Algal Calcification and Silicification


The algae represent major producers of calcium carbonate and silica among the world's biota. Calcification involves the precipitation of CaCO3 from Ca2+ and CO32 ions. Algal calcification by coccolithophores may account for up to half of global oceanic CaCO3 production. Silicification, the transformation of silicic acid into skeletal material, occurs in a few algal groups. The abundant diatoms represent the major silicifiers, playing a key role in marine silica cycling. Fossilised diatomaceous deposits have long been exploited for building and filling materials. Biomineralisation of calcium and silicon require homeostatic ion controls that are well characterised for Ca2+ and H+ in coccolithophores. Calcification occurs in an alkalinised vesicle, while silicification requires an acidic pH. Research on silicification remains focused upon cell wall development. Initiation and development of structures that are mineralised intracellularly requires initiation and regulation by organic components within the vesicles. Low‐temperature, low‐pressure biogenic formation of silica and calcite has potential for biotechnological application in novel industrial processes.

Key Concepts

  • Organisms across the tree of life are capable of biomineralisation.
  • Understanding the process of biomineralisation is required to interpret the information contained in biomineral geochemical proxies.
  • Mechanisms of calcification and silicification include uptake of inorganic ions via protein transporters, intracellular sequestration of those ions and the association of crystal initiation with an organic matrix.
  • The maintenance of metabolic homeostasis, especially relative to proton flux, is linked to calcification, but less is known about the effect of silicification on homoeostasis.
  • The adaptive functions of mineralised cellular structures remain hypothetical and are open topics of research.
  • The biological mechanisms of crystal formation have widespread applications in nanotechnology.

Keywords: calcite; silica; coccolithophores; diatoms; biogeochemistry

Figure 1. Alternative models for the mechanism of external calcifying band production in the giant‐celled alga Chara. (a) Calcification is driven by the diffusion of H+ into the cell in localised regions, producing localised alkalinisation of the cell surface and precipitation of CaCO3. In an adjacent region, H+ is actively pumped out of the cell, producing localised acidification of the cell surface. (b) Calcification is driven by the extrusion of Ca2+ in exchange for H+ in the alkaline zone. CO2 diffusion from the cell's interior provides the carbon source for CO32− formation and CaCO3 precipitation. In both models, H+ extrusion in the acidic zone facilitates the production of CO2 from HCO3 that can be used by photosynthesis in the chloroplasts (green).
Figure 2. Model for fluxes of Ca2 +, HCO3 and H+ during calcification in coccolithophores. (a) Scanning electron micrographs of heterococcoliths on the surface of Coccolithus pelagicus and Emiliania huxleyi cells (cell diameter = 20 µm). (b) Ca2 + and HCO3 uptake into a Golgi‐derived coccolith vesicle (CV) leads to the production of CaCO3 and H+ during calcite precipitation.
Figure 3. (a) Scanning electron micrograph showing complete silica frustules of the diatom Thalassiosira eccentrica (cell diameter = 30 µm; photo: Grant Diedrich, University of North Carolina Wilmington Microscopy Facility). (b) Working model of silica biogenesis in a diatom. Uptake of silicic acid is via Na+‐dependent specialised silicate transporters (SIT) at the plasma membrane. Soluble silica is delivered to the silica deposition vesicle (SDV), possibly involving SIT isoforms. Excess silicic acid may be removed from the cell via putative efflux proteins. Silica polymerisation occurs in the acidic SDV to form insoluble amorphous silica. Silaffin proteins and long‐chain polyamines play a key role in regulating the formation of insoluble silica within the SDV. Actin filaments and microtubules associated with the SDV mediate formation of microscale shape and architectural features. Frustulins and other glycoproteins coat the surface of the frustule and likely play a role in stabilisation of the mature silica wall structure. The entire process is likely coordinated by cell–cell communication between the two cells, forming new valves. On maturity, the complete valve is released onto the cell surface by exocytosis. Proteins and arrows in white are hypothesised from mRNA and protein expression (Shrestha and Hildebrand, ; Shrestha et al., ).


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Further Reading

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Simkiss K and Wilbur KM (1989) Biomineralization: Cell Biology and Mineral Deposition. San Diego: Academic Press.

Stoermer EF and Smol JP (eds) (1999) The Diatoms: Applications for the Environmental and Earth Sciences. Cambridge: Cambridge University Press.

Thierstein HR and Young JR (2004) Coccolithophores: From Molecular Processes to Global Impact. New York: Springer.

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Koester, Julie, Brownlee, Colin, and Taylor, Alison R(Feb 2016) Algal Calcification and Silicification. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000313.pub2]