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

ABSTRACT: 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.

SUBMITTER: Rahman MA

PROVIDER: S-EPMC4141250 | biostudies-other | 2014

REPOSITORIES: biostudies-other

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Project description:RESEARCH PURPOSE AND FINDINGS:Coralline algae are key biological substrates of many carbonate systems globally. Their capacity to build enduring crusts that underpin the formation of tropical reefs, rhodolith beds and other benthic substrate is dependent on the formation of a calcified thallus. However, this important process of skeletal carbonate formation is not well understood. We undertook a study of cellular carbonate features to develop a model for calcification. We describe two types of cell wall calcification; 1) calcified primary cell wall (PCW) in the thin-walled elongate cells such as central medullary cells in articulated corallines and hypothallial cells in crustose coralline algae (CCA), 2) calcified secondary cell wall (SCW) with radial Mg-calcite crystals in thicker-walled rounded cortical cells of articulated corallines and perithallial cells of CCA. The distinctive banding found in many rhodoliths is the regular transition from PCW-only cells to SCW cells. Within the cell walls there can be bands of elevated Mg with Mg content of a few mol% higher than radial Mg-calcite (M-type), ranging up to dolomite composition (D-type). MODEL FOR CALCIFICATION:We propose the following three-step model for calcification. 1) A thin (< 0.5 μm) PCW forms and is filled with a mineralising fluid of organic compounds and seawater. Nanometer-scale Mg-calcite grains precipitate on the organic structures within the PCW. 2) Crystalline cellulose microfibrils (CMF) are extruded perpendicularly from the cellulose synthase complexes (CSC) in the plasmalemma to form the SCW. 3) The CMF soaks in the mineralising fluid as it extrudes and becomes calcified, retaining the perpendicular form, thus building the radial calcite. In Clathromorphum, SCW formation lags PCW creating a zone of weakness resulting in a split in the sub-surface crust. All calcification seems likely to be a bioinduced rather than controlled process. These findings are a substantial step forward in understanding how corallines calcify.

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

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