Biomacromolecules in bivalve shells with crossed lamellar architecture

We present an in-depth characterisation of shells from two bivalve species with crossed lamellar microstructure, namely Tridacna gigas and Fulvia tenuicostata. High-resolution scanning electron microscopy and confocal microscopy imaging reveal a fine structure of nanogranular particles that are inorganic–bioorganic nanocomposites for both shells. In F. tenuicostata, inorganic–organic components are arranged in a polycrystalline fibre-like fabric. T. gigas consists of up to four hierarchical lamellar structural orders and the second-order lamellae consist of elongated nanometre-sized laths. The inorganic matrix is intimately intergrown with the total amount of organic matter (1.8 and 1.5 wt%), and the composition of the shell macromolecules is variable between the two calcareous biominerals. This work shows for the first time the presence of polysaccharide-based compounds that could be essential for the construction of bio-organics as well as many prominent protein bands, glycoproteins and/or glycosaminoglycans of unknown sizes far above 260 kDa in bivalve shells with crossed lamellar microstructure. Chitosan (deacetylated chitin) with apparent molecular weights from 18 to 110 kDa for T. gigas and from 12 kDa till above 110 kDa for F. tenuicostata are detected in gel electrophoresis after Calcofluor staining. In each of the shell extracts, the infrared spectroscopy shows polysaccharides, proteins and lipids. Our findings from two crossed lamellar shells representing two genera of Mollusca: Cardiidae indicate that chitin–protein complexes and lipid–lipoproteins are not restricted only to bivalves with nacroprismatic shells.