Axial growth of hexactinellid spicules: formation of cone-like structural units in the giant basal spicules of the hexactinellid Monorhaphis

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.

Produce, carry/position, and connect: morphogenesis using rigid materials

Animal morphogenesis can be summarized as a reconfiguration of a mass of cells. Although extracellular matrices that include rigid skeletal elements, such as cartilage/bones and exoskeletons, have important roles in morphogenesis, they are also secreted in situ by accumulated cells or epithelial cells. In contrast, recent studies of the skeleton construction of sponges (Porifera) illuminate a conceptually different mechanism of morphogenesis in which cells manipulate rather fine rigid materials (spicules) to form larger structures. Here, two different types of sponge skeleton formation using calcareous spicules or siliceous spicules are compared with regard to the concept of the production of rigid materials and their use in skeletons. The comparison highlights the advantages of their different strategies of forming sponge skeletons.

Localization and characterization of ferritin in Demospongiae: a possible role on spiculogenesis

Iron, as inorganic ion or as oxide, is widely used by biological systems in a myriad of biological functions (e.g., enzymatic, gene activation and/or regulation). In particular, marine organisms containing silica structures—diatoms and sponges—grow preferentially in the presence of iron. Using primary sponge cell culture from S. domuncula–primmorphs—as an in vitro model to study the Demospongiae spiculogenesis, we found the presence of agglomerates 50 nm in diameter exclusively inside sponge specialized cells called sclerocytes. A clear phase/material separation is observed between the agglomerates and the initial stages of intracellular spicule formation. STEM-HRTEM-EDX analysis of the agglomerates (30–100 nm) showed that they are composed of pseudohexagonal nanoparticles between 5 and 15 nm in size, displaying lattice parameters corresponding to hematite (Fe₂O₃) and mixed iron oxide phases typically attributed to ferritin. Further analysis, using western blotting, inductively coupled plasma mass spectrometry (ICP-MS), sequence alignment analysis, immunostaining and magnetic resonance imaging (MRI), of mature spicule filaments confirm the presence of ferritin within these organic structures. We suggest that S. domuncula can be classified as a dual biomineralizating organism, i.e., within the same cellular structure two distinct biomineralizing processes can occur as a result of the same cellular/metabolic function, spiculogenesis.

Distribution of microfossils within polymetallic nodules: biogenic clusters within manganese layers

Biomineralization is the process by which living organisms or organic matrices produced by them initiate and structure deposition of inorganic polymers/minerals. Deep-sea polymetallic nodules and crusts have recently been recognized as biominerals that are formed around bio-seeds; these deposits are of economic value. A detailed understanding of their formation will contribute to their sustainable exploitation in the future. Polymetallic nodules grow concentrically around discrete nuclei that have recently been described as bio-seeds formed from microorganisms, diatoms, or coccoliths. In the present study, polymetallic nodules from the Clarion-Clipperton Zone have been analyzed. It is described that the approximately 5-cm large polymetallic nodules are composed of micronodules (size of 100-450 μm) that aggregated to nests (2-3 mm). High-resolution scanning electron microscopy (HR-SEM) and high-resolution energy dispersive X-ray (HR-EDX) spectroscopic analyses revealed that the micronodules are composed of discrete layers of Mn and Fe. Imprints of microorganisms/microbe-like assemblies are found in the Mn-rich regions of the micronodules. HR-SEM/EDX analyses confirmed that these microorganisms are surrounded by a Mn-rich environment. These findings strongly suggest that those organisms acted as bio-seeds that allowed the deposition of Mn(IV) minerals which in turn helped Fe minerals to associate. Hence, these data support the concept that the growth of the polymetallic nodules starts as a biomineral and is completed by genuine mineralic depositions. It is expected that these data will contribute to the development of strategies for a sustainable exploitation of the polymetallic nodules.

The role of proteins in biosilicification

Although the use of silicon dioxide (silica) as a constituent of living organisms is mainly restricted to diatoms and sponges, the ways in which this process is controlled by nature continue to inspire and fascinate. Both diatoms and sponges carry out biosilificiation using an organic matrix but they adopt very different strategies. Diatoms use small and heavily modified peptides called silaffins, where the most characteristic feature is a modulation of charge by attaching long chain polyamines (LCPAs) to lysine groups. Free LCPAs can also cooperate with silaffins. Sponges use the enzyme silicatein which is homologous to the cysteine protease cathepsin. Both classes of proteins form higher-order structures which act both as structural templates and mechanistic catalysts for the polycondensation reaction. In both cases, additional proteins are continuously being discovered which modulate the process further. This paper concentrates on the role of these proteins in the biosilification process as well as in various applications, highlighting areas where focus on specific protein properties may provide further insight. The field of biosilification is a crossroads of different disciplines, where insight into the energetics and mechanisms of molecular self-assembly combine with fundamental biology, complex multicomponent colloidal systems, and an impressive array of potential technological applications.

Circumferential spicule growth by pericellular silica deposition in the hexactinellid sponge Monorhaphis chuni

The giant basal spicule of the hexactinellid sponge Monorhaphis chuni represents the longest natural siliceous structure on Earth. This spicule is composed of concentrically arranged lamellae that are approximately 10 μm thick. In the present study, we investigated the formation of outer lamellae on a cellular level using microscopic and spectroscopic techniques. It is shown that the formation of an outermost lamella begins with the association of cell clusters with the surface of the thickening and/or growing spicule. The cells release silica for controlled formation of a lamella. The pericellular (silica) material fuses to a delimited and textured layer of silica with depressions approximately 20-30 μm in diameter. The newly formed layer initially displays 40 μm wide, well-structured banded ribbons and only attains its plain surface in a final step. The chemical composition in the depressions was studied using energy dispersive X-ray spectroscopy and by staining with Texas Red. The data suggest that those depressions are the nests for the silica-forming cells and that silica formation starts with a direct association of silica-forming cells with the outer surface of the spicule, where they remain and initiate the development of the next lamellae.

Molecular biomineralization: toward an understanding of the biogenic origin of polymetallic nodules, seamount crusts, and hydrothermal vents

Polymetallic nodules and crusts, hydrothermal vents from the Deep Sea are economically interesting, since they contain alloying components, e.g., manganese or cobalt, that are used in the production of special steels; in addition, they contain rare metals applied for plasma screens, for magnets in hard disks, or in hybrid car motors. While hydrothermal vents can regenerate in weeks, polymetallic nodules and seamount crusts grow slowly. Even though the geochemical basis for the growth of the nodules and crusts has been well studied, the contribution of microorganisms to the formation of these minerals remained obscure. Recent HR-SEM (high-resolution scanning electron microscopy) analyses of nodules and crusts support their biogenic origin. Within the nodules, bacteria with surface S-layers are arranged on biofilm-like structures, around which Mn deposition starts. In crusts, coccoliths represent the dominant biologically formed structures that act as bio-seeds for an initial Mn deposition. In contrast, hydrothermal vents have apparently an abiogenic origin; however, their minerals are biogenically transformed by bacteria. In turn, strategies can now be developed for biotechnological enrichment as well as selective dissolution of metals from such concretions. We are convinced that the recent discoveries will considerably contribute to our understanding of the participation of organic matrices in the enrichment of those metals and will provide the basis for feasibility studies for biotechnological applications.

Giant siliceous spicules from the deep-sea glass sponge Monorhaphis chuni

Only 13 years after realizing, during a repair of a telegraph cable pulled out from the deep sea, that the depth 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 had 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. Soon after, during the German Deep Sea Expedition "Valdivia" (1898-1899), Schulze could describe the largest siliceous hexactinellid sponge on Earth, the up to 3-m high Monorhaphis chuni, which develops the equally largest bio-silica structure, the giant basal spicules (3 mx10 mm). Using these spicules as a model, basic knowledge on the morphology, formation, and development of the skeletal elements could be achieved. They are formed by a proteinaceous scaffold (composed of a 27-kDa protein), which mediates the formation of the siliceous lamellae, into which the proteins are encased. The high number of 800 of 5-10 microm thick lamellae is concentrically arranged around the axial canal. The silica matrix is composed of almost pure silicon oxide, providing it with unusually optophysical properties, which are superior to those of man-made waveguides. Experiments might suggest that the spicules function in vivo as a nonocular photoreception system. In addition, the spicules have exceptional mechanical properties, combining mechanical stability with strength and stiffness. Like demosponges, also the hexactinellids synthesize their silica enzymatically, via the enzyme silicatein (27-kDa protein). It is suggested that these basic insights will surely contribute to a further applied utilization and exploration of silica in bio-material/biomedical science.

Sponge spicules as blueprints for the biofabrication of inorganic-organic composites and biomaterials

While most forms of multicellular life have developed a calcium-based skeleton, a few specialized organisms complement their body plan with silica. However, of all recent animals, only sponges (phylum Porifera) are able to polymerize silica enzymatically mediated in order to generate massive siliceous skeletal elements (spicules) during a unique reaction, at ambient temperature and pressure. During this biomineralization process (i.e., biosilicification) hydrated, amorphous silica is deposited within highly specialized sponge cells, ultimately resulting in structures that range in size from micrometers to meters. Spicules lend structural stability to the sponge body, deter predators, and transmit light similar to optic fibers. This peculiar phenomenon has been comprehensively studied in recent years and in several approaches, the molecular background was explored to create tools that might be employed for novel bioinspired biotechnological and biomedical applications. Thus, it was discovered that spiculogenesis is mediated by the enzyme silicatein and starts intracellularly. The resulting silica nanoparticles fuse and subsequently form concentric lamellar layers around a central protein filament, consisting of silicatein and the scaffold protein silintaphin-1. Once the growing spicule is extruded into the extracellular space, it obtains final size and shape. Again, this process is mediated by silicatein and silintaphin-1, in combination with other molecules such as galectin and collagen. The molecular toolbox generated so far allows the fabrication of novel micro- and nanostructured composites, contributing to the economical and sustainable synthesis of biomaterials with unique characteristics. In this context, first bioinspired approaches implement recombinant silicatein and silintaphin-1 for applications in the field of biomedicine (biosilica-mediated regeneration of tooth and bone defects) or micro-optics (in vitro synthesis of light waveguides) with promising results.

Marine biominerals: perspectives and challenges for polymetallic nodules and crusts

Deep sea minerals in polymetallic nodules, crusts and hydrothermal vents are not only formed by mineralization but also by biologically driven processes involving microorganisms (biomineralization). Within the nodules, free-living and biofilm-forming bacteria provide the matrix for manganese deposition, and in cobalt-rich crusts, coccolithophores represent the dominant organisms that act as bio-seeds for an initial manganese deposition. These (bio)minerals are economically important: manganese is an important alloying component and cobalt forms part of special steels in addition to being used, along with other rare metals, in plasma screens, hard-disk magnets and hybrid car motors. Recent progress in our understanding of the participation of the organic matrices in the enrichment of these metals might provide the basis for feasibility studies of biotechnological applications.