Response of major bacterial groups to dust deposition during a mesocosm experiment in the Mediterranean Sea (DUNE) Blain, S.1,2 , Besseling1,2 , M., Lagdass1,2 , M., Deschâtre3 , M., Guieu, C.3 , Obernosterer, I.1,2 1 UPMC Univ Paris 06, UMR 7621, LOBB, F-66651 Banyuls/mer, France. 2 CNRS, UMR 7621, LOBB, F-66651 Banyuls/mer, France. 3 LOV UPMC Univ Paris 06, UMR CNRS 7093, quai de la darse, F-06238 Banyuls/mer, France. The biogeochemical response of the oligotrophic marine environment to dust deposition is likely in part driven by heterotrophic bacteria as they benefit from the input of new nutrients, and they also mediate important biogeochemical processes like iron dissolution or changes in iron bioavailability. In this context, the impact of Saharan dust deposition on changes in the bacterial community composition was investigated in 6 large mesocosms (33 m3 ) deployed at an oligotrophic coastal site in the Mediterranean Sea in June 2008 (DUNE project, P.I. C. Guieu). Applying the fingerprinting technique CE-SSCP, we observed a temporal evolution of the bacterial community composition over the 8d experimental period in both the dust- amended and control treatments. At the end of the experiment, the bacterial community composition in the dust-amended mesocosms revealed clear differences to the control treatments. The relative contribution of the major bacterial groups Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes to bulk bacterial abundance, as determined by Fluorescence In Situ Hybridization (FISH), did not change over time in any of the mesocosms. The Roseobacter clade, belonging to the Alphaproteobacteria, however, displayed a substantial increase in abundance over time in the mesocosms amended with dust, while their relative abundance remained constant in the control treatment. By contrast, the SAR11 cluster did not respond in terms of relative abundance to dust addition. Further analyses will allow to determine the time scale of these changes and how they are related to changes in biogeochemical parameters or processes observed during the experiment.
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