Cold ecosytems sequester sequester a very large pool of organic carbon, possibly vulnerable to global warming. Bacteria play a key role in carbon biogeochemical cycling, and understanding their origin and function in alpine soils is therefore important. With the aim of unravelling the phylogenetic basis of the observed bacterial diversity and its seasonal dynamics, we sampled soils of two alpine meadows in June, August, October and May. These two meadows, ESM and LSM, contrast by their early or late snow-melting date, and also by their vegetation and pedology (see also LZ\\\\\\\'s poster). Total soil DNA was extracted, and a library of bacterial 16S rDNA clones was constructed and sequenced. Sequences related to Acidobacteria were abundant, and strikingly more represented in the LSM- (42%) than in the ESM library (22%), consistent with a lower soil pH. Conversely, Actinobacteria sequences were more frequent in the ESM library (18% vs 6%), along with Alpha-, Beta- and Deltaproteobacteria. In particular, rhizosphere-associated bacteria like Frankia or Rhizobium were better represented in the ESM dataset. Some of the Acidobacteria subgroups and Actinobacteria generae that contribute most to the seasonal fluctuations have been identified. An analysis of cultivable bacteria is now underway. In conclusion, our study suggests that although bacterial communities show significant seasonal variations, the spatial shift between ESM and LSM communities is clearly wider.

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