UN PROBLEME POSE PAR DES PALEOBACTERIES : LA CRISTALLISATION DE LA GALENE DES DEPOTS STRATIFORMES EST-ELLE MICROBIOLOGIOUE ?

Abstract

SEM study of divers rocks (sandstones, clays, marls, dolomites. etc...) mineralised by galena has resulted jn the discovery of fossil bacteria within the galena crystals. Two significant facts result trom these studies : the first concerns the clearly developped and exclusive association of galena and certain paleobacteria and their absence within surrounding sediment. Current studies confirm that this mutual attraction is not an exception; it is already established that certain species of aerobic, anaerobic, autotroohic and heterotrophic bacteria are not adversly affected by leadrich environments. Our research has enabled us to follow the absorption of this cation within individual germs of Sarcina flava. Certain results obtained in vitro suggest that this metal may be utilised by organisms during their microbio-synthetic activities prior to its rejection in the form of black, cryptocrystalline sulphur, PbS. and in other cases, either as carbonate, as an oxide, or an organo-metallic complex. The second significant fact concerns the imprisoning of paleobacteria within the galena crystals irrespective of their age and mmposition or environment of denosition? By varying the focus of the SEM it was possible to penetrate several microns jnto the crystals. Invariably, this has lead to the observation that the crystals of galena have envelopped a multitude of bacteria. The absence of all traces of physial imperfections (fractures, etc..) which could have facilited their postcrystallisation emplacement seems to indicate not only that the bacteria in question have lived in this environment and are themselves enriched in Pb but also that the cristallisation of the galena is an early 108 J.-P. Devigne diagenetic phenomena. These observations pose the following questions to which we have attempted to reply : should one consider this crystallisation as a simple chemical or physical reaction, or should we consider that the paleobacteria are the instigators of this process, at least during the initial phases of crystal germ formation ? In this latter case the imprisoning of the lead-associated paleomicrobes within the crystal structure is only the result of progressive crystal qrowth.