Molecular microbial ecology of pyroxene weathering and carbonate precipitation : a case study

The alteration of pyroxenes by CO2 followed by precipitation of carbonates is of great geochemical and geodynamical importance for the global carbon cycle. We have studied both reactions, observed in the context of an aridic soil from Tataouine (Tunisia) in order to evaluate the importance of such non-classical but widespread environments. At Tataouine, pyroxene crystals were delivered by a meteorite fall which occurred 71 years ago, thus providing a precise timing for the alteration reactions. Results of the mineralogical study coupling scanning and electron microscopies will be presented. The conjunction of pyroxene dissolution with carbonate precipitation, as well as the implication of micro-organisms in the reactions will be demonstrated. Then we will show how the study of the molecular ecology of the same samples can be processed to bring valuable information on the processes involved in the investigated mineralogical reactions. These samples are a challenge for molecular ecology because of relatively low biomass. The microbial diversity of the Tatahouine sand was analysed by sequencing the DNA fragments (rrs gene) encoding the 16S ribosomal subunit. An important result of this study was to show the importance, among others, of Cyanobacteria on the meteorite fragments. Moreover we observed that the microbial diversity of the pyroxene fraction was different from the diversity of other mineral fractions in the sand. A differential colonization of pyroxenes has thus been evidenced; this observation has interesting connections with ecological questions among which the possibility of the existence of a pressure of selection linked with the mineralogical nature of the substrate.