First evidence of chitin in calcified coralline algae: new insights into the calcification process of Clathromorphum compactum.

Interest in calcifying coralline algae has been increasing over the past years due to the discovery of extensive coralline algal dominated ecosystems in Arctic and Subarctic latitudes, their projected sensitivity to ocean acidification and their utility as palaeoenvironmental proxies. Thus, it is crucial to obtain a detailed understanding of their calcification process. We here extracted calcified skeletal organic matrix components including soluble and insoluble fractions from the widely-distributed Subarctic and Arctic coralline alga Clathromorphum compactum. The lyophilized skeletal organic matrix fractions showed comparatively high concentrations of soluble and insoluble organic matrices comprising 0.9% and 4.5% of skeletal weight, respectively. This is significantly higher than in other skeletal marine calcifiers. Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and X-Ray Diffraction (XRD) results indicate that chitin is present in the skeletal organic matrices of C. compactum. This polymer exhibits similar hierarchical structural organizations with collagen present in the matrix and serves as a template for nucleation and controls the location and orientation of mineral phases. Chitin contributes to significantly increasing skeletal strength, making C. compactum highly adapted for living in a shallow high-latitude benthic environment. Furthermore, chitin containing polysaccharides can increase resistance of calcifiers to negative effects of ocean acidification.