Ca-carbonates precipitation and limestone genesis — the microbiogeologist point of view


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July 1999


Experiments show that the production of carbonate particles by heterotrophic bacteria follows different ways. In heterotrophy, the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles. It is induced by several metabolic pathways of the nitrogen cycle (ammonification of amino-acids, degradation of urea and uric acid, dissimilatory reduction of nitrates) and of the sulphur cycle (dissimilatory reduction of sulphates). The active carbonatogenesis is independent of the mentioned metabolic pathways. The carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms. In autotrophy, non-methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates (autotrophic carbonates). As carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments, it has been an ubiquitous phenomenon since Precambrian times. Carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter. After a phase of latency, there is an exponential increase of bacterial numbers together with the accumulation of metabolic end-products. These induce a pH increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium. This phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations. Particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state. The active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles. In eutrophic conditions, the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and/o


Biomineralisation, Bacteria, Experiments, Metabolism, Limestones

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