High Magnesium Calcite and Dolomite composition carbonate in Amphiroa (Lithophyllaceae, Corallinales, Rhodophyta): further documentation of elevated Mg in Corallinales with climate change implications.

Species of the calcified, articulate coralline Amphiroa are key components of many shallow marine ecosystems. Understanding their mineral composition is important as their susceptibility to dissolution, due to ocean acidification, may vary with mineral composition. We studied the distribution of Mg-calcite, very high magnesium calcite (VHMC), and dolomite within Amphiroa species to elucidate their mineral properties and susceptibility to dissolution. Results revealed that the asymmetrical X-ray diffraction (XRD) pattern typical of Amphiroa globally represents high levels of VHMC and dolomite composition carbonate. The dolomite seems most likely to be disordered, but higher resolution XRD is required for confirmation. The calcified long sides of medullary cells have predominantly VHMC/dolomite and the corners have bands of VHMC/dolomite. Epithallial cell walls are low Mg-calcite, and cortical cells are low Mg-calcite with bands of VHMC. VHMC/dolomite is more stable than Mg-calcite, and this may provide a competitive advantage for Amphiroa species as seawater pH declines. Further work is required to determine the metabolic controls on VHMC/dolomite mineral formation.

Coralline algal calcification: A morphological and process-based understanding.

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.