Mechanism of internal stratification of siliceous sponge spicules

Insights into the structure and morphogenesis of the giant basal spicule of the glass sponge Monorhaphis chuni

BACKGROUND

A basal spicule of the hexactinellid sponge Monorhaphis chuni may reach up to 3 m in length and 10 mm in diameter, an extreme case of large spicule size. Generally, sponge spicules are of scales from micrometers to centimeters. Due to its large size many researchers have described its structure and properties and have proposed it as a model of hexactinellid spicule development. Thorough examination of new material of this basal spicule has revealed numerous inconsistencies between our observations and earlier descriptions. In this work, we present the results of detailed examinations with transmitted light and epifluorescence microscopy, SEM, solid state NMR analysis, FTIR and X-ray analysis and staining of Monorhaphis chuni basal spicules of different sizes, collected from a number of deep sea locations, to better understand its structure and function.

RESULTS

Three morphologically/structurally different silica layers i.e. plain glassy layer (PG), tuberculate layer (TL) and annular layer (AL), and an axial cylinder (AC) characterize adult spicules. Young, immature spicules display only plain glassy silica layers which dominate the spicule volume. All three layers i.e. PG, TL and AL can substitute for each other along the surface of the spicule, but equally they are superimposed in older parts of the spicules, with AL being the most external and occurring only in the lower part of the spicules and TL being intermediate between AL and PG. The TL, which is composed of several thinner layers, is formed by a progressive folding of its surface but its microstructure is the same as in the PG layer (glassy silica). The AL differs significantly from the PG and TL in being granular and porous in structure. The TL was found to display positive structures (tubercles), not depressions, as earlier suggested. The apparent perforated and non-perforated bands of the AL are an optical artefact. The new layer type that we called the Ripple Mark Layer (RML) was noted, as well as narrow spikes on the AL ridges, both structures not reported earlier. The interface of the TL and AL, where tubercles fit into depressions of the lower surface of the AL, represent tenon and mortise or dovetail joints, making the spicules more stiff/strong and thus less prone to breaking in the lower part. Early stages of the spicule growth are bidirectional, later growth is unidirectional toward the spicule apex. Growth in thickness proceeds by adding new layers. The spicules are composed of well condensed silica, but the outermost AL is characterized by slightly more condensed silica with less water than the rest. Organics permeating the silica are homogeneous and proteinaceous. The external organic net (most probably collagen) enveloping the basal spicule is a structural element that bounds the sponge body together with the spicule, rather than controlling tubercle formation. Growth of various layers may proceed simultaneously in different locations along the spicule and it is sclerosyncytium that controls formation of silica layers. The growth in spicule length is controlled by extension of the top of the axial filament that is not enclosed by silica and is not involved in further silica deposition. No structures that can be related to sclerocytes (as known in Demospongiae) in Monorhaphis were discovered during this study.

CONCLUSIONS

Our studies resulted in a new insight into the structure and growth of the basal Monorhaphis spicules that contradicts earlier results, and permitted us to propose a new model of this spicule's formation. Due to its unique structure, associated with its function, the basal spicule of Monorhaphis chuni cannot serve as a general model of growth for all hexactinellid spicules.

Involvement of aquaporin channels in water extrusion from biosilica during maturation of sponge siliceous spicules

Aquaporins are a family of small, pore-forming, integral cell membrane proteins. This ancient protein family functions as water channels and is found in all kingdoms (including archaea, eubacteria, fungi, plants, and animals). We discovered that in sponges aquaporin plays a novel role during the maturation of spicules, their skeletal elements. Spicules are synthesized enzymatically via silicatein following a polycondensation reaction. During this process, a 1:1 stoichiometric release of water per one Si-O-Si bond formed is produced. The product of silicatein, biosilica, is a fluffy, soft material that must be hardened in order to function as a solid rod. Using the model of the demosponge species Suberites domuncula Olivi, 1792, which expresses aquaporin, cDNA was cloned and the protein was heterologously expressed. The sponge aquaporin is grouped with the type 8 aquaporins. The function of the sponge aquaporin can be blocked by Mn-sulfate (MnSO4) and mercury chloride (HgCl2). Microscopic and functional studies suggest that aquaporin is involved in removal of the reaction water at the site where siliceous spicules are formed. Another molecule that is likely to be involved in biosilica maturation is the mucin/nidogen-like polypeptide. cDNA has also been cloned from S. domuncula. Experimental studies suggest that water extrusion/suctioning from biosilica after enzymatic synthesis during spicule formation involves both aquaporin-mediated water channeling and "polymerization-induced phase separation" facilitated by the mucin/nidogen-like polypeptide.

Mesostructure from hydration gradients in demosponge biosilica

Organisms of the phylum Porifera, that is, sponges, utilize enzymatic hydrolysis to concatenate bioavailable inorganic silicon to produce lightweight, strong, and often flexible skeletal elements called spicules. In their optical transparency, these remarkable biomaterials resemble fused silica, despite having been formed under ambient marine biological conditions. Although previous studies have elucidated the chemical mechanisms of spicule formation and revealed the extensive hydration of these glasses, their precise composition and local and medium-range structures had not been determined. We have employed a combination of compositional analysis, (1) H and (29) Si solid-state nuclear magnetic resonance spectroscopy, and synchrotron X-ray total scattering to characterize spicule-derived silica produced by the demosponge Tethya aurantia. These studies indicate that the materials are highly hydrated, but in an inhomogeneous manner. The spicule-derived silica is, on average, perfectly dense for the given extent of hydration and regions of fully condensed and unstrained SiO networks persist throughout each monolithic spicule. To accommodate chemical strain and defects, the extensive hydration is concentrated in distinct regions that give rise to mesostructural features. The chemistry responsible for producing spicule silica resembles hydrolytic sol-gel processing, which offers exceptional control over the precise local atomic arrangement of materials. However, the specific processing involved in forming the sponge spicule silica further results in regions of fully condensed silica coexisting with regions of incomplete condensation. This mesostructure suggests a mechanism for atomistic defect tolerance and strain relief that may account for the unusual mechanical properties of the biogenic spicules.

Silicateins--a novel paradigm in bioinorganic chemistry: enzymatic synthesis of inorganic polymeric silica

The inorganic matrix of the siliceous skeletal elements of sponges, that is, spicules, is formed of amorphous biosilica. Until a decade ago, it remained unclear how the hard biosilica monoliths of the spicules are formed in sponges that live in a silica-poor (<50 μM) aquatic environment. The following two discoveries caused a paradigm shift and allowed an elucidation of the processes underlying spicule formation; first the discovery that in the spicules only one major protein, silicatein, exists and second, that this protein displays a bio-catalytical, enzymatic function. These findings caused a paradigm shift, since silicatein is the first enzyme that catalyzes the formation of an inorganic polymer from an inorganic monomeric substrate. In the present review the successive steps, following the synthesis of the silicatein product, biosilica, and resulting in the formation of the hard monolithic spicules is given. The new insight is assumed to open new horizons in the field of biotechnology and also in biomedicine.

Inorganic materials using 'unusual' microorganisms

A promising avenue of research in materials science is to follow the strategies used by Mother Nature to fabricate ornate hierarchical structures as exemplified by organisms such as diatoms, sponges and magnetotactic bacteria. Some of the strategies used in the biological world to create functional inorganic materials may well have practical implications in the world of nanomaterials. Therefore, the strive towards exploring nature's ingenious work for designing strategies to create inorganic nanomaterials in our laboratories has led to development of biological and biomimetic synthesis routes over the past decade or so. A large proportion of these relentless efforts have explored the use of those microorganisms, which are typically not known to encounter these inorganic materials in their natural environment. Therefore, one can consider these microorganisms as 'unusual' for the purpose for which they have been utilized - it is in this context that this review has been penned down. In this extensive review, we discuss the use of these 'unusual' microorganisms for deliberate biosynthesis of various nanomaterials including biominerals, metals, sulfides and oxides nanoparticles. In addition to biosynthesis approach, we have also discussed a bioleaching approach, which can provide a noble platform for room-temperature synthesis of inorganic nanomaterials using naturally available raw materials. Moreover, the unique properties and functionalities displayed by these biogenic inorganic materials have been discussed, wherever such properties have been investigated previously. Finally, towards the end of this review, we have made efforts to summarize the common outcomes of the biosynthesis process and draw conclusions, which provide a perspective on the current status of the biosynthesis research field and highlights areas where future research in this field should be directed to realize the full potential of biological routes towards nanomaterials synthesis. Furthermore, the review clearly demonstrates that the biological route to inorganic materials synthesis is not merely an addition to the existing list of synthesis routes; biological routes using 'unusual' microorganisms might in fact provide an edge over other nanomaterials synthesis routes in terms of their eco-friendliness, low energy intensiveness, and economically-viable synthesis. This review has significant importance for colloids and interface science since it underpins the synthesis of colloidal materials using 'unusual' microorganism, wherein the role of biological interfaces for controlled synthesis of technologically important nanomaterials is clearly evident.

Intra-epithelial spicules in a homosclerophorid sponge

Attempts to understand the intricacies of biosilicification in sponges are hampered by difficulties in isolating and culturing their sclerocytes, which are specialized cells that wander at low density within the sponge body, and which are considered as being solely responsible for the secretion of siliceous skeletal structures (spicules). By investigating the homosclerophorid Corticium candelabrum, traditionally included in the class Demospongiae, we show that two abundant cell types of the epithelia (pinacocytes), in addition to sclerocytes, contain spicules intracellularly. The small size of these intracellular spicules, together with the ultrastructure of their silica layers, indicates that their silicification is unfinished and supports the idea that they are produced "in situ" by the epithelial cells rather than being incorporated from the intercellular mesohyl. The origin of small spicules that also occur (though rarely) within the nucleus of sclerocytes and the cytoplasm of choanocytes is more uncertain. Not only the location, but also the structure of spicules are unconventional in this sponge. Cross-sectioned spicules show a subcircular axial filament externally enveloped by a silica layer, followed by two concentric extra-axial organic layers, each being in turn surrounded by a silica ring. We interpret this structural pattern as the result of a distinctive three-step process, consisting of an initial (axial) silicification wave around the axial filament and two subsequent (extra-axial) silicification waves. These findings indicate that the cellular mechanisms of spicule production vary across sponges and reveal the need for a careful re-examination of the hitherto monophyletic state attributed to biosilicification within the phylum Porifera.

The biology of glass sponges

As the most ancient extant metazoans, glass sponges (Hexactinellida) have attracted recent attention in the areas of molecular evolution and the evolution of conduction systems but they are also interesting because of their unique histology: the greater part of their soft tissue consists of a single, multinucleate syncytium that ramifies throughout the sponge. This trabecular syncytium serves both for transport and as a pathway for propagation of action potentials that trigger flagellar arrests in the flagellated chambers. The present chapter is the first comprehensive modern account of this group and covers work going back to the earliest work dealing with taxonomy, gross morphology and histology as well as dealing with more recent studies. The structure of cellular and syncytial tissues and the formation of specialised intercellular junctions are described. Experimental work on reaggregation of dissociated tissues is also covered, a process during which histocompatibility, fusion and syncytialisation have been investigated, and where the role of the cytoskeleton in tissue architecture and transport processes has been studied in depth. The siliceous skeleton is given special attention, with an account of discrete spicules and fused silica networks, their diversity and distribution, their importance as taxonomic features and the process of silication. Studies on particle capture, transport of internalised food objects and disposal of indigestible wastes are reviewed, along with production and control of the feeding current. The electrophysiology of the conduction system coordinating flagellar arrests is described. The review covers salient features of hexactinellid ecology, including an account of habitats, distribution, abundance, growth, seasonal regression, predation, mortality, regeneration, recruitment and symbiotic associations with other organisms. Work on the recently discovered hexactinellid reefs of Canada's western continental shelf, analogues of long-extinct Jurassic sponge reefs, is given special attention. Reproductive biology is another area that has benefited from recent investigations. Seasonality, gametogenesis, embryogenesis, differentiation and larval biology are now understood in broad outline, at least for some species. The process whereby the cellular early larva becomes syncytial is described. A final section deals with the classification of recent and fossil glass sponges, phylogenetic relationships within the Hexactinellida and the phylogenetic position of the group within the Porifera. Palaeontological aspects are covered in so far as they are relevant to these topics.

Some aspects of silica deposition in lithistid demosponge desmas

Transmitted light microscope and SEM observations of various growth stages, including very young forms, of lithistid demosponge spicules called desmas provided a wealth of new observations on silica deposition in desmas of most lithistid demosponge groups. In typical (pachastrellid) demosponges a basic feature of silica deposition in young spicules is the formation of silica granules (100-160 nm in diameter) deposited in more or less regular concentric layers. Further growth stages in typical demosponges are similar, only silica granules are smaller and more densely packed. The shape of the spicule is controlled by an organic axial filament, while features of the outer spicule surface are also determined by silicalemma. In lithistid desmas the early stage of silica deposition is controlled by an organic axial filament or, in some cases, dispersed organic molecules only. The next step, after early arrest of axial filament growth and its total encasing by silica, is the deposition of various silica granules (40-300 nm in diameter), spheres, and/or cylinders (1,300-3,330 nm), which are either the result of precipitation or the effect of templating by organic molecules (proteins and polysaccharides), without direct control by the silicalemma. The later stages of desma growth are under direct control of the silicalemma, which molds secondary branches and/or elements of sculpture of the desmas. The tips of desmas, which articulate with older desmas, are also controlled by local spatial relationships. Differentiation of morphological forms of silica in desmas, which is at least genus-specific, clearly supports the polyphyletic nature of lithistid sponges.

Biological glass fibers: correlation between optical and structural properties

Biological systems have, through the course of time, evolved unique solutions for complex optical problems. These solutions are often achieved through a sophisticated control of fine structural features. Here we present a detailed study of the optical properties of basalia spicules from the glass sponge Euplectella aspergillum and reconcile them with structural characteristics. We show these biosilica fibers to have a distinctive layered design with specific compositional variations in the glass/organic composite and a corresponding nonuniform refractive index profile with a high-index core and a low-index cladding. The spicules can function as single-mode, few-mode, or multimode fibers, with spines serving as illumination points along the spicule shaft. The presence of a lens-like structure at the end of the fiber increases its light-collecting efficiency. Although free-space coupling experiments emphasize the similarity of these spicules to commercial optical fibers, the absence of any birefringence, the presence of technologically inaccessible dopants in the fibers, and their improved mechanical properties highlight the advantages of the low-temperature synthesis used by biology to construct these remarkable structures.

Physical and chemical analysis of the siliceous skeletons in six sponges of two groups (demospongiae and hexactinellida)

The siliceous skeletons of six hexactinellids and demosponges were compared using a series of physical and chemical tests. The sponges were two hermit-crab sponges (Class Demospongiae, family Suberitidae), one from Scotland, Suberites domuncula, and the other, Pseudospongosorites suberitoides, from the Gulf of Mexico, and four hexactinellids, Hyalonema sp., Euplectella aspergillum, Rhabdocalyptus dawsoni, and Aphrocallistes vastus. The operating hypothesis was that differences in the amorphous hydrated silica skeletons in Demosponges and Hexactinellids might prove taxonomically useful. Physical properties studied included SEM, glass density, glass transition temperature (Tg), TG/DTA to determine water content, and FTIR spectra. Chemical determinations were made using energy dispersive X-ray fluorescence (EDXRF). Spicular skeletal material is deposited in concentric layers around the axial canal. With the exception of differences at several bands in the IR spectra, the siliceous skeletons in demosponges and hexactinellids are largely indistinguishable. Density was similar in all sponges (range 2.03-2.13 g/cc) and similar to the density of opal (SiO2 x 1.5H2O) (mean = 2.09 g/cc). IR spectra were similar, with prominent absorption bands at 460-470, 800, and 1,090-1,100 cm(-1) (due to different vibrational modes of Si-O-Si linkages) and at 1,650 and 3,450-3,560 cm(-1) (due to water). The skeletons of all six showed similar spectra to that of silica gel both before and after heating to 1,200 degrees C. Water comprised 10-14% of the skeleton by weight, slightly higher in the demosponges. Average spicule chemical composition was 85.2% SiO2, 12.3% water, and 2.5% other elements (mainly S, Al, K, Ca, and Na). The percent amounts of Si did not differ significantly between the demosponges and the hexactinellids.

Molecular biology of demosponge axial filaments and their roles in biosilicification

For hundreds of years, the skeletal elements of marine and freshwater sponges have intrigued investigators with a diverse array of remarkably complex morphologies. Early studies of demosponge monaxonal megascleres revealed the presence of a central organic axial filament running their entire length. Until recently, however, the precise function of these axial filaments was largely unknown. The spicules from the temperate Eastern Pacific demosponge, Tethya aurantia, comprise approximately 75% of the dry weight of this species, facilitating the large-scale isolation and purification of the biosilica-associated proteins. Silicateins, the most abundant proteins comprising the axial filaments of these spicules, prove to be members of a well-known superfamily of proteolytic and hydrolytic enzymes and can be easily collected after silica demineralization with hydrofluoric acid. Consistent with these findings, the intact filaments are more than simple, passive templates; in vitro, they actively catalyze and spatially direct the hydrolysis and polycondensation of silicon alkoxides to yield silica at neutral pH and low temperature. Catalytic activity also is exhibited by the monomeric subunits obtained by disaggregation of the protein filaments and those produced from recombinant DNA templates cloned in bacteria. These proteins also can be used to direct the polymerization of organosilicon polymers (silicones) from the corresponding organically functionalized silicon alkoxides. Based on these observations, the silicateins are currently being used as models for the design of biomimetic agents with unique catalytic and structure-directing properties. The presence of axial filaments in a diversity of spicule types and the evolutionary implications of these findings are also discussed.

Silicatein filaments and subunits from a marine sponge direct the polymerization of silica and silicones in vitro

Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators, optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis experiments. The catalytic activity of the "silicatein" (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.