Effects of Saharan dust on the microbial community during a large in situ mesocosm experiment in the NW Mediterranean Sea

The response of the microbial community to Saharan dust deposition was investigated in 6 large mesocosms (52 m 3 ) deployed at an oligotrophic coastal site in the NW Mediterranean Sea in June 2008 (DUNE project). The mesocosms represented well the environmental conditions observed at the study site during the 8 d experimental period, and the triplicate mesocosms exhibited high reproducibility for each treatment. Dust deposition resulted in an increase in chlorophyll a con- centration (0.22 ± 0.03 µg l -1 ), as compared to that in the control treatments (0.12 ± 0.01 µg l -1 ), but no treatment effect was observed for bacterial heterotrophic abundance at 5 m depth. Results from the fingerprinting technique CE-SSCP indicate a temporal evolution of the structure of the total (16S rRNA gene) and active (16S rRNA transcripts) bacterial community, and Saharan dust deposition had a noticeable structuring effect on the active bacterial community. Combining results from 16S rRNA gene clone libraries and CE-SSCP indicates that the relative contribution of Alteromonas macleodii to the active bacterial community was enhanced 2-fold following dust addition. The 2 operational tax- onomic units (OTUs) Thiothrix and Alteromonas, belonging to Gammaproteobacteria, and the Bac- teroidetes OTU NS5-1 were specific to the clone libraries from the dust-amended mesocosms or more abundant in these than in the control ones. CARD-FISH analyses, however, indicate that these OTUs had overall low abundances (1 to 5% of total DAPI-counts). Despite the pronounced temporal trend observed during the experimental period, dust deposition had a small, but noticeable structuring effect on the heterotrophic bacterial community that was detectable only at the OTU level at 99% similarity of the 16S rRNA gene.