The cement of the tube-dwelling polychaete <i>Sabellaria alveolata</i> : a complex composite adhesive material

Adhesives produced by marine organisms offer remarkable performance and serve as a major source of inspiration for developing biomimetic adhesives. However, a thorough understanding of their composition and operating mechanism is essential for advancing such applications. Sabellariid tubeworms are model organisms in bioadhesion research, and their adhesive system has been characterized in several studies. However, some aspects of cement formation are still poorly understood and several differences have been pointed out between the two main model species. This study aims to investigate the adhesive system of <i>Sabellaria alveolata</i> by identifying new potential adhesive proteins, as well as describing the ultrastructure and elemental composition of the cement cells and their secretion. Different adhesive proteins are packaged in one or the other of two types of cement cells, namely, those containing homogeneous granules and those containing heterogeneous granules with lamellar inclusions. Phosphoserine has been identified as one of the main modified amino acids in tubeworm cement and, using in situ hybridization, we propose that FAM20C kinases would be the enzymes responsible for the phosphorylation of serine residues in adhesive proteins. Comparison between the ultrastructure of the granules and that of the cement suggests that the inclusions of the heterogeneous granules would inflate through a still unexplained process to form hollow spheroids dispersed in the cement matrix, leading to the formation of a complex composite material.