Research Article | Open Access
Structural Arrangement and Properties of Spicules in Glass Sponges
The morphology, chemical composition, and optical properties of long monoaxonic spicules were studied in several species of marine deep-sea hexactinellid sponges of different orders and families: Asconema setubalense (Hexasterophora, Lyssacinosida) and Monorhaphis chuni Schulze (Monorhaphiidae). Their macrostructural organization is a system of thin layers laid around the central cylinder containing a square canal filled with organic matter. A significant role in spicule organization is played by the organic matrix. The macrostructural of organization of the spicule in Monorhaphis chuni is a system of the “cylinder-within-a-cylinder” type. However the spicule surface is covered with ridges. They penetrate a few layers into the spicule. Analysis of the elemental composition of the basalia spicule of Monorhaphis chuni demonstrates a heterogeneous allocation of C, O, Si on the spicule surface, subsurface layers, and on ridges. All studied spicules have the properties of anisotropic crystals and they demonstrate a capability to the birefrigence. On the other hand we discovered unique property of spicules—their capacity for triboluminescence. The discovery of triboluminescence in composite organosilicon materials of which the spicules of hexactinellid sponges are built may contribute to the creation of biomimetic materials capable of generating light emission.
There are 5,000 species of sponges and around 600 of them belong to the classes of so named glass (Hyalospongia) or six radial (Hexactinellid) sponges. Not much more than a dozen years ago a small note by Italian authors  drew attention to the spicules of Hexactinellids, because they are similar to artificial optical fibers. For the last ten years the Hexactinellids spicules have been intensively studied by very different investigators: physicists, chemists, and biologists of different specialties (zoologists, cytologists, molecular biologists, biotechnologists, etc. [2–8]). Glass sponges are exclusively marine animals; they occur at depths from 10 to 6,770 m in all oceans. They have skeletons of siliceous (glass) spicules with a distinctive triaxonic (cubic three rayed) symmetry . Furthermore, glass sponges are highly unusual in that their major tissue component is a giant “syncytium” that ramifies throughout the entire body. They have amazing diversity of spicules. Only for their morphological description more than 400 terms have been suggested, which are used in taxonomy of Hexactinellids .
Study of basalia spicules of Euplectella aspergillum shows that the spicules of glass sponges can function as single-mode, few-mode, or multimode fibers. They have 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. These spicules are similar to commercial optical fibers [11, 12].
At the same time, the organization of corporeal spicules of skeletons of glass sponges is extremely diverse. Principal studies of the physical and chemical properties of spicules of hexactinellid sponges are carried out on basal monoaxonic megascleres, which are diactine or pentactine spicules with a hypertrophied proximal ray and reduced other rays.
The principal function of the spicules of hexactinellids is certainly a skeletal one. Still, their organizational traits imply the presence of other functions. In particular, a hypothesis was suggested that these spicules perform the function analogous to that of the nervous system. The spicules may serve as waveguides transmitting optical, electric, or chemical signals and carry information on the state of the environment to cells within a sponge [6, 7].
Previously, it was shown on the basis of investigation of the morphology of spicules, of absorption spectra, and fluorescence, as well as of the composition of fatty acids, that the glass hexactinellid sponge Pheronema raphanus possesses photosynthesizing cyanobacteria . This fact implies that the spicules of these sponges may have unusual optical properties providing the function of photosynthesizing systems in complete darkness.
This study deals with investigation of the structural organization of corporeal and basalia spicules and their characteristics with special traits of birefringence and their luminescence in the normal state and upon destruction.
2. Material and Methods
The morphology, chemical composition, and optical properties of long monoaxonic spicules were studied in several species of marine deep-sea hexactinellid sponges of different orders and families: Asconema setubalense (Hexasterophora, Lyssacinosida, Rossellidae Schulze, 1885) and Monorhaphis chuni Schulze (Monorhaphiidae). The material was taken from the collection of the Institute of Oceanology of the Russian Academy of Sciences (Moscow, Russia) or collected during the voyage of research ship Akademik Oparin in the South China Sea. The spicules were sputtered with platinum and examined under a LEO 430 scanning electron microscope (SEM).
Elemental composition was then obtained by using an energy-dispersive X-ray (EDX) module attached to the SEM. Platinum-coated specimens were analyzed for the presence of carbon, and carbon-coated specimens were analyzed for the presence of C, O, Na, Si, cu.
For destruction, the middle part of a spicule was heated. The heated part exploded. The explosion was accompanied by a popping sound and luminescence. Linear heating of a sample was provided by a miniature ceramic heater. The basis of the luminometer was an R928 photoelectric amplifier in configuration E717-22 (Hamamatsu Photonics, Japan) loaded at the input resistance of an ASK 3107 digital four-channel oscillograph (ASK, Russia). Acoustic emission was taken directly in the working chamber of the triboluminometer with a built-in electret microphone. Selection, synchronization, amplification, and processing of signals were provided by the working program of an ASK-3107-PO1 oscillograph.
We studied the birefringence, or double refraction, with an optical microscope, rotating the table with specimens, through which the polarized light passed.
3.1. Organization of Spicules
The spicules of the investigated species of glass sponges from different orders and families are similar in structure. Their macrostructural organization is a system of thin layers laid around the central cylinder containing a square canal filled with organic matter (Figures 1(a) and 1(b)). The diameter of the central cylinder is ca 50 μm and the thickness of layers laid around it is ca. 40 nm. The layers are penetrated by channels, usually in pairs (Figure 1(c)).
A significant role in spicule organization is played by the organic matrix. It may be revealed after delicate extraction of silicon oxide during long time, around two months, in 1 M NaOH. After such procedure the spicules assume a honeycomb structure, composed by the organic component (Figure 2).
Analysis of the elemental composition of C, O, Na, Si, Cu in percentages by weight of the corporal monoaxonic spicule of A. setubalense (Figure 3, Table 1) shows that the spatial organization of spicules is complicated and elements are disposed on the spicule incoherently. On the spicule surface in points 1–5 the composition of elements C, O, Na, Si, Cu is approximately equal. Silicon content is 34–36%, carbon: 25–29%, sodium: about 1% and trace amounts of copper: not more than 0.58%. Oxygen is the most variable: 47.96–54.47%. Highly variable amounts of carbon (68.89–97.44%), oxygen (18.04–65.2%), and silicon (1.64–26.47%) are found on the cross-section.
3.1.1. The Ultrastructure of the Basalia Spicule of Monorhaphis chuni
This glass sponge has a single basalia spicule. It may be very long (up to 3 m) and thick (up to 30 mm in diameter) . The central part of such spicule has the classic structure. There is a well-defined organic axial filament, located in the central channel. Around the filament are silica layers alternating with layers of the organic matrix. As a result, the macrostructural organization of the spicule is a system of the “cylinder-within-a-cylinder” type.
However the surface layers of this spicule have different structure (Figure 4). The spicule surface is covered with ridges having a length of approximately 50 μm and height of approximately 10 μm. They penetrate a few layers into the spicule.
Analysis of the elemental composition of the basalia spicule of M. chuni (Figure 5, Table 2) demonstrates a heterogeneous allocation of C, O, Si, Cl, K on the spicule surface, subsurface layers, and on ridges. This is evidence of a very complicated structure of this basalia spicule.
3.2. Spicule Properties
The intensity of the passed polarized light transforms during rotation of corporeal spicule (Figures 6(a)–6(d)). When passing polarized light through a cross-section of the basalia spicule, a characteristic conoscopic figure in the form of an obscure cross is observed, located exactly in the center of picture (Figure 6(e)).
3.2.2. Triboluminescence of Spicules
The spicules may be destroyed by various methods. Local heating of a spicule was used in the present case. When spicules are heated, they keep their optical transparency to 120°C. The spicule begins to be destroyed at approximately 150°C. Swellings appear on the spicules. Above 320°C, the spicules are destroyed. In some cases, when a spicule was destroyed, the integrity of outer concentric layers was explosively disrupted, but the central cylinder is not destroyed and retains its integrity (Figure 7).
The process of destruction of the peripheral layers of spicules is accompanied by a popping sound and a luminescent flash (Figure 8).
In spite of a lot of studies devoted to the structure and physicochemical properties of spicules in Hexactinellids, (e.g., [3–8, 11, 12, 15–17], etc.), their biogenesis, organization, function, and biological meaning are far from being perfectly understood.
Furthermore in the last dozen years certain mythology related to spicules has taken root in the scientific community, which has been repeated and practically not called into question. It concerns both the spicule organization as well as spicule properties and function. Macrostructural spicule organization was interpreted as an intercalation of silica layers and organic matrix of a “cylinder-within-a-cylinder” type [3–5, 8]. Spicule biogenesis was represented as a result of absorption of silica from marine water around proteins silicateins [15, 17–20]. This process was suggested to be repeated and resulted in a multilayer structure with alternating mineral and organic layers.
Our data on spicule ultrastructure and elemental composition indicate a very complicated spicule organization. The organic matrix has a three-dimensional arrangement. It is located not only in the central spicule channel but also in the intermediate layers between silica layers. It also permeates these layers with radial rays. In other words, the organic spicule components form the volumetrical three-dimensional skeleton of the whole of spicule, which organizes the silica mineral component of a spicule. Both these constituents form a composite material possessing anisotropic crystal characteristics.
Glass sponge spicules are able to bifurcate polarized light rays, that is, they have birefringence. At this point our data contradict the prevailing opinion that birefringence in glass sponge spicules is absent .
Another unique property of spicules, that we suggest now, is their capacity for triboluminescence. Triboluminescence (mechanoluminescence), the emission of light at destruction of materials, has been known for more than four hundred years. Triboluminophores are a rather wide class of organic and inorganic substances in a crystalline form. The nature of this physical phenomenon is not quite clear . As a rule, the spectrum of triboluminescence nearly or completely coincides with the spectrum of photoluminescence .
There are no available published studies on triboluminophores among composite materials. Triboluminescence occurring upon destruction of the composite organosilicon material composing a part of sponge spicules has been described here for the first time. Destruction may be attained in various ways. In our experiments, the thermal blasting of spicules was used. Due to local heating of a spicule, it explodes, which is accompanied by a popping sound and a subsequent photoemission.
Triboluminescence of organosilicon spicules seems to be due to their complex multilayered organization. In different parts of spicules, there is mechanical tension between layers that is removed upon destruction of the spicule. The discovery of triboluminescence in spicules of deep-sea hexactinellid sponges suggests that the spicules may provide photons to the photosynthesizing symbionts living in them. It is likely that, under certain conditions, the sponge itself may provide its own symbionts with light energy .
Recently, the interest in new materials capable of triboluminescence has increased due to the creation of light-emitting sensors of destruction [21, 22]. The discovery of triboluminescence in composite organosilicon materials of which the spicules of hexactinellid sponges are built may contribute to the creation of biomimetic materials capable of generating light emission. This, in turn, may be used in various technical constructions for the transformation of energy.
Additional investigations are required for understanding of biological meaning of complicated and unique properties of hexactinellids spicules.
The authors are grateful to K. R. Tabachnik for providing them with the material of spicules. The work was supported by grant of Russian government for national supporting of scientific investigations under direction of leading scientists in Russian universities: Contract 11.G34.31.0010.
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Copyright © 2011 Anatoliy L. Drozdov and Alexander A. Karpenko. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.