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Deep geological disposal in a multibarrier cementitious facility is being developed by a number of countries for the safe, long-term disposal of intermediate-level radioactive wastes. Intermediate-level radioactive waste, which dominates the radioactive waste inventory in the United Kingdom on a volumetric basis, is a heterogeneous wasteform that contains organic materials including cellulosic materials, encased in concrete. Under the alkaline conditions expected in the cementitious geological disposal facility (GDF), these materials will undergo abiotic, chemical hydrolysis, and will produce degradation products including isosaccharinic acid (ISA) or gluconic acid (GA) that can form soluble complexes with radionuclides. Alkaliphilic microorganisms sampled from a hyperalkaline site contaminated with lime-kiln waste, were able to degrade cellulosic material (tissue paper) in Ca(OH)2 saturated microcosms at a starting pH of 12. Enzymatic processes in these microcosms caused the production of acetate, acidification of the microcosms and a cessation of ISA production. Enrichment cultures prepared at pH 10 and inoculated with a sediment from the same hyperalkaline site were able to degrade ISA, and couple this degradation process to the reduction of electron acceptors that will dominate as the GDF progresses from an aerobic ‘open phase’ through nitrate- and Fe(III)-reducing conditions post closure. A strictly alkaliphilic bacterium belonging to the Bacillus genus was isolated from the nitrate-reducing enrichment culture, and was found to degrade a variety of organic molecules that are expected to be found in a cementitious GDF. Detailed investigation into the growth of this bacterium suggested that different mechanisms are involved in the biodegradation of ISA and GA, and that bacterial growth is coupled to a decrease in soluble U(VI) concentrations. This implies that microorganisms could have a role in attenuating the mobility of radionuclides in and around a GDF via (i) the biodegradation of cellulose and cessation of ISA production, (ii) the biodegradation of the ligands (ISA and GA) and (iii) the immobilisation of radionuclides. This should facilitate the reduction of undue pessimism in the long-term performance assessment of suchfacilities.

Nous avons étudié l’écologie microbienne d’un site hydrothermal alcalin sous-marin peu profond (0-50 m) localisé dans la baie de Prony (PHF) dans le sud de Nouvelle-Calédonie (Pacifique Sud-Ouest) comparable par son fonctionnement au site hydrothermal alcalin profond (800m) de Lost City (LCHF) (Dorsale médio-atlantique). PHF au même titre que LCHF est un système hydrothermal ultramafique associé à des réactions de serpentinisation des roches du manteau terrestre, libérant des fluides anoxiques très alcalins (jusqu'à pH 11) riche en calcium, en hydrogène et en méthane dissous. Le site hydrothermal de Prony (PHF) est caractérisé par de grandes cheminées de carbonate émettant des fluides dont la température n’excède pas 40°C. Les approches moléculaires ont démontré une prédominance des bactéries (Firmicutes, Deltaproteobacteria …) sur les archées (Methanosarcinales). En ce qui concerne les mises en culture, elles sont restées vaines pour la plupart des grands groupes trophiques recherchés (bactéries sulfato-réductrices, archées méthanogènes) excepté pour les fermentaires relevant du phylum des Firmicutes. Deux nouvelles bactéries appartenant à ce phylum ont été isolées. Il s’agit (i) de Vallitalea pronyensis vraisemblablement associée aux cheminées hydrothermales et (ii) d’Alkaliphilus hydrothermalis qui serait plutôt indigène aux fluides alcalins émis par les cheminées si l’on en juge par ses conditions physico-chimiques optimales de croissance en adéquation avec celles des fluides. Ces deux microorganismes représentent les premiers anaérobies stricts isolés de systèmes hydrothermaux alcalins serpentinisés à ce jour. / We studied the microbial ecology of an alkaline hydrothermal submarine shallow field (0-50 m) located in Prony Bay (PHF) in the south of New Caledonia (SW Pacific) similar to the deep alkaline hydrothermal site (800m) of Lost City (LCHF) (Mid-Atlantic Ridge). Similarly to LCHF, PHF is an ultramafic hydrothermal system functioning on the basis of serpentinization reactions of the mantle rocks, releasing anoxic, highly alkaline fluids (to pH 11) rich in calcium, and in dissolved hydrogen and methane. The Prony hydrothermal field (PHF) is characterized by large carbonate chimneys emitting fluids with temperatures not exceeding 40 °C. Molecular approaches revealed a prevalence of Bacteria (Firmicutes, Deltaproteobacteria…) over Archaea (Methanosarcinales). Regarding microbial cultures, they were unsuccessful for most major trophic groups (sulfate-reducing bacteria, methanogens) with the exception of fermentative representatives of the phylum Firmicutes. Two novel bacteria belonging to this phylum were isolated. They include (i) Vallitalea pronyensis which is likely associated with hydrothermal vents and (ii) Alkaliphilus hydrothermalis which should be indigenous to alkaline fluids emitted from chimneys since its growth optimal physicochemical conditions match those of fluids. These two bacteria represent the first anaerobic microorganisms isolated from alkaline hydrothermal serpentinized systems so far.

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