Molecular signatures of biomacromolecules at micron and submicron scales in Arctica islandica shells

Arctica islandica (Linnaeus, 1767) is a bivalve mollusk with a lifespan of up to 500 years. Its shell is a complex inorganic-organic nanocomposite, which is still not fully characterized. Here, we used Optical Photothermal Infrared (O-PTIR) microspectroscopy to characterize its organic phase, i.e. the hydrophobic insoluble extracts of both modern and fossil A. islandica shells. We compared the spectra from O-PTIR technique with those obtained from microRaman, microFTIR and ATR-FTIR spectroscopies. The structure of the shells was also studied using scanning electron microscope imaging. The spectroscopic techniques confirmed the presence of β-chitin and chitin-binding protein in the hydrophobic insoluble organic matrix. The results of element and light stable isotope analyses (δ¹³C, δ¹⁵N) from the hydrophobic insoluble extracts of both the modern and fossil shells were consistent with a shallow marine environment. The results demonstrate how O-PTIR can be applied in-situ for the study of biomaterials in shells, and points towards how it can be applied for the elucidation of intricate structure-function relationships. Furthermore, the approach of using the hydrophobic insoluble extracts to probe conditions and the nature of organic remains in fossil shells demonstrate an alternative to the use of soft tissues in isotopic studies and hence opens new possibilities for environmental reconstruction using fossil material when soft tissues are unavailable.