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 concentration (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 taxonomic units (OTUs) Thiothrix and Alteromonas, belonging to Gammaproteobacteria, and the Bacteroidetes 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.
KEY WORDS: Saharan dust deposition · In situ mesocosms · CE-SSCP · CARD-FISH · Clone libraries · Alteromonas macleodii · NW Mediterranean SeaResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 62: 201-213, 2011 dust addition markedly stimulated phytoplankton primary production and induced a shift from Cyanobacteria to diatoms, while the response in microbial community respiration was minor, resulting in an increase in the ratio of primary production to respiration (Duarte et al. 2006). In contrast, estuarine heterotrophic bacterial and phytoplankton primary production were both stimulated by organic nitrogen introduced by rainfall (Seitzinger & Sanders 1999). These studies demonstrate that atmospheric inputs can impact autotrophic and heterotrophic microbial communities with important consequences for ocean biogeochemistry.In the Mediterranean Sea, which is during the stratified period mostly a LNLC region, Saharan dust events represent a dominant source of atmospheric deposition. Previous studies have demonstrated that, during the summer stratification period, atmospheric dust is the main external source of phosphorus (Ridame & Guieu 2002) and of iron (Bonnet & Guieu 2006) in the surface mixed ...