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

Merinda C. Nash,Adela S. Harvey,Walter Adey,Guillermo Diaz-Pulido,Frank Melzner

doi:10.1371/journal.pone.0221396

Abstract

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 (< 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.

Highlights

Calcifying red algae are key components of many marine ecosystems globally
Based on the absence of impermeable barriers other than the plasmalemma, we propose that seawater penetrates down the interfilament, through the outer perimeter of the cell wall, mixing with the organism-produced fluids resulting in a mixed matrix fluid that enables Mg-calcite mineral formation
We propose that in the secondary cell wall (SCW) (Fig 3A and 3B), as the cellulose microfibrils (CMF) is extruded, it soaks in the mixed matrix fluid and becomes mineralised while it is still perpendicular to the cell wall membrane, forming the radial Mg-calcite (Fig 3)

Summary

Introduction

Calcifying red algae are key components of many marine ecosystems globally. One of their main values is substrate provision [1,2] via their formation of calcified structures. Fine branching articulated (i.e. geniculate) coralline algae are key ecosystem components of many shallow, nearshore exposed and tide-pool environments [20,21,22]. It is calcification in the cell wall of the coralline algae that enables provision of these ecosystem components. Some rhodolith-forming genera, e.g. Lithothamnion, can form internal bandings of alternating thin-walled elongate cells, high in Mg, to shorter, thicker walled cells, lower in Mg content [18,26]

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Discussion

Conclusion