polypeptide disclosed its relationship to be related to carbonic anhydrases. Recombinant silicase displays besides a carbonic anhydrase activity the ability to dissolve
amorphous silica under formation of free silicic acid (Schr€ oder et al. 2003).
9.9 Biosintering
As outlined above, the basic pattern of silica growth around axial filaments is
identical in spicules of demosponges and hexactinellids, with one exception:
While in demosponges, all silica lamellae fuse to a “solid” structure, a similar
process occurs in hexactinellids only in the central part of spicules restricted to
Amphidiscosida (e.g., the giant basal spicules from M. chuni; Wang et al. 2009);
Fig. 9.8. In most other hexactinellid taxa, the lamellae remain separated. In
demosponges, the individual lamellae merge by fusion of the 70–300 nm large
silica nanospheres that compose the lamellae (Tahir et al. 2004). Whereas fusion of
glass of quartz grade by melting processes would require temperatures well above
1,800
C, this process occurs in the living organism at ambient temperature. Accordingly, the product of this biological fusion of silica lamellae resembles the product
of a technical process termed sintering, i.e., a thermally activated material transport
in a powder or porous compact, decreasing the specific surface by growth of the
particle contacts, shrinkage of pore volume, and change of the pore geometry
(Th€ ummler and Oberacker 1993; Wakai and Aldinger 2004). In general, the
material is densified below its melting point. Sintering is widely used for the
densification of oxide-based ceramic powders including silicon oxide and requires,
in general, temperatures above 1,000
C for thermal activation. The free enthalpy
(Gibb’s energy; DG) of sintering is negative, implying that during the reaction
energy is released, provided that the activation energy (E a ; reaction minimum
energy required to start a chemical reaction) has been overcome. Enzymes work
by lowering the activation energy for a reaction and thus dramatically increase the
rate of the reaction. Considering the fact that within the silica mantel of spicules, the
enzyme silicatein exists [either within (M€ uller et al. 2008a, d] or between (Woesz
et al. 2006) the lamellae), silicatein would be a prime candidate to reduce the
activation energy of this exergonic reaction (Fig. 9.10a). Consequently, it acts in
principle like the sintering additives used in conventional powder technology
processes. Therefore, we propose that the fusion of silica lamellae in demosponge
spicules follows a newly defined biocatalytically mediated process, “biosintering”.
Accordingly, bio-sintering occurs during formation of poriferan siliceous spicules.
Similar to demosponges, in hexactinellids fusion between spicules is frequently
observed in the orders Hexactinosida, Lyssacinosida, and Lychniscosida (Uriz
2006). There, the initial skeletal elements, composed of hexactine spicules, are
subsequently reinforced by additional silica. The large choanosomal spicules from
Euplectella aspergillum fuse together to a complex silica network (Fig. 9.10b, c).
272
W.E.G. M€ uller et al.
amorphous silica under formation of free silicic acid (Schr€ oder et al. 2003).
9.9 Biosintering
As outlined above, the basic pattern of silica growth around axial filaments is
identical in spicules of demosponges and hexactinellids, with one exception:
While in demosponges, all silica lamellae fuse to a “solid” structure, a similar
process occurs in hexactinellids only in the central part of spicules restricted to
Amphidiscosida (e.g., the giant basal spicules from M. chuni; Wang et al. 2009);
Fig. 9.8. In most other hexactinellid taxa, the lamellae remain separated. In
demosponges, the individual lamellae merge by fusion of the 70–300 nm large
silica nanospheres that compose the lamellae (Tahir et al. 2004). Whereas fusion of
glass of quartz grade by melting processes would require temperatures well above
1,800
C, this process occurs in the living organism at ambient temperature. Accordingly, the product of this biological fusion of silica lamellae resembles the product
of a technical process termed sintering, i.e., a thermally activated material transport
in a powder or porous compact, decreasing the specific surface by growth of the
particle contacts, shrinkage of pore volume, and change of the pore geometry
(Th€ ummler and Oberacker 1993; Wakai and Aldinger 2004). In general, the
material is densified below its melting point. Sintering is widely used for the
densification of oxide-based ceramic powders including silicon oxide and requires,
in general, temperatures above 1,000
C for thermal activation. The free enthalpy
(Gibb’s energy; DG) of sintering is negative, implying that during the reaction
energy is released, provided that the activation energy (E a ; reaction minimum
energy required to start a chemical reaction) has been overcome. Enzymes work
by lowering the activation energy for a reaction and thus dramatically increase the
rate of the reaction. Considering the fact that within the silica mantel of spicules, the
enzyme silicatein exists [either within (M€ uller et al. 2008a, d] or between (Woesz
et al. 2006) the lamellae), silicatein would be a prime candidate to reduce the
activation energy of this exergonic reaction (Fig. 9.10a). Consequently, it acts in
principle like the sintering additives used in conventional powder technology
processes. Therefore, we propose that the fusion of silica lamellae in demosponge
spicules follows a newly defined biocatalytically mediated process, “biosintering”.
Accordingly, bio-sintering occurs during formation of poriferan siliceous spicules.
Similar to demosponges, in hexactinellids fusion between spicules is frequently
observed in the orders Hexactinosida, Lyssacinosida, and Lychniscosida (Uriz
2006). There, the initial skeletal elements, composed of hexactine spicules, are
subsequently reinforced by additional silica. The large choanosomal spicules from
Euplectella aspergillum fuse together to a complex silica network (Fig. 9.10b, c).
272
W.E.G. M€ uller et al.