vendredi 27 février 2015

Introduction


Numerous living organisms are able to produce mineralized hard-parts with species-specific morphologies and architectures. The remarkable adaptation of these mineralized organs to various biological functions* provides immediate evidence of the precise control exerted on localized deposition of these mineral phases**.


Figure 1

*Supporting structures of corals (a), bryozoa (b), sponges (c, d); Protective shells of Nautilus (Mollusca Cephalopoda) with also positioning function; Nutrition by particle filtration (feather star arms, Echinodermata) or chewing (Vertebrate tooth: g); Skeletal frameworks: Foraminifera (h), Diatom (i).
** From a quantitative view point Ca-carbonate is the most used mineral phase (a,b,c,e,f,h), followed by silica (d,i), and calcium phosphate (g). Among these three main mineral phases a striking contrast exists between crystallized materials (calcium carbonate [=calcareous: a,b,c,e,f,h] and Ca phosphate structures (g) and the amorphous silica structures (d,i).
 
In addition these mineralized structures are so widely produced and in such quantities (specifically for calcareous and siliceous ones) that they play a major role in the world ocean as sedimentary contributors. Tropical reefs exemplify this geological importance of materials resulting from various mineralizing capabilities of the living organisms: e.g. the Marutea atoll in Polynesia (Fig. 2a).  Facing wave energy a solid protective zone is built by cemented red algae nodules (2b) whereas reef framework is made by coral colonies distributed according to their trophic requirements and environmental conditions. A multitude of associated organisms contribute to completion of this entirely biological build up.



Figure 2

However, with regard to involvement of these biologically produced materials into the sedimentary processes the question is not only quantitative but, and even more importantly, qualitative also. Owing to the biological origin of these materials the question arises whether they do have physical properties (e.g. sensitivity to dissolution) strictly similar to their purely chemical equivalents. Far beyond the geographical aspect illustrated by formation of solid geological structures, the balance between formation of calcium carbonate by biological calcification and post mortem dissolution of these materials is a key point to reach a precise understanding of the chemical equilibrium in the world ocean. This cannot be obtained without reliable data and relevant concepts regarding structures, compositions and resulting properties of biominerals.  Regarding the most important of them, the calcium carbonate skeletons, important to note that the concern is not only sea water composition but also, through CO2 exchange between ocean and atmosphere, the composition of the whole Earth fluid envelopes.

Calcification of deep-sea corals clearly suggests that some specific mechanism is at work in the biomineralization (or biomineralisation) process. Deep sea corals (e.g. Fig. 3: Lophelia ) are able to create their aragonite corallites (b,c) at such temperature and CO2 partial pressures that  the calcium carbonate particles falling from sea surface are dissolved before reaching sea-bottom.

Figure 3: Colony (a), branches (b) and individual corallite (c) of the deep sea coral Lophelia


Obviously aragonite of the Lophelia skeletons cannot be produced by a chemical precipitation that follows the usual thermodynamic rules. 
When, instead of simply looking at the morphology of these calcareous skeletons a top-down approach to their internal organizations is carried out at progressively increasing resolutions we obtain a consistent series of structural and biochemical patterns revealing the method by which living organisms control the formation of their calcareous skeletons.

 Since several decades the calcareous skeletons of many organisms play a major role in climate studies and reconstruction of earth environments through time. They are used as environmental archives because during skeletal growth the calcification mechanism was sensitive to the physical or chemical properties of local environment. Thus, through various physical or chemical “proxies” (e.g. isotopic fractionation or elemental ratio such as Mg/Ca) any calcareous skeleton is a record of the environmental changes that have occurred during life of the producing organism.

Since the very beginning of this physically based environmental method (initiated by H.C. Urey) it was noted that biologically produced Ca-carbonates do not follow the thermodynamic rules, providing additional evidence of a non regular crystallization process at the origin of the calcareous biocrystals. In his pioneering paper (Urey et al.  Bull. Geol. Soc. America 1951) Urey formulated the surprising note: “we may ask whether there is a vital effect”. More than six decades later the species-specific mechanism of environmental recording in the calcium carbonate of living organisms is not fully understood.

In addition to environmental application, many sectors of economic life are linked to biomineralization mechanisms. For shells grown in aquaculture production centers (oysters, or Pecten for instance) quality of shell formation is obviously dependant of the conformity of environmental life conditions to the standard requested by each species. Every year pathological diseases in shell mineralization cause important economical losses in this sector.

Since the beginning of 20th century pearl production by grafting and cultivation method epitomizes the importance of an in-depth understanding of biomineralization mechanisms. In pearl production centers around the world greatest attention is paid to these small fragments of mineralizing tissue (the grafts) that are in charge of producing the next generation of pearls.  Prosperity or collapse of the whole enterprise depends on the correct balance in activity of genes in the transplanted mineralizing cells, resulting in the complex blend of organic compounds that control the mineralizing process.