Response of geological resident microbial communities to additions of nitrates and/or acetate, mimicking leachates of bituminized intermediate-level radioactive waste

Background
Clay deposits such as the Opalinus Clay formation are studied as host rocks for geological disposal of high- and intermediate-level long-lived radioactive waste in several European countries. Bituminized intermediate-level long-lived radioactive waste contains, besides bitumen and radionuclides, also large amounts of organics, nitrates and sulphates. Over time, these salts will dissolve, leach and diffuse into the surrounding clay host rock, together with water soluble organic substances. To mimic the potential effect of such inorganic– and organic leachates, pulses of nitrate and/or acetate have been injected in intervals of an in situ experiment, called Bitumen-Nitrate-Clay interaction (BN) experiment, running at the Mont Terri geological laboratory (St. Ursanne, Switzerland).
Objectives
To elucidate whether the microbial communities, present in the BN experiment, are affected or involved in the observed biogeochemical changes.
Methods
Beside classical microbial analyses, at pivotal moments (i.e. before, during and after these pulse injection tests), priority was given to DNA-based molecular biology analysis methods, as these methods provide very accurate information on the composition, the metabolic capacity and possible evolution of bacterial communities in response to the nitrate and/or acetate injections.
Conclusions
The changes observed in the bacterial populations appeared to correlate well with the imposed physico-chemical changes. As soon as nitrate was added an overwhelming community shift appeared to nitrate reducing bacteria. If in parallel acetate was offered, the community composition did not alter that much, but the speed of nitrate reduction was increased twentyfold. This high nitrate removal speed was maintained as long as the easily consumable carbon source was abundant.