Handbook of Marine Natural Products Volume 426 || From Biosilica of Sponges (Demospongiae and Hexactinellida) to Fabricated Biomedical Materials

From Biosilica of Sponges (Demospongiaeand Hexactinellida) to FabricatedBiomedical Materials

25

Xiaohong Wang, Heinz C. Schroder, Matthias Wiens, Lu Gan,Wolfgang Tremel, and Werner E. G. M€uller

Contents

25.1 Introduction: Sponges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1260

25.2 The Power of “Nature as Model” in Bioinorganic Material Science: Shift of a

Paradigm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261

25.3 Levels of Biomineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1263

25.4 Spicule Network in Siliceous Sponges: A Unique Skeleton . . . . . . . . . . . . . . . . . . . . . . . . . . 1265

25.4.1 Spicule Diversity: Case Study Hexactinellid Monorhaphis . . . . . . . . . . . . . . . . . 1268

25.4.2 Giant Basal Spicules from Monorhaphis: The Largest Bio-Silica

Structure on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1268

25.4.3 Chemical Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1271

25.4.4 Giant Basal Spicules – Protein Scaffold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1272

25.4.5 Biochemical Properties and Intracellular Localization of Silicatein . . . . . . . . 1272

25.4.6 Cloning of the Hexactinellid Silicatein cDNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273

25.5 Modeling of the Morphology of the Spicules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1274

25.5.1 Phases of Silica Deposition During Spicule Formation . . . . . . . . . . . . . . . . . . . . . 1276

25.5.2 Giant Basal Spicules – Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1277

X. Wang

National Research Center for Geoanalysis, 26 Baiwanzhuang Dajie, Beijing, China

and

Institute for Physiological Chemistry, Medical Center of the Johannes Gutenberg University,

Duesbergweg 6, Mainz, Germany

e-mail: [email protected]

H.C. Schroder • M. Wiens • W.E.G. M€uller (*)

Institute for Physiological Chemistry, Medical Center of the Johannes Gutenberg University,

Duesbergweg 6, Mainz, Germany

e-mail: [email protected]

L. Gan

National Research Center for Geoanalysis, 26 Baiwanzhuang Dajie, Beijing, China

W. Tremel

Institute for Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, Mainz,

Germany

E. Fattorusso, W. H. Gerwick, O. Taglialatela-Scafati (eds.),

Handbook of Marine Natural Products, DOI 10.1007/978-90-481-3834-0_25,# Springer Science+Business Media B.V. 2012

1259

mailto:[email protected]

mailto:[email protected]

25.5.3 Giant Basal Spicules – Optophysical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1279

25.6 Biomimetic Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1281

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1282

Abstract

Only 13 years after realizing, during a repair of a telegraph cable pulled out from

the deep sea, that the bottom of the ocean is plentifully populated with a highly

diverse fauna and flora, the Challenger expedition (1873–1876) treasured up a rich

collection of vitreous sponges (Hexactinellida). They have been described by

Schulze and represent the phylogenetically oldest class of siliceous sponges

(phylum Porifera); they are eye-catching because of their distinct body plan,

which relies on a filigree skeleton. It is constructed by an array of morphologically

determined elements, the spicules. During the German Deep Sea Expedition

“Valdivia” (1898–1899), Schulze could describe the largest siliceous hexactinellid

sponge on Earth, the up to 3 m highMonorhaphis chuni, which likewise forms the

largest biosilica structure, the giant basal spicule. Using such spicules as a model,

basic knowledge on the morphology, formation, and development of the skeletal

elements could be acquired. They are formed by a proteinaceous scaffold (com-

posed of a 27-kDa protein), which mediates the formation of siliceous lamellae

that encase the protein. The 27-kDa protein represents an enzyme that forms

polysilicate from silicic acid monomers. The silica matrix is composed of almost

pure silicon and oxygen, providing it with unusual optophysical properties that are

superior to those of man-made waveguides. Experiments suggest that the spicules

function in vivo as a nonocular photoreception system. In addition, the spicules are

provided with exceptional mechanical properties, combining mechanical stability

with strength and stiffness. These basic insights, obtained from the spicule for-

mation in sponges, will surely contribute to a further applied utilization and

exploration of silica in biomaterial/biomedical science.

25.1 Introduction: Sponges

The sponges (phylum Porifera) have occupied – since Aristotle (384–322 BC) –

a distinguished position among the animals because of their biomedical potential,

their beauty, and their enigmatic evolutionary origin. Difficulties in their systematic

positioning and in the elucidation of their relationship to other multicellular organ-

isms have resulted in their designation as “zoophytes” or “plant-animals” (a taxon

placed between plants and animals) until they were finally recognized as genuine

metazoans, which diverged first from the animal ancestor, the urmetazoan [1]. By

then it became clear that sponges are not “simple blobs of cells” but contain and

express a variety of metazoan-like transcription factors and in turn form sophisti-

cated tissue assemblies [2]. After the discovery/appreciation of the glass sponges

[3], the sponges have been grouped into three classes: Demospongiae,Hexactinellida

(both have a siliceous skeleton), and Calcarea (calcareous skeleton) [4].

1260 W.E.G. M€uller et al.

Figure 25.1 gives examples of hexactinellids (Hyalonema sieboldi, Fig. 25.1A;Monorhaphis chuni, Fig. 25.1F) and one example of demosponge (Suberitesdomuncula, Fig. 25.1G). Sponges were united to the phylum Porifera due to the

characteristic and distinct pores on the surface of the animals. They represent the

evolutionary oldest, still extant taxon which testifies the developmental level of

animals living in the Neoproterozoic Eon (1,000–520 million years ago [MYA]);

hence they can be termed “living fossils” [5].

The Hexactinellida are the oldest group of sponges in fossil records of the

Sansha section in Hunan (Early Cambrian; China), where more or less completely

preserved sponge fossils, like Solactiniella plumata (Fig. 25.1B), have been found

[6]. This fossil is noteworthy since it shows, besides an unusual body preservation,

also very intact siliceous spicules (skeletal elements). The approximately 40-mm-

large specimen comprises 0.5–5-mm-long spicules with a diameter of 0.1 mm

(Fig. 25.1C); some of them are broken and present the open axial canals. It should

be mentioned that high-resolution scanning electron microscopic (HR-SEM) ana-

lyses (Fig. 25.1C), coupled with energy-dispersive X-ray, identified those spicules

of S. plumata as to be composed of the original amorphous silica. In contrast, the

characteristic fossil hexactin spicules from hexactinellids found in the Xinjiang

Province from the Tarim basin, and dated back to the Ordovician (510–445 MYA),

have not conserved their silica state but have been diagenetically converted to

calcite microcrystals (Fig. 25.1D, E).

The siliceous sponges disclosed – after application of modern molecular bio-

logical and cell biological techniques – a hitherto unknown biochemical reaction,

the synthesis of a polymeric inorganic molecule (silica) via an organic molecule, an

enzyme (reviewed in [7–9]). Such an activity/reaction was not known before.

A thorough elucidation of this process resulted in a paradigm shift which might

contribute also to new directions in biomaterial sciences.

25.2 The Power of “Nature as Model” in Bioinorganic MaterialScience: Shift of a Paradigm

In 1828 Wohler [10] succeeded to copy nature by producing an organic compound

from inorganic reactants. He synthesized urea from the inorganic ammonium

cyanate, establishing the first rules in organic chemistry. However, to copy organic

(bio)synthetic reactions comprehensively, the existence of enzymes had to be

discovered, an important step which dates back to Pasteur [11]. He found that lactic

acid is a fermentation product, and thus proved the basis for the discipline bio-

chemistry. In fact, already in 1784 Spallanzani [12] described that gastric juice,

soaked into a horny sponge that he had swallowed and subsequently removed

from his own stomach, had the potency to digest meat. This experiment can be

taken as a first demonstration of (enzymatic) reactions inside living organisms.

The causal-analytical understanding of organic reactions in biological systems

became possible after the deciphering of the genetic code and the subsequent

elucidation and application of molecular biological, recombinant techniques.

25 From Biosilica to Fabricated Materials 1261

Fig. 25.1 Fossil and present-day siliceous sponges. (A) Hexactinellida: Hyalonema sieboldi.Lateral view of a specimen showing the body and the stalk (size 500 mm). Specimens of this

hexactinellid can be morphologically divided into the cylindrical upper body (bo) which is

attached to 30-cm-long basal stalk spicules, also termed basalia (ba), that fix the specimens to

the substratum. (B) Earliest sponge in body preservation, Solactiniella plumata (Lowermost

Cambrian Sansha section [Hunan, China]) (size 40 mm). (C) High-resolution scanning electron

microscope (HR-SEM) analysis was performed to visualize the monaxial spicules (mo) from S.plumata. (D) Characteristic fossil hexactin spicules from a hexactinellid, found in the Tarim basin

(Xinjiang Province, China); Ordovician (510–445 million years). (E) The silica material in these

spicules had been diagenetically converted to calcite microcrystals; HR-SEM. (F)Monorhaphis inits natural soft bottom habitat of bathyal slopes off New Caledonia (Photograph taken by Michel

Roux, University of Reims; reproduced with permission). (G) Demospongiae: example Suberitesdomuncula. This specimen lives on the hermit crab Paguristes oculatus which resides in shells of

the mollusk Trunculariopsis trunculus; size 30 mm


However, only recently first strategies could be formulated and experimentally

proven, to outline the biosynthesis of inorganic structures formed in uni- and

multicellular organisms. At present, in a self-accelerating progress, the matri-

ces (templates), e.g., collagen, and the organic catalysts (enzymes, e.g.,

silicatein) required for the synthesis of such inorganic structures and skeletal

elements have been illuminated with the help of inorganic/organic chemists,

biochemists, molecular biologists, and material scientists. These interacting

and cooperating activities established the discipline of “bioinorganic material

science.” The first opportunities have been touched in biomedicine and elec-

tronics, providing us with a first indication about the power and potential of

this new technology [13–16].

25.3 Levels of Biomineralization

Biomineralization describes processes by which living organisms produce min-

erals. These widely discovered reactions include also the formation of silica

(biosilica) in algae, plants, and invertebrates or of calcium phosphates and car-

bonates in vertebrates [17–20]. The biologically mediated reactions result in

the formation of sea shells or the bones in mammals and birds. Less frequently

than silicon- and calcium-based skeleton formation occurs the biomineral forma-

tion using copper, iron, and gold deposits. It is one current challenging task

to mimic the natural ways of producing minerals/“inorganic polymers” (biomi-

metics). While related man-made processes, performed by chemical reactions

only, require elevated temperatures and strong chemicals, the organisms are

able to lay down and form mineral structures at ambient temperatures. Most

frequently, those mineralization processes in biological systems allow the forma-

tion of composites which comprise, in addition to the inorganic polymer,

an organic part, often proteins, which controls those reactions (biomineralization;

[19, 20]).

It is the achievement of Lowenstam and Weiner [17] to have systematized

processes of biomineralization into the following two categories: the biologically

induced mineralization and the biologically controlled mineralization. These two

groups are distinguished by the properties of the guiding functions of the organic

components. In biologically induced mineralization processes, organic surfaces

act as nucleation for the biomineralization. The processes are driven either by the

chemical and/or physical properties of organic surfaces only (Fig. 25.2, left), or

involve also biochemical reactions (Fig. 25.2, middle). An example for biomin-

eralization processes, occurring on organic surfaces and mediated by their chem-

ical and/or physical properties, is the formation of ferromanganese crusts

on marine basaltic seamounts. The deposition of the minerals is facilitated by

the pH and redox environment in the vicinity of the coccolith/coccolithophore

algae. The formation of manganese nodule involves a biochemical reaction which

is based on exoenzymes. Those enzymes facilitate the oxidation of manganese and

iron under formation of insoluble reaction products. Finally, the biologically


Categories of biomineralization

biologically-induced

(chemical) (biological)

biologically-controlled

Fig. 25.2 Categories of biomineralization. Left: Biologically induced mineralization (based on

chemical and physical properties of organic surfaces). Those processes had been elucidated in

ferromanganese crusts, formed on basaltic seamounts in the deep sea (top). The formation of those

mineral deposits is (passively) initiated by the surfaces of calcareous algae, the coccoliths/

coccolithophores (co) (middle). These organisms act with their surfaces in the initial stage of mineral

formation, the induction phase of nucleation (n). The inorganic monomers are deposited, after the

nucleation phase, without help of an organic component (bottom). Middle: Biologically induced

mineralization (basing on biochemical reactions, which proceed on organic surfaces). In manganese

nodules (as likely examples), exoenzymes (e) mediate oxidation ofMn(II) toMn(IV) and/or Fe(II) to

Fe(III) (top). The insoluble reaction products precipitate onto organic surfaces. Those oxidation

reactions occur on S-layers of bacteria (ba) (middle). The further growth of themineral is independent

of organic components (bottom). Right: Biologically controlled mineralization (the deposition of the

minerals is guided by an organic scaffold). This type of mineralization is controlled by an organic

matrix and results in the formation of a composite biomineral. The organic component (o) is

intimately involved in the formation of polymeric inorganicmolecules (bottom). Prominent examples

for an enzymatically, biologically controlled mineralization are the siliceous skeletons from sponges

(top) with an example of Lubomirskia baicalensis. The enzymatic/proteinaceous matrix is composed

of silicatein, which forms the axial filament (af), that resides in the axial canal (ac) and mediates the

formation of the silica layers (si) of the spicules (middle)


controlled mineralization (the deposition of the minerals guided by an organic

scaffold) must be highlighted since in this category the morphology of the

inorganic deposition is controlled by an organic matrix. Hence those biominerals

represent a composite, formed from the inorganic polymer and an organic com-

ponent (protein, polysaccharide, glycoprotein). The associations of inorganic

monomers are linked together by organic polymers (Fig. 25.2, right). A promi-

nent/unique example of this category is represented by the siliceous sponges.

In these organisms the skeletal siliceous elements is enzymatically formed

by association of the silicic acid monomers to amorphous biosilica (Fig. 25.2,

bottom right).

25.4 Spicule Network in Siliceous Sponges: A Unique Skeleton

In living organisms four major groups of biominerals exist: (1) iron compounds,

which are restricted primarily to Prokaryota; (2) calcium phosphates, found in

Metazoa; (3) calcium carbonates, used by Prokaryota, Protozoa, Plantae, Fungi,

and Metazoa; and (4) silica (opal), present in sponges and diatoms [21]. The

formation of the skeleton is a multifaceted process and can be explained in an

exemplary way for the siliceous sponges. Even though these animals comprise the

simplest body plan, their biomineral structure formation is already highly complex

and by far not completely understood. Like the skeletons in triploblasts, the

skeleton formation in the diploblastic Porifera also is intimately connected with

morphogenesis [8, 22]).

The formation of siliceous spicules in sponges is a genetically controlled process

that initiates the morphogenesis phase. Data demonstrated that suitable silicate

concentrations induce genes, e.g., those encoding collagen, silicatein, or

myotrophin [23]. A major step to elucidate the formation of the siliceous spicules

on molecular level was the finding that the “axial organic filament” of siliceous

spicules is in reality an enzyme [24–26], silicatein, which mediates the apposition

of amorphous silica and hence the formation of spicules.

The skeletal framework of the sponges is highly ordered. In the examples

of the demosponge Suberites domuncula (Fig. 25.1G) and the hexactinellid

Monorhaphis chuni (Figs. 25.1F and 25.3A, B), as well as the freshwater sponge

Lubomirskia baicalensis (Fig. 25.2, upper right), it can strikingly be seen that the

growth of the sponges proceeds in a highly organized architecture, mostly in

a radiate accretive manner. Most siliceous sponges comprise larger megascleres

(>100 mm long) (Fig. 25.3F), and smaller microscleres (<100 mm) (Fig. 25.3D).

Some microscleres have bizarre shapes (Figs. 25.3D and 25.4G), while the

megascleres usually have a long central rod (Fig. 25.3A). Importantly, the spic-

ules (Fig. 25.4) contain, both in the central hole and within the outer concentric

silica layers, the enzyme silicatein. The silica shell of the spicules is synthesized

in the initial stage by the silicatein fiber which exists in the central hole, the axial

canal. Around this canal a >3-mm-thick siliceous hollow fiber is finally formed

(Fig. 25.4B, C, H) [27, 28].


Fig. 25.3 Extreme size and morphology of the spicules in the hexactinellid spongeMonorhaphis.(A) A 270-cm-long giant basal spicule (gbs) having a diameter of up to 11 mm. (B) Schematic

representation of the growth phases of the sessile animals with their giant basal spicule (gbs) which

anchors them to the substratum and holds their surrounding soft body (bo). The characteristic

habitus displays linearly arranged large atrial openings (at) of approximately 2 cm in diameter.

With growth the soft body dies off in the basal region and exposes the bare giant basal spicule

(a–c). (C) Part of the body (bo) with its atrial openings (at). The body surface is interspersed with

ingestion openings allowing a continuous water flow though canals in the interior which open

into oscules that are centralized in atrial openings, the sieve plates. (D) An ornate mesamphidisk.

(E) Grilles forming the atrial openings (at) are composed of different types of spicules, tauactines,

framing of lattices, on which the pentactines are arranged in a phalanx. (F) Giant basal spicule(80 cm long), collected 1898/1899 at a depth of 1,079 m off the coast of Somalia

(06�18.8000N–049�32.5000E) (Museum f€ur Naturkunde Berlin, Germany; ZMB Por 12700). The

spicule (gbs) is surrounded by stony corals (co). (G) The 270-cm-long giant basal spicule was used

as an optical waveguide; green light was used as a light source (LS)


mm to µm

µm

µm

µm to nm

nm

Suberites Monorhaphis

Fig. 25.4 Morphology of the spicules from S. domuncula (A–E) and Monorhaphis (F–O), as

analyzed by their top-down hierarchical organizations. Sequentially, images from the cm to mmlevel, then around the mm range, proceeding from the mm to nm scale and finally from the nm level

are given. Organization of the S. domuncula tylostyles (�400 mm): (A) nest of some tylostyles.

(B and C) Cross breaks through tylostyles. In (B) a lamellar organization (la) of the silica spicule

becomes visible after etching with hydrofluoric acid (HF) vapor. This zonation of the siliceous

mantel (si) is absent in those spicules which remained native (C). In both samples the axial canal

(ac) is seen. (D) Young tylostyle, approximately 20 mm in length, displays an open aperture (ap)

which runs into the axial canal. (E) Higher magnification of the apical aperture (ap).Monorhaphisspicules: (F) HR-SEM image of the lattice of a grille. The pentactines (pen) are oriented toward

the exterior of the body thus forming a mechanical and relative sealing of the atrial opening.

The strutting columns are formed from tauactines (tau). (G) Tip of a dermal pentactine pinnule.

(H and I) Structural arrangement of the lamellae in the giant basal spicules; cross sections. In the

center of the spicule, the axial canal (ac) is surrounded by the axial cylinder (cy), a region formed

from electron-dense homogeneous silica. The third and major part of the spicules is composed of

the outer 300–800 concentrically arranged lamellae (la) (H). (I) Higher magnification of the image

in (H), taken from the lamellar region. (J andK) Cross breaks through a spicule, showing the piled


25.4.1 Spicule Diversity: Case Study Hexactinellid Monorhaphis

The spongeMonorhaphis chuni is distributed in the Indo-West Pacific region and is

found in depths between 516 and 1,920 m. For most of the studies, specimens

dredged from 800 m in the Okinawa Trough and from 1,000 m in the South China

Sea have been used. Monorhaphis inhabits muddy substrata and is fixed there by

a single giant basal spicule. Photographs taken from the natural environment by

Roux et al. [29] are available (Fig. 25.1F). Young specimens have been imagined to

comprise a continuous body; one giant basal spicule anchors the specimen to the

substratum and carries the cylindrical body (Fig. 25.3B). The body is interspersed

with many atrial openings which are located along one side (Fig. 25.3B, C, E).

Special dermal pinnular pentactine spicules exist (Fig. 25.4F, G) that are aligned

along tauactines that frame the atrial openings, the sieve plates (Figs. 25.3E and

25.4F). These tauactines form the grille of the atrial openings and are armed by

pentactines, allowing a mechanical sealing of the body toward the aqueous envi-

ronment. The size of a mesamphidisk (microsclere) is approximately 100 mm(Fig. 25.3D). The diameter of the body reaches in larger specimens 8 cm. During

growth the specimens elongate together with an extension of their giant basal

spicules (Fig. 25.3B). Older specimens apparently lose the basal portions of their

soft body and expose the bare giant basal spicule. While recently giant basal spicules

of a size of 1.7 m in maximum have been discovered, now giant spicules with a length

of 2.7 m have been collected that are used by the author for the studies. The maximum

diameter of that spicule is at its middle 12 mm and at its base 3 mm [27, 30].

25.4.2 Giant Basal Spicules from Monorhaphis: The LargestBio-Silica Structure on Earth

The spicules consist of an organic scaffold and an inorganic silica layer/mantel. The

silica mantel is formed from individual lamellae. These lamellae had been analyzed

mainly by SEM. The morphological description here proceeds from the millimeter

to the nanometer scale. Initial studies on the morphology of siliceous spicules were

performed with the spicules from S. domuncula [26]. This demosponge comprises

only one type of spicules, the tylostyles. They reach in the outer layer of the animals

a size of 200 � 5 mm and in the center of the specimens 300–500 � 5–8 mm(Fig. 25.4A). All spicules have a 0.3–1.6-mm-wide axial canal in their center. As

shown in SEM images (Fig. 25.4B, C), the central canal is surrounded by lamellated

�

Fig. 25.4 (continued) lamellae (la) (J). (K) A tangential view to a cross break of a giant basal

spicule displaying the stepwise layering of the silica lamellae. (L andM) In the axial canal (ac) the

square axial filament (af) is seen (L). (M) Higher magnification of the wall of the axial canal

toward the center of the spicule. (N andO). Appearance of the proteinaceous palisade-like scaffold

(pr) within one lamella (la) after exposure of the spicule to HF vapor. The fibrous structures are

interrupted by holes (N). (O) The proteinaceous palisade-like scaffold protein (pr) is composed of

fibers that are interconnected and leave open holes (h) whose rims are reinforced


Fig. 25.5 Composite characteristics of the giant basal spicule (Monorhaphis). (A) Comparison

between the tip of a giant basal spicule (diameter of 7 mm) and a pin/needle. (B) Polished cross


silica layers (Fig. 25.4B). In young, just growing spicules, their tips are open and

display an aperture of a size of 100 nm (Fig. 25.4D, E). Based on these exploratory

studies, a detailed analysis of the giant basal spicules from Monorhaphis became

possible [27, 30, 31].

A view of a broken giant basal spicule fromMonorhaphis, with a large diameter

of up to 12 mm (Fig. 25.5A), discloses at the fracture surface the lamellar-wise

organization of the silica mantel. A cut through a giant basal spicule shows that the

lamellae are arranged in perfect concentric rings around the central axial cylinder

(Fig. 25.4H, I).

Millimeter to micrometer scale: Studies were performed by high-resolution

scanning electron microscopy (HR-SEM), allowing a resolution of the morphol-

ogy of the giant basal spicules. A diagonal SEM analysis of a polished cross

section of a spicule shows the structural division of the spicule into three zones

(Fig. 25.4H). In the center of the spicules lies the axial canal which is surrounded

by a region of electron-dense homogeneous silica constituting the axial cylinder

of a diameter of 100–150 mm (Fig. 25.4H). The major part of the spicules is

composed of 300–800 regularly and concentrically arranged lamellae (each

3–10 mm thick; Fig. 25.4H, I, as well as Fig. 25.5B, C, E). Cross breaks of those

spicules show piles of lamellae that open like pages of a book (Fig. 25.4J).

A lateral view opens the stepwise arrangement of the layers, reminiscent of the

�

Fig. 25.5 (continued) section through a giant basal spicule, displaying its three parts, the axial

canal (ac), the axial cylinder (cy), and the lamellar region (la). (C) A longitudinal cut through the

spicule shows the foliate, lamellar (la) piling of the layers within a spicule. (D) MicroCT analysis

of a giant basal spicule. 3D reconstruction of the spicule close to the tip. There, the spicule (sp) is

surrounded by an organic envelope (env). The surface of the lamellae shows that protrusions are

regularly arranged on the spicule. (E) Polished cross section through a spicule pointing to the

discontinued gaps between the lamellae (la). It appears that the connections between the lamellae

have an ordered arrangement. One of those connections is marked (> <). (F) A collagen net,

perforated with holes (h), surrounds the spicules. (G) During the process of stepwise dissolution of

the silica shell around the axial filament (by HF), the lamellar zone (la) starts already after 1–3 min

to dissolve. (H) In a later phase the proteins in the axial cylinder (cy) are released that surrounds

the axial filament (af); stained with Sirius red. (I) Analysis of the proteins in total extract of the

spicules by two-dimensional gel electrophoresis (first isoelectric focusing and then size separa-

tion). The gel has been stained with Coomassie brilliant blue. A set of �27 kDa proteins are

stained reflecting their different phosphorylation status. (J–L) Stepwise release of the protein

matrix within the lamellae (la), giving rise to the palisade-like proteinaceous scaffold (pr);

exposure to HF for 1 min (J), 30 min (K), or 180 min (L). (M) Higher magnification of the

fibrous structure from the lamellae, the palisade-like scaffold (pr), disclosing holes (h). (N)A slightly etched cross section through a spicule. Vermicular structures (vs) within the lamellae

are disclosed. (O) Analysis of those spicular surfaces by atomic force microscope (AFM),

displaying the vermicular structures (vs) as concentric arranged cables. In addition, one gap

between two lamellae is visible (g). (P) Scheme summarizing the morphological zones of the

giant basal spicules, outlining their composite property. The axial canal harbors the organic axial

filament. This structure is surrounded by the silica mantel which comprises within its lamellae the

protein/enzyme, silicatein. This silica mantel is surrounded by a collagen net


seat rows in an amphitheater (Fig. 25.4K). The interlamellar space of the spicules

does surprisingly not constitute a continuous open slit. The slit is in average

0.1–0.2 mm wide and is composed of alternating fusion zones and open gaps;

apparently the fusion zones allow a continuum between silica lamellae

(Fig. 25.5E). The axial canal harbors the organic axial filament which has a char-

acteristic rectangular shape (Fig. 25.4L). The wall of the axial canal is

not completely smooth but displays a fine-structured network, composed of

20–40-nm-large silica granules (Fig. 25.4M).

Nanometer scale: Studies, to obtain an insight into the structural organization ofthe spicules at the nm scale, have been performed after mild, partial, and limited

dissolution of the silica material by hydrofluoric acid (HF). Under such conditions

the organic matrix is released from the interior of the lamellae (Fig. 25.4N, O). In

light microscopic studies it could be shown that the dissolution kinetics (caused by

HF) is different in the region of the axial cylinder and the lamellar region. The

dissolution starts from cracks in the spicule, primarily following the gaps between

the lamellae (Fig. 25.5G). During this process the lamellar zone can be distin-

guished from the axial cylinder by the protruding ends of dissolving individual

lamellae (Fig. 25.5G). The dissolution of the axial cylinder proceeds from the

periphery without revealing individual lamellae (Fig. 25.5H). Application of dif-

ferent dyes allows a differential visualization of the proteinaceous components.

While Coomassie brilliant blue stains the released protein from the lamellae and the

axial cylinder, the polypeptides released from the axial cylinder react selectively

with Sirius red (Fig. 25.5H).

Next the HR-SEM technique was applied to detect/identify the proteinaceous

scaffold that is released after HF treatment (Fig. 25.5J–L). In the transition phase,

until a complete dissolution by HF is reached, the siliceous surfaces of the lamellae

remain visible (Figs. 25.4N and 25.5K); they are held up by palisade-like pillar

structures composed of proteinaceous material. The individual pillars have

a diameter of 0.1–0.2 mm and a length of 5–10 mm (Figs. 25.4O and 25.5M). The

proteinaceous material obtained was characterized biochemically as one distinct

protein with an apparent size of �27 kDa. Slightly etched spicules show on the

surfaces vermicular structures of diameters of 20–30 nm (Fig. 25.5N), likely to

represent the proteinaceous matrix. The different mechanical properties of these

vermicular structures within a lamella can be visualized by atomic force micro-

scope (AFM) analysis (Fig. 25.5O); these structures follow the concentric arrange-

ment/orientation of the lamellae [8].

25.4.3 Chemical Composition

The gross chemical composition of sponge spicules has been described both for

Demospongiae and for Hexactinellida. Already Schulze [32] determined that

besides Si (silicon) and O (oxygen) (96%), only trace amounts of Na and

K contribute to the inorganic material. Hence the silica (opal) of the spicules

possesses quartz glass quality [33].


25.4.4 Giant Basal Spicules – Protein Scaffold

Surprisingly it was found that the siliceous lamellae, composing the spicules from

Monorhaphis, contain also a proteinaceous scaffold. The proteinaceous material

obtained was characterized by NaDodSO4–PAGE and found to represent one

distinct protein with an apparent size of �27 kDa (Fig. 25.5I) [31, 34, 35]. If

such a protein sample is subjected to two-dimensional gel electrophoresis, the

proteins are separated into acidic and alkaline sets of protein (Fig. 25.5I). Work

had been focused especially on the identification of the function of the �27-kDa

protein. By applying biochemical and immunobiochemical techniques, it could be

established that this �27-kDa protein represents the silica-forming enzyme,

silicatein. This molecule constitutes the axial filament as well as the matrix of the

silica lamellae. Hence, the “inorganic” silica material of the spicules represents

a composite, formed of protein and silica (Fig. 25.5P).

Collagen is surely, also in sponges, an important component in the morphogen-

esis of the spicules. These fiber polypeptides surround the spicules as a fibrous

sheath. By application of the SEM technique, the collagen net was found to be

regularly interrupted by circular holes (7–10 mm; Fig. 25.5F). Microtomography

(MicroCT) analysis revealed that this organic envelope (collagen fibrils) surrounds

the inorganic spicule (Fig. 25.5D). In addition, it could be visualized that the

surface of the silica spicule is not even but has a serrated relief structure. The

protrusions are arranged in an organized pattern, an almost regular helical succes-

sion of small projections (Fig. 25.5D, P) [36].

25.4.5 Biochemical Properties and Intracellular Localization ofSilicatein

The kinetic parameters for silicatein follow those of usual enzymes; the Michaelis

constant (Km) for silicatein was determined to be 22.7 mM, and the corresponding

Vmax (maximal velocity) was 9.6 nM/min [9].

The spicules are formed in the first stage intracellularly and are completed

extracellularly. Silicic acid is taken up intracellularly via a Na+/HCO3� [Si

(OH)4]-cotransporter system [37]. There silicic acid is accumulating in special

organelles, the silicasomes (Fig. 25.6). Within this compartment, the first imma-

ture spicules are formed from silicic acid under formation of polysilica around the

silicatein/axial filament (Fig. 25.6A). Subsequently those immature spicules are

extruded into the extracellular space where the spicules are completed by the

(also) extracellularly existing silicatein. This biomineralization pathway, enzy-

matic formation of intracellular polysilica structures and subsequent extracellular

completion of the spicules by silicatein, is unique and found only in sponges

(Fig. 25.6A).

These data, first the selective uptake of silicic acid by membrane-associated

transporters and then formation of polysilica structures by the enzyme silicatein,

allows also a biochemical explanation of the ability of sponges to form high-quality


quartz glass spicules. This high purity is the result of a selective uptake [Si(OH)4-

cotransporter system] and selective deposition of polysilica (silicatein) (Fig. 25.6B).

25.4.6 Cloning of the Hexactinellid Silicatein cDNA

Only sponges are provided with the polysilica-forming enzyme, silicatein. After

cloning of the silicateins from demosponges, it became obvious that silicatein

belongs to the class of cathepsins, with cathepsin L as the most prominent member

[23, 25]. This lysosomal enzyme belongs to the family of papain-like peptidases

that are characterized by the catalytic triad of cysteine (Cys), histidine (His), and

asparagine (Asn). The silicateins are distinguished from the cathepsins by the

exchange of the first amino acid (aa) residue in the catalytic triad, Cys by Ser. In

addition, the silicateins are distinguished from the cathepsins by the presence of

NBCSA contransporterBANa-HCO3/

Si(OH)4

Na+ Na+

selectivitybarrier

selectivitybarrier

Na+ K+

ATPase

ATP

ADP

[-Si(OH)2-O-]n

[SiO2]nTransport /depositionSpicule formation

extracellularpoly-silica formation

poly-silica

intracellularpoly-silica formation

silicasome

poly-silica

silicatein

silicate

template Silica

SerThr His

SerSilicatein Thr His

K+

K+

Fig. 25.6 Schematic outline of the enzymatic synthesis of polysilica structures (biosilica) and of

the transport of silicic acid into sponge cells. (A) Orthosilicate (silicic acid) is taken up by

a membrane-associated transporter. From there the silicic acid monomers are transported into

the silicasomes and undergo association with the silicatein enzyme. After formation of the first

silica structures, the immature spicules are released from the cells and completed extracellularly

by the existing silicatein molecules and silicic acid. (B) Scheme outlining the two selectivity

barriers that control the synthesis of the pure biosilica product. The Na+-bicarbonate�/silicic acidcotransporter mediates the uptake of silicate, allowing a strong selective enrichment of silicate in

the cells and in the tissue of the sponges (first selectivity control). Na+ is pumped out via the

Na+/K+ ATPase transporter under the consumption of ATP. The second selectivity control is on

the level of silicatein, which will accept – substrate specifically – only silicate under formation of

biosilica


a Ser stretch that precedes the second aa in the catalytic triad, His. Like

demosponges (Fig. 25.7), also hexactinellids contain the silicatein enzyme.

Recently, also the gene encoding this enzyme in Hexactinellida has been isolated

from Crateromorpha meyeri [34].

25.5 Modeling of the Morphology of the Spicules

The determination of the spicule morphology remains one of the most enigmatic

processes in sponges. Surely, the sizes and forms of the spicules are under species-

specific control. However, only little information is presently known to define those

control systems, governing the growth and form processes. The basis of the spicule

formation is defined by the axial filament that elongates by a controlled interaction

of silicatein molecules. Silicatein, especially if it is extracted from the spicules

under mild, nonharsh conditions, readily forms dimers through noncovalent link-

ages. Subsequently, filaments/aggregates are formatted from monomeric silicatein,

suggesting a reassembly through fractal-like structures [28, 38, 39]. Those native

protein samples reveal after NaDodSO4–PAGE (one-dimensional separation) anal-

ysis only one band of a size of 27 kDa (Fig. 25.8A–a). If the sample is subjected to

two-dimensional gel electrophoresis, the proteins are separated into a series of

isoenzymes, due to different phosphorylation states. The number of silicatein

spots on the gel is considerably larger if extracts obtained by mild (short) HF

dissolution (Fig. 25.8A-b) are analyzed, than with extracts obtained after extensive

(>24 h) HF dissolution (Fig. 25.8A-c). This finding reflects the fact that during HF

treatment, phosphate groups are split off from the protein.

We ascribe to the axial filament a major role as a form factor for the determi-

nation of the morphology of the spicules. From the S. domuncula silicatein it is

known that the enzyme exists in at least two isoforms, transcribed from two

different genes, silicatein-a and silicatein-b [28, 38, 39]. These proteins readily

form dimers, tetramers, as well as hexamers. Furthermore, it could be worked out

that the two isoforms associate in a defined stoichiometric ratio: four molecules of

silicatein-a with one molecule of silicatein-b. It has been suggested that the four

silicatein-a molecules form a platform with serine clusters directed to the center.

These serines of the coaxially arranged silicateins interact with silicatein-b. In orderto attribute a function to the obviously crucial phosphate units linked to the

silicatein, it is proposed that the orientation of the 4:1 silicatein assembly undergoes

alterations driven by different phosphorylation levels of the enzyme (Fig. 25.8C).

The first support came from studies with the different silicatein molecules isolated

from the demosponge Geodia cydonium. This species comprises two types of

spicules, sterrasters (which are microscleres) and anatriaenes (megascleres)

(Fig. 25.8D, E). It could be shown that these spicules comprise not only a different

set of silicateins but also silicateins with a distinct and different phosphorylation

pattern. We propose that cells adjacent to the spicules secrete both silicatein and

collagen that forms a well-controlled proteinaceous tube into which the axial

cylinder and the lamellar zone are imbedded.


Fig.25.7

Silicatein:sequence.Thesilicatein

protein

from

thehexactinellidC.meyeriwas

deducedfrom

theisolatedcD

NA

andaligned

withsilicatein-a

from

S.do

muncula

(SILCAa_SUBDO)andsilicatein-ß

from

S.do

mun

cula

(SILCAb_SUBDO)andoneisoform

ofsilicatein-a

from

L.ba

icalensis(a-3)

(SILCAa3_LUBAI)

aswellas

withthecathepsinLsequencesfrom

S.do

mun

cula

(CATL_SUBDO)andhuman

(CATL_HUMAN).Residues

conserved

(sim

ilar

orrelatedwithrespectto

theirphysicochem

icalproperties)in

allsequencesareshownin

whiteonblack,andthose

inatleastfoursequencesin

black

ongray.T

hecharacteristicsitesinthesequencesaremarked;thecatalytictriad(CT)am

inoacids,SerinsilicateinsandCysincathepsin,andHisandAsn.T

he

borderswithin

themature

silicatein

(mature

peptide),thesignalpeptideas

wellas

thepropeptidearegiven.Thestretchoftheenzymatically

activesilicatein

molecule

has

beenusedforthepreparationofrecombinant,activesilicatein.The“conventional”serinecluster

(Ser)is

marked.Finally

thepotential

phosphorylationsiteswithin

thesequencesarelisted

abovethealignment


25.5.1 Phases of Silica Deposition During Spicule Formation

The process of spicule formation can be divided into the following phases: the

initial intracellular steps and the extracellular final and shaping phase.

The intracellular phase in the sclerocytes: In the first steps silicatein is synthe-

sized as a proenzyme and processed via the 34.7-kDa form (propeptide-mature

enzyme) to the 23-kDa mature enzyme [26, 27]. Very likely during the transport

through the endoplasmic reticulum and the Golgi complex, silicatein undergoes

phosphorylation and is transported into silicasomes where it forms rods, the axial

filaments (Fig. 25.6A). After assembly to filaments the first layer(s) of silica is (are)

formed. Silica deposition occurs in two directions: first from the axial canal to the

surface (centrifugal orientation) and second from the mesohyl to the surface of the

Fig. 25.8 Silicatein protein, space model and backbone for the synthesis of the diverse silica

structures, giving rise to the spicules. (A-a) Analysis of the total spicule extract by one-

dimensional gel electrophoresis. The line marks the position of the 27 kDa protein. The gel was

stained with Coomassie brilliant blue. (A-b and A-c) Analysis of a total spicule extract by two-

dimensional gel electrophoresis (first isoelectric focusing and then size separation). Protein extract

was collected after mild (short) (A-b) or extensive (>24 h) (A-c) HF-dissolution. The lines mark

the positions of the 27-kDa polypeptides. (B) Schematic representation of planar-oriented

silicatein-a tetramer (a; green-gray-blue-pink) with one silicatein-b (b) in the center (yellow).The amino acids involved in the active centers (ac) of the silicateins are marked. In this orientation

the Ser-rich clusters (sc) are directed toward the center of tetramer (B-a). (B-b) Graphical model of

the silicatein-a tetramer with the silicatein-b in the center. (C) Working model proposing that the

orientation of the 4:1 silicatein assembly is changed by an alteration of the level of phosphoryla-

tion state of the enzyme molecules (arrow). (D) Sterrasters (microscleres) from the demosponge

Geodia cydonium. (E) The tip of a megasclere (anatriaene)


spicule (centripetal). Finally the spicules are released into the extracellular space

where they grow in length and diameter by appositional growth.

Extracellular phase (appositional growth): Silicatein is present also in the

extracellular space. It came surprising that also there the silicatein molecules are

organized to larger entities. The immunogold electron microscopical analysis

showed that the silicatein molecules are arranged along strings, which are organized

in parallel to the surfaces of the spicules. In the presence of Ca2+, silicatein

associates with galectin and allows the appositional growth of the spicules

(Fig. 25.9). Since the surface of a new siliceous spicule is also covered with

silicatein, the appositional growth/thickening of a spicule hence proceeds from

two directions (centrifugal and centripetal). In the next step, the galectin-containing

strings are organized by collagen fibers to net-like structures. It is very likely that

collagen, which is released by the specialized cells the collencytes, provides the

organized platform for the morphogenesis of the spicules. The longitudinal growth

of the spicules can be explained by the assumption that at the tips of the spicules, the

galectin/silicatein complexes are incorporated into deposited biosilica under for-

mation and elongation of the axial canal [39].

25.5.2 Giant Basal Spicules – Mechanical Properties

Inspired by studies withMonorhaphis, a new concept of natural composite material

in rigid biological systems was born and fundamentally outlined by Mayer [40].

The organic phase controls energy dissipation especially in systems that are inter-

spersed by very thin organic layers. In continuation of this topic, it had been

proposed from their load–displacement studies that breakage of hexactinellid

spicules follows a telescope-like pattern [30, 31].

Images from a typical load–displacement experiment with spicules from

Monorhaphis are given in Fig. 25.10. The pattern of fractures within the spicule

was correlated with the organization of the lamellar zone and the axial cylinder,

since both areas are characterized by different bioorganic/inorganic hybrid compo-

sitions (Fig. 25.10A–D). With increasing load, a sequential breakage of the lamel-

lae occurred. At first the outer, concentrically arranged silica layers burst, followed

by the more inwardly located layers. If a sample from an unbroken, nontreated

spicule was subjected to the classical bend test, the resulting load–displacement

curve showed at first a linear increase reflecting the elastic response of the outer

lamellar layers of the spicule. The following deflection does not drop to zero but is

fixed according to the viscoelastic view by the more central lamellar layers.

The stepwise breakage of an untreated giant spicule (from the hexactinellid

Hyalonema sieboldi; diameter of 0.8 mm) is shown in Fig. 25.10E–L. The spicule

was inserted into a traction device between two moving rubber wheels, and

continuous pulling was achieved by a motor. If the breaking process is recorded

with a high-speed camera, the first crack of the outer lamellae is not visible even if

the spicule has been completely bent by one circumvolution, leaving a diameter of

the circular, wrapped-around spicule of 4.5 cm (Fig. 25.10E). However, after


pulling the circle down to 4 cm, the first set of the outer lamellae blasts off

(Fig. 25.10F). Then another period of flexible bending proceeds until the second

breakage point is reached (Fig. 25.10H). Again, a further flexible bending phase

takes place until a third spalling of the lamellae occurs, followed by a final fracture

of the spicule (Fig. 25.10K). In comparison, the breakage of a quartz glass fiber of

Fig. 25.9 Schematic outline of the appositional spicule formation in sponge (example S.domuncula). The process of the appositional growth of the spicules occurs in the extracellular

space (mesohyl). (A and B) There, galectin molecules associate in the presence of Ca2+ to strings

(nets) that allow binding of silicatein molecules. (C) Collagen fibers orient the silicatein-galectin

strings concentrically round the growing spicules. (D–F) In the last steps, the silicatein-galectin

strings oriented by collagen fibers circle and allow biosilica deposition within the formed cylinder

(> <). After completion of one biosilica lamella (si), the next biosilica lamella starts in the same

manner by apportion onto the previous lamella (E). The initial biosilica layer is formed around the

silicatein rod (axial filament), existing in the axial canal (ac) of the spicules. Below the graphs the

corresponding electron microscopic images are given


a similar diameter has been recorded (Fig. 25.10M–P). As expected, the glass fiber

cannot even be bent to a large circle but cracks almost immediately after an

inflection of about 30�.

25.5.3 Giant Basal Spicules – Optophysical Properties

Sponges can react fast to physical stimulation from the environment with contrac-

tion or expansion. These observations could imply that the coordinate reactions are

governed by a nerve system. However, until now no nerve fibers or synapses could

be identified in sponges. Nevertheless, our previous studies showed that the

siliceous demosponges S. domuncula and G. cydonium contain and express

genes coding for neuronal molecules, e.g., a metabotropic glutamate/GABA

Fig. 25.10 Force displacement relationship of a giant basal spicule, as measured in a 3-point

bending assay. (A) SEM analysis of a partially noncracked spicule. (B–D) After increased loading,the silica layers crack starting from the surface of the spicules. (E–L) Flexible breakage of a giantspicule (from H. sieboldi), (M–P) in comparison to quartz glass fibers. The diameter of both fibers

was about 0.8 mm. The fibers were inserted into a traction device between two moving rubber

wheels and were continuously pulled toward the fixation point (F) using a motor. The breakage

was recorded with a high-speed camera. The spicule had been completely bent by one circumvo-

lution; the diameter of the circular, wrapped-around spicule was initially 4.5 cm. After proceeding

traction, three stepwise breaking steps can be recorded (F, H, and K) which are interrupted by

phases of flexible bending. In contrast, an abrupt and total fracture of a quartz glass fiber is

recorded. Even before a complete circle could be wound, the fiber blasted


Fig. 25.11 “Nature as a Model” – a biomimetic approach. (Left panel: A–D) Synthesis of

spicules in the extracellular space. (A) Scheme; cross section through a growing spicule with

the silica mantel (si) which surrounds the axial canal (ac) that harbors the axial filament (af). In the

extracellular space collagen (col), fibers are indicated. (B and C) SEM images showing the

respective sections; silicatein molecules are marked (sil). (D) Immunogold labeling with anti-

bodies raised against silicatein to visualize the organic cylinders which guide and mediate the

synthesis of the silica layers in an appositional manner (marked > <). (Right panel: A0–E0) The


(g-aminobutyric acid)-like receptor. Furthermore, it is well established that sponge

embryos and adult sponges react to light. So far all attempts to identify

photosensory cells in sponges failed. Therefore, the question was raised, if

noncellular structures, like spicules, might function as biological receiver for

light energy. The first prerequisite was already discovered to a great extent by the

findings that spicules from sponges can transmit light. In these elegant studies, the

hexactinellids Rossella racovitzae and E. aspergillum, H. sieboldi, and

Monorhaphis (Fig. 25.3G) were used. It could be demonstrated that these large

spicules transmit light with high efficiency. The blue light with a wavelength

between 400 and 600 nm is filtered out. Based on this finding, it has been suggested

that the spicules from hexactinellids act as an optical absorbent in a novel photo-

reception. This assumption has been supported by the recent finding that sponges

possess the luciferase enzyme [41].

25.6 Biomimetic Approaches

Electron microscopy studies revealed that spicule formation occurs by appositional

layering. In the extracellular space, the spicules grow through apposition of lamel-

lar silica layers up to their final size of 450 mm (Demospongiae) or 2.5 m

(Hexactinellida). Based on the experimental studies on the extracellular growth of

the spicules, the natural principle (“Nature as a Model”) was applied in

a biomimetic way. As mentioned, growth of the spicules is guided by an organic

cylinder formed from silicatein and galectin that results in the formation of biosilica

lamellae (Fig. 25.11; left panel) [15, 16].

This concept from the elucidation of the different phases of spiculogenesis and

the molecules involved in it allows a wide application of silicatein in the field of

nanobiotechnology. This principle has been utilized by the group of Tremel

to synthesize on an “inert” surface (matrix), biosilica from monomeric precursors

[42–44]. The matrix used had been activated by cysteamine to allow binding to

a reactive polymer; in turn this polymer is able to chemisorb the nitrilotriacetic acid

ligand. Finally, this architecture allowed the binding of recombinant histidine-

tagged silicatein (Fig. 25.11, right panel). Surprisingly enough, silicatein

immobilized onto this matrix has the capacity to catalyze biosilica and even

biotitania and biozirconia from the monomeric precursors. This is a further striking

example to show that nature can be used as a biological blueprint for nanobiotech-

nological applications. In addition, this technology introduces a new concept, the

synthesis of inorganic polymers by an organic molecule (silicatein). Just in oppo-

site, in 1828, Wohler succeeded with his epochal experiments on the synthesis of

�

Fig. 25.11 (continued) biomimetic approach. (A0 and B0) The template is functionalized with

a reactive ester polymer and (C0) then with the NTA linker. (D0) The recombinant silicatein is

bound via the His-tagged to the NTA polymer. (E0) The recombinant silicatein mediates polysilica

formation and forms a silica lamella like nature does in sponge spicules


urea, an organic compound, from inorganic basic materials (“die k€unstlicheErzeugung eines organischen, und zwar animalischen, Stoffes aus unorganischen

Stoffen”).

Acknowledgments W.E.G.M. is holder of an ERC Advanced Investigator Grant (No. 268476 –

BIOSILICA). This work was supported by grants from the German Bundesministerium f€urBildung und Forschung (Project “Center of Excellence BIOTECmarin”), the Deutsche

Forschungsgemeinschaft (Schr 277/10-1), the International Human Frontier Science Program,

the European Commission (Grant No. 031541-BIO-LITHO [biomineralization for lithography

and microelectronics]), the consortium BiomaTiCS at the Universit€atsmedizin of the Johannes

Gutenberg-Universit€at Mainz, and the Public Welfare Project of Ministry of Land and Resources

of the People’s Republic of China (Grant No. 201011005–06).

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Documents

Handbook of Marine Natural Products Volume 426 || From Biosilica of Sponges (Demospongiae and Hexactinellida) to Fabricated Biomedical Materials