## 1 Introduction  
The current work was devoted to the study of the carbonization process of marine sponges belonging exclusively to the Keratosa subclass, which has a mineral-free skeleton that consists of \"keratinized\" spongin fibers [1, 2]. This sponge subclass is characterized by well-developed, anastomosing spongin skeletons that are hierarchically organized into primary, secondary, and sometimes tertiary fiber structures and constitute a significant part of the body volume [3]. The skeletal structures cleaned from mineralized components, waste products, and \"contaminants\" via depigmentation and demineralization have been called \"bath sponges\" since ancient times, and in the scientific literature they are defined as \"commercial sponges\" [4]. Commercial sponges include a number of different members of the phylum _Porifera_, which differ in fiber morphology, porosity, and size. Their structure includes networks of spongin fibers, which can range in diameter from 5 to 100 microns, and the cells range in size from 100 microns to a millimeter, depending on the type of sponge. Unfortunately, the spongin biosynthesis pathways in bath sponges and the protein sequences of this unique biopolymer are still unknown and, to date, spongin has no clear chemical definition [5]. However, according to [6], spongin from sponges of the _Demosopones_ class has an amino acid composition similar to that of collagen type XIII (COL13) found in vertebrates. Studies [7; 8; 9] showed that spongin contains the classic collagen motif Gly-Xaa-Yaa, where hydroxyproline (Hyp) occupies any position in the triplet motif except for the glycine (Gly) position. This biopolymer with an unknown chemical structure contains sulfur, bromine, and iodine [10; 11; 12; 13; 14]. Due to their biocompatibility and their unique physicochemical, structural, and mechanical properties [15], spongin-based bath sponge scaffolds have been widely used in medicine [16] and tissue engineering [17; 18]. Spongin-containing marine sponges are examples of renewable resources due to their ability to be grown under marine breeding conditions in their natural habitat [19; 20]. This property makes marine sponges an attractive biological material with great biomimetic potential for various applications. Recently, it was shown [21; 22; 23; 24] that the use of 3D spongin-based scaffolds from natural marine sponges is promising regarding carbonization. Specifically, spongin carbonization in argon at temperatures up to 1200 \({}^{\circ}\)C leads to the formation of 3D nanoporous structure of turbostatic graphite, which can be applied to obtain new composite materials and, in particular, catalysts for seawater purification [21]. Many researchers previously attempted to manufacture carbon materials with a controlled microstructure and morphology from renewable and biodegradable natural sources. Although heat treatment of a variety of proteins usually causes their decomposition without the formation of carbonaceous materials [25], fiber-based structural proteins, such as keratin [26], collagen [27], and silk [28; 29], appear to be suitable for carbonization at temperatures between 200 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, and in some cases, even up to 2800 \({}^{\circ}\)C [30]. However, to date, there are no published data on the formation of ready-to-use carbon frameworks with a hierarchical structure. For example, the fibers of dried Luffe fruit, which has a spongy network of xylem fibers similar to a sponge, are completely destroyed after carbonization and only the carbonized material powder can be used for further applications [31]. Similar results were obtained for silk carbon fibers, which were too brittle for further processing [32]. In our previous works, we studied the physical and chemical properties of carbonized spongin, such as its structure and elemental composition; characterized the outer surface; and determined the presence of water and carbon oxides on it [21; 33]. However, the question about the process of spongin carbonization itself remains open.