Coralline algal calcification: A morphological and process-based understanding

Research purpose and findingsCoralline 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 calcificationWe propose the following three-step model for calcification. 1) A thin (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.研究目的与研究发现 珊瑚藻（coralline algae）是全球众多碳酸盐系统的关键生物基质。它们能够形成持久外壳，支撑热带珊瑚礁、珊瑚藻砾石床及其他底栖基质的形成，这一能力依赖于钙化叶状体的形成。然而，这一重要的骨骼碳酸盐形成过程目前仍未得到充分解析。我们针对细胞层面的碳酸盐特征开展研究，以期建立钙化作用模型。本研究描述了两类细胞壁钙化模式：1） 薄壁伸长细胞中的钙化初生细胞壁（PCW），例如具关节珊瑚藻的中央髓细胞，以及壳状珊瑚藻（CCA）的下表皮层细胞；2） 厚壁圆形皮层细胞中的、带有放射状镁方解石晶体的钙化次生细胞壁（SCW），这类细胞存在于具关节珊瑚藻及壳状珊瑚藻（CCA）的围周层细胞中。许多珊瑚藻砾石床所特有的条带结构，正是仅含初生细胞壁的细胞与次生细胞壁细胞之间的规律性过渡。在细胞壁内部，可存在镁含量升高的条带，其镁摩尔百分比相较于放射状镁方解石（M型）高出数个百分点，最高可达白云石组分（D型）。 钙化作用模型 我们提出如下三步钙化作用模型：1） 针对薄外壳类群（如克拉通藻属Clathromorphum），次生细胞壁（SCW）的形成滞后于初生细胞壁，进而形成薄弱区域，导致表层下外壳出现裂隙。所有钙化作用似乎更倾向于是生物诱导过程，而非受控过程。本研究的发现为理解珊瑚藻的钙化机制迈出了重要一步。