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[1] A simple model of the CaCO 3 saturation state of the ocean is presented. It can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration. It should also attract researchers unfamiliar with complex biogeochemical models. Despite its limitations, the modelcalculated CaCO 3 saturation state of today's ocean agrees well with observations. In general, the model reveals three distinctly different modes of operation: The ''Strangelove Ocean'' of high supersaturation which is dominated by inorganic CaCO 3 precipitation, (2) the ''Neritan Ocean'' of indefinite saturation dominated by biogenic shallow-water CaCO 3 precipitation, and (3) the ''Cretan Ocean'' of low saturation dominated by biogenic pelagic CaCO 3 precipitation. In the latter mode, the deep ocean [CO 3 2À ] is remarkably stable, provided that the biogenic production of CaCO 3 exceeds the riverine flux of Ca 2+ and CO 3 2À. This explains the overall constancy of the saturation state of the ocean documented over the last 100 Ma. The model is then used to address diverse questions. One important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification.

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We used the combination of preparative electrophoresis and immunological detection to isolate two new proteins from the shell calcitic prisms of Pinna nobilis, the Mediterranean fan mussel. The amino acid composition of these proteins was determined. Both proteins are soluble, intracrystalline, and acidic. The 38-kDa protein is glycosylated; the 17-kDa one is not. Ala, Asx, Thr, and Pro represent the dominant residues of the 38-kDa protein, named calprismin. An N-terminal sequence was obtained from calprismin. This sequence, which comprises a pattern of 4 cysteine residues, is not related to any known protein. The second protein, named caspartin, exhibits an unusual amino acid composition, since Asx constitutes by far the main amino acid residue. Preliminary sequencing surprisingly suggests that the first 75 N-terminal residues are all Asp. Caspartin self-aggregates spontaneously into multimers. In vitro tests show that it inhibits the precipitation of calcium carbonate. Furthermore, it strongly interferes with the growth of calcite crystals. A polyclonal antiserum raised against caspartin was used to localize this protein in the shell by immunogold. The immunolocalization demonstrates that caspartin is distributed within the prisms and makes a continuous film at the interface between the prisms and the surrounding insoluble sheets. Our finding emphasizes the prominent role of aspartic acid-rich proteins for the building of calcitic prisms among molluscs.The secretion of a shell by molluscs is a striking example of a selfassembling process performed outside living tissues. When a mollusc builds its shell, the calcifying epithelium of its mantle extrudes mineral ions, mainly calcium and bicarbonate. In addition, it secretes an extracellular matrix composed of proteins, glycoproteins, proteoglycans, and polysaccharides (1). This mixture is released into the extrapallial space, a microvolume delimited by the epithelium, the growing shell, and the leathery periostracum (2). In this supersaturated space, the released macromolecules interact with bicarbonate and calcium to form biocrystallites that self-aggregate in an orderly manner. The end product is a densely packed organomineral assembly, the shell, in which the mineral phase represents more than 95% by weight (3). As shell proteins exert a control on the biomineralization process, they can be used in the synthesis of biomimetic materials (4 -6). However, the resolution of the primary structure of molluscan shell proteins has been seriously impaired for decades by their polydispersity, their polyanionic properties, and finally their post-translational modifications (3, 7).Despite these obstacles, investigations on molluscan shell proteins have made significant advances in the last 6 years, with the characterization of 16 proteins and, for most of them, the identification of the corresponding transcript (8, 9). Among the different shell textures that molluscs use to produce their shell, mother-of-pearl has received the greatest deal of attention. Its intrinsic bea...

Coccoliths of the unicellular marine alga Emiliania Izuxleyi are formed intracellularly in a specialized vesicle. Closely associated with the CaC0, crystals of the coccoliths is an acid CaZ +-binding polysaccharide. The latter is considered to fulfil a regulatory function in CaC03 crystallization. In this study it is demonstrated that the coccolith polysaccharide is able to inhibit CaCO, precipitation in vitro. The degree of inhibition is dependent on the nature of the cations bound to the acid groups of the polysaccharide. After substitution of Na' by Ca2+ ions the polysaccharide is far less effective in inhibiting CaC0, precipitation.The coccolith polysaccharide contains two types of acid groups: uronic acids and sulphate esters. Only the uronic acids are responsible for the inhibition of CaCO, precipitation. Desulphated polysaccharide inhibits precipitation to the same extent as the native molecule whereas the carboxyl-reduced polysaccharide is unable to block CaCO, crystallization.Inhibition of CaC0, precipitation can be suppressed by the presence of ethanol. Presumably the conformation of the molecule is altered under these conditions. Alginic acid [poly(mannuronic-iduronic acid)] and poly(ga1acturonic acid) also inhibit CaC0, precipitation. Inhibition of precipitation is effected by the coccolith polysaccharide and alginic acid at comparable concentrations. The concentration of poly(ga1acturonic acid) needed to obtain the same result is about 20-times higher. Contrary to the coccolith polysaccharide, both poly(uronic acid)s inhibit CaC0, precipitation in the absence as well as in the presence of ethanol. These results suggest that conformational changes of the coccolith polysaccharide may play a role in CaCO, crystallization in vivo.

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