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

Abstract

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/or particles excreted from the cell. All these tiny particles assemble into biomineral aggregates which often display `precrystalline' structures. These aggregates grow and form biocrystalline build-ups which progressively display more crystalline structures with growth. In oligotrophic conditions, the primary solid products are rapidly smoothed in the crystalline structure and leave no trace. In present aqueous environments, apart from deep ocean, the potential efficiency of heterotrophic bacterial carbonatogenesis in Ca-carbonate sedimentation is much higher than autotrophic or abiotic processes. It much more likely accounts for extensive apparently abiotic limestone formation than any of the latter. As far as biodetrital particles are concerned, it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria. Thus, apart from (probably mythical) purely evaporitic and autotrophic ones, most limestones must be considered as principally of heterotrophic bacterial origin. As the carbon of limestones is issued from organic matter, bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the Earth.