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Christaki U.,University of Lille Nord de France | Van Wambeke F.,Aix - Marseille University | Lefevre D.,Aix - Marseille University | Lagaria A.,University of Lille Nord de France | And 16 more authors.
Biogeosciences | Year: 2011

The abundance and activity of the major members of the heterotrophic microbial community ĝ€" from viruses to ciliates ĝ€" were studied along a longitudinal transect across the Mediterranean Sea in the summer of 2008. The Mediterranean Sea is characterized by a west to-east gradient of deepening of DCM (deep chlorophyll maximum) and increasing oligotrophy reflected in gradients of biomass and production. However, within this well documented longitudinal trend, hydrological mesoscale features exist and likely influence microbial dynamics. Here we present data from a W-E transect of 17 stations during the period of summer stratification. Along the transect the production and fate of organic matter was investigated at three selected sites each one located in the centre of an anticyclonic eddy: in the Algero-Provencal Basin (St. A), the Ionian Basin (St. B), and the Levantine Basin (St. C). The 3 geographically distant eddies showed low values of the different heterotrophic compartments of the microbial food web, and except for viruses in site C, all integrated (0ĝ€"150 m) stocks were higher in reference stations located in the same basin outside the eddies. During our study the 3 eddies showed equilibrium between GPP (Gross Primary Production) and DCR (Dark Community Respiration). Integrated PPp (Particulate Primary Production) values at A, B and C varied from ∼140 to ∼190 mg C mĝ̂'2. © Author(s) 2011.

Roux S.,University Blaise Pascal | Roux S.,Laboratoire Microorganismes Genome et Environnement | Krupovic M.,Institute Pasteur Paris | Poulet A.,University Blaise Pascal | And 5 more authors.
PLoS ONE | Year: 2012

Recent studies suggest that members of the Microviridae (a family of ssDNA bacteriophages) might play an important role in a broad spectrum of environments, as they were found in great number among the viral fraction from seawater and human gut samples. 24 completely sequenced Microviridae have been described so far, divided into three distinct groups named Microvirus, Gokushovirinae and Alpavirinae, this last group being only composed of prophages. In this study, we present the analysis of 81 new complete Microviridae genomes, assembled from viral metagenomes originating from various ecosystems. The phylogenetic analysis of the core genes highlights the existence of four groups, confirming the three sub-families described so far and exhibiting a new group, named Pichovirinae. The genomic organizations of these viruses are strikingly coherent with their phylogeny, the Pichovirinae being the only group of this family with a different organization of the three core genes. Analysis of the structure of the major capsid protein reveals the presence of mushroom-like insertions conserved within all the groups except for the microviruses. In addition, a peptidase gene was found in 10 Microviridae and its analysis indicates a horizontal gene transfer that occurred several times between these viruses and their bacterial hosts. This is the first report of such gene transfer in Microviridae. Finally, searches against viral metagenomes revealed the presence of highly similar sequences in a variety of biomes indicating that Microviridae probably have both an important role in these ecosystems and an ancient origin. © 2012 Roux et al.

PubMed | Laboratoire Microorganismes Genome et Environnement
Type: Journal Article | Journal: Applied and environmental microbiology | Year: 2010

With the aim of explaining the variations in microcystin (MC) concentrations during cyanobacterial blooms, we studied several Microcystis aeruginosa populations blooming in different freshwater ecosystems located in the same geographical area. As assessed by real-time PCR, it appeared that the potentially MC-producing cells (mcyB(+)) were predominant (70 to 100%) in all of these M. aeruginosa populations, with the exception of one population in which non-MC-producing cells always dominated. Apart from the population in the Grangent Reservoir, we found that the proportions of potentially MC-producing and non-MC-producing cells varied little over time, which was consistent with the fact that according to a previous study of the same populations, the intergenic transcribed spacer (ITS) genotype composition did not change (38). In the Grangent Reservoir, the MC-RR variant was the dominant microcystin variant throughout the bloom season, despite changes in the ITS composition and in the proportions of mcyB(+) cells. Finally, the variations in total MC concentrations (0.3 to 15 microg liter(-1)) and in the MC cellular quotas (0.01 to 3.4 pg cell(-1)) were high both between and within sites, and no correlation was found between the MC concentrations and the proportion of mcyB(+) cells. All of these findings demonstrate that very different results can be found for the proportions of potentially MC-producing and non-MC-producing cells and MC concentrations, even in M. aeruginosa populations living in more or less connected ecosystems, demonstrating the importance of the effect of very local environmental conditions on these parameters and also the difficulty of predicting the potential toxicity of Microcystis blooms.

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