## 4 Conclusions  
Studies using a complementary set of methods demonstrated that during pyrolysis of the marine sponge, its three-dimensional architecture at the macro- and microlevels was preserved, and at the nanolevel, the graphitization of the interfibril medium occurred with the formation of turbostratic graphite. The current research for the first time demonstrated that the graphitization process and turbostratic graphite formation directly depended  
Figure 5: Raman spectra of the marine sponge before (Native sponge) and after (Carbonized sponge) thermal modification. on the composition and properties of the matrix containing a large number of aromatic compounds that were located around the spongin fibers. EDX, XPS, and NEXAFS studies of the atomic and molecular composition showed that the native sponge composition included carbon (74-77 at.%); oxygen (16-20 at.%); nitrogen (2-3 at.%); calcium (-2 at.%); silicon (-2 at.%); and small amounts of potassium, sodium, magnesium, and sulfur. From a comparative analysis of the spectral data, the contents of carbon atoms in the atomic groups were estimated at 63.2% (C-C and C=C), 6.3% (C-ON), and 7.3% (C=O), and for oxygen at 13.7% (C=O) and 2.3% (C-OH). Based on the literature data on the contributions of amino acids in the spongin and assuming that all nitrogen was concentrated in the peptide groups, the atomic contents of carbon and oxygen in the spongin were \(\sim\)5 at.% and \(\sim\)4 at.%, respectively. The matter concentrated in the space between the spongin fibrils included -70 at.% carbon and -11 at.% oxygen, of which -63 at.% carbon was involved in the formation of aromatic and C-C bonds, while the rest was in carbonyl, carboxyl, phenolic, and epoxy (C-O-C) atomic groups. The analysis of TG, DTGA, and DSC dependences during the marine sponge pyrolysis in an argon flow at temperatures of 24-1200 \({}^{\circ}\)C showed that the process of sample modification proceeded in three stages: stage I involved a mass loss of -10% (80-140 \({}^{\circ}\)C) due to water evaporation, stage II involved a 40% mass loss (210-450 \({}^{\circ}\)C) due to peptide bond decomposition and carbon combustion with a release of nitrogen-containing gases and carbon dioxide, and stage III involved a -20% mass loss (800-1100 \({}^{\circ}\)C) and was characterized by a sequence of alternating exothermic and endothermic narrow peaks in the DSC curve due to the auto-oscillatory course of the transformation reaction (3) in the presence of CO\({}_{2}\). It was shown that by the end of stage II at 450 \({}^{\circ}\)C after the complete decomposition of spongin, the process of graphitization with the formation of turbostratic graphite had already proceeded with the release of nitrogen-containing gases, carbon dioxide, and water. This led to the preservation of the 3D structure of the native sponge and the transformation of the aromatic bond into an sp2 bond with the formation of graphite nanocrystallites that were \(\approx\)3 nm in size. The results of the studies showed extreme informativity of spectral research methods at macro- (TGA, DTGA, DSC), micro- (SEM, EDX, Raman spectroscopy), and nanolevels (XPS, NEXAFS) when analyzing the graphitization process of biomaterials and also demonstrated the prospects of further studies of marine sponges modification. The obtained data and developed techniques can stimulate the search for other promising biological materials for carbonization that have a complex 3D structure and contain a large number of aromatic atomic groups in their composition. Conceptualization, O.V.P. and D.V.S. ; research supervision, O.V.P., D.V.S., V.N.S. and A.S.V. ; writing--original draft, O.V.P., D.V.S., A.S.V. and V.N.S. ; TGA, DTGA and DSC interpretation, R.N.S., K.A.B. and V.N.S. ; Raman spectroscopy measurements, S.I.I. ; XPS and NEXAFS spectroscopy measurements and its data interpretation, O.V.P., S.V.N., D.V.S., A.S.V., P.M.K., K.A.B., R.N.S. and V.N.S. All the authors were actively involved in discussing the results and writing and drafting the manuscript.