seems abundant, and it exists on the microbes’ surface
forming the mats’ micro-laminated layers [144]. This silica biomineralization in
microbial mats seems linked to Fe precipitation. In line with this, the biomineralization of silica by Thermus thermophilus TMY has been studied, and the results
indicated that the microbe produces a silica-induced protein [138]. Analysis of the
protein amino acid sequence revealed that it is related to solute-binding protein of the
Fe
3+ ATP-binding cassette (ABC) transporter. There is a possibility that this silicainduced protein contributes to silica mineralization by mediating protein-Fe-silica
interaction. However, some researchers argue that the contribution of bacteria like
Thermus in biomineralization of silica is not significantly important. According to
these researchers, silica and iron precipitation is dominated by non-microbially
mediated process. They suggested that microorganisms contribute only marginally
to silicification [145] which is an inorganically controlled process [146].
As there is controversy over how important microorganisms are in silica biomineralization, there has also been disagreement regarding silica sorption capacity of
bacterial cell wall and exopolysaccharides. Studies made on silica precipitation by
the cyanobacteria Calothrix sp. indicated that the silica precipitation role of the cell
surface is limited [146]. The conclusion drawn from this study is that precipitation of
silica is largely non-biogenic, and the microbial surfaces have a minor effect on silica
nucleation. However, there is evidence that bacterial surfaces are good sorption
interfaces to bind silicate ions [140]. Although bacterial cell surface has net negative
charges, there are also positively charged amine groups. At near-neutral pH, these
positively charged residues can interact with silicate anions. However, the available
amine groups may not be enough to adsorb large amount of silicate that leads to
precipitation. On the other hand, more silicate binding can occur due to cross
bridging involving metal ions which link silicate anions to the negatively charged
groups within the cell wall matrix [147]. The binding of silicate anions results in
deposition of poorly ordered silicate mineral on the surface of the bacteria, which
eventually become clay-like crystalline particles. The formation of these particles
increases the sorption interface for metal ions and hence increases the overall metalbinding ability of the bacteria [148, 149]. However, it seems that the bacterial surface
has a greater affinity for metal ions than the clay particles [150–152].
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
305
complex biogenic silica using biological machineries [139]. Although Schröder
et al. [33] demonstrated the possibility of producing biosilica glass using sponge
enzymes, there has been little progress made so far. Understanding and use of
biosilicification process could be of immense potential application in concrete.
Eukaryotic organisms are not the only group known with ability of biomineralization of silica. Prokaryotes also demonstrate this process. In fact, it is not unusual
to find bacteria associated with clay-like silicate minerals in hot springs [47, 140–
143] and surface sulfide springs [41]. Clay-like bacterial surface precipitates have
also been reported even in places such as freshwater lakes and rivers with low
concentrations of dissolved silica [41]. It is believed that microbes are instrumental
in the formation of these minerals.
In microbial mats, silica seems abundant, and it exists on the microbes’ surface
forming the mats’ micro-laminated layers [144]. This silica biomineralization in
microbial mats seems linked to Fe precipitation. In line with this, the biomineralization of silica by Thermus thermophilus TMY has been studied, and the results
indicated that the microbe produces a silica-induced protein [138]. Analysis of the
protein amino acid sequence revealed that it is related to solute-binding protein of the
Fe
3+ ATP-binding cassette (ABC) transporter. There is a possibility that this silicainduced protein contributes to silica mineralization by mediating protein-Fe-silica
interaction. However, some researchers argue that the contribution of bacteria like
Thermus in biomineralization of silica is not significantly important. According to
these researchers, silica and iron precipitation is dominated by non-microbially
mediated process. They suggested that microorganisms contribute only marginally
to silicification [145] which is an inorganically controlled process [146].
As there is controversy over how important microorganisms are in silica biomineralization, there has also been disagreement regarding silica sorption capacity of
bacterial cell wall and exopolysaccharides. Studies made on silica precipitation by
the cyanobacteria Calothrix sp. indicated that the silica precipitation role of the cell
surface is limited [146]. The conclusion drawn from this study is that precipitation of
silica is largely non-biogenic, and the microbial surfaces have a minor effect on silica
nucleation. However, there is evidence that bacterial surfaces are good sorption
interfaces to bind silicate ions [140]. Although bacterial cell surface has net negative
charges, there are also positively charged amine groups. At near-neutral pH, these
positively charged residues can interact with silicate anions. However, the available
amine groups may not be enough to adsorb large amount of silicate that leads to
precipitation. On the other hand, more silicate binding can occur due to cross
bridging involving metal ions which link silicate anions to the negatively charged
groups within the cell wall matrix [147]. The binding of silicate anions results in
deposition of poorly ordered silicate mineral on the surface of the bacteria, which
eventually become clay-like crystalline particles. The formation of these particles
increases the sorption interface for metal ions and hence increases the overall metalbinding ability of the bacteria [148, 149]. However, it seems that the bacterial surface
has a greater affinity for metal ions than the clay particles [150–152].
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
305