Abstract
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.
Highlights
Dry and wet atmospheric deposition represents a significant source of inorganic and organic nutrients and organic carbon to the ocean (Jickells et al 2005, Jurado et al 2008)
The lack of in situ observations is due to the episodic nature of atmospheric dust deposition that makes it difficult to follow related in situ changes at appropriate temporal and spatial scales
For the biogeochemical parameters determined during the DUNE project and the bacterial community structure, the control mesocosms represented the temporal changes that occurred in the Bay of Elbo well (Guieu et al 2010), and the 3 replicate mesocosms exhibited high reproducibility in this study
Summary
Dry and wet atmospheric deposition represents a significant source of inorganic and organic nutrients and organic carbon to the ocean (Jickells et al 2005, Jurado et al 2008). In high-nutrient low-chlorophyll regions, dust deposition enhances iron availability and stimulates phytoplankton primary production and potentially carbon dioxide drawdown (Boyd et al 2007, Cassar et al 2007). In low-nutrient low-chlorophyll (LNLC) regions, the simultaneous input of phosphate and iron from dust could create a favourable environment for enhanced dinitrogen fixation (Mills et al 2004). 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
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