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PAH biodegradation in plant rhizosphere has been investigated in many studies, but the timescale of degradation and degrading bacteria activity was rarely considered. We explored the impact of plants on the temporal variability of PAH degradation, microbial abundance, activity, and bacterial community structure in a rhizotron experiment. A historically contaminated soil was spiked with PAHs, planted or not with alfalfa, over 22 days with sampling once a week. In both conditions, most of the spiked PAHs were dissipated during the first week, conducting to polar polycyclic aromatic compound production and to decreased richness and diversity of bacterial communities. We showed a rapid impact of the rhizosphere on PAH degradation via the increased activity of PAH-degrading bacteria. After 12 days, PAH degradation was significantly higher in the planted (100% degradation) than in unplanted (70%) soil. Gram-negative (Proteobacteria) PAH-dioxygenase genes and transcripts were higher in planted than unplanted soil and were correlated to the spiked PAH degradation. Conversely, Gram-positive (Actinobacteria) PAH-dioxygenase gene transcription was constant over time in both conditions. At 12 days, plant growth favored the activity of many Gammaproteobacteria (Pseudomonadaceae, Stenotrophomonas, and Acinetobacter) while in unplanted soil Alphaproteobacteria (Sphingomonadaceae, Sphingobium, and Magnetospirillum) and Actinobacteria (Iamia, Geodermatophilaceae, and Solirubrobacterales) were more active.

We examined the relationship between viruses and co-occurring bacterial communities across spatiotemporal scale in two contrasting freshwater lakes, namely meromictic Lake Pavin and dimictic Lake Aydat (Central France). Next-generation sequencing of 16S rRNA genes suggested distinct patterns in bacterioplankton community composition (BCC) between the lakes over depths and seasons. BCC were generally dominated by members of Actinobacteria, Proteobacteria, and Bacteroidetes covering about 95% of all sequences. Oxygen depletion at the bottom waters in Aydat and existence of permanent anoxia in the monimolimnion of Pavin resulted in the occurrence and dominance of lesser known members of lake communities such as Methylotenera, Methylobacter, Gallionella, Sulfurimonas, and Syntrophus in Pavin and Methylotenera and Sulfuritalea in Aydat. Differences in BCC appeared strongly related to dissolved oxygen concentration, temperature, viral infection, and virus-to-bacteria ratio. UniFrac analysis indicated a clear distinction in BCC when the percentage of viral infected bacterial cells and virus-to-bacteria ratio exceeded a threshold level of 10% and 5, respectively, suggesting a link between viruses and their potential bacterial host communities. Our study revealed that in both the lakes, the prevailing environmental factors across time and space structured and influenced the adaptation of bacterial communities to specific ecological niches.

Microorganisms dwelling in sediments have a crucial role in biogeochemical cycles and are expected to have a strong influence on the cycle of arsenic, a metalloid responsible for severe water pollution and presenting major health risks for human populations. We present here a metagenomic study of the sediment from two harbours on the Mediterranean French coast, l'Estaque and St Mandrier. The first site is highly polluted with arsenic and heavy metals, while the arsenic concentration in the second site is below toxicity levels. The goal of this study was to elucidate the potential impact of the microbial community on the chemical parameters observed in complementary geochemical studies performed on the same sites. The metagenomic sequences, along with those from four publicly available metagenomes used as control data sets, were analysed with the RAMMCAP workflow. The resulting functional profiles were compared to determine the over-represented Gene Ontology categories in the metagenomes of interest. Categories related to arsenic resistance and dissimilatory sulphate reduction were over-represented in l'Estaque. More importantly, despite very similar profiles, the identification of specific sequence markers for sulphate-reducing bacteria and sulphur-oxidizing bacteria showed that sulphate reduction was significantly more associated with l'Estaque than with St Mandrier. We propose that biotic sulphate reduction, arsenate reduction and fermentation may together explain the higher mobility of arsenic observed in l'Estaque in previous physico-chemical studies of this site. This study also demonstrates that it is possible to draw sound conclusions from comparing complex and similar unassembled metagenomes at the functional level, even with very low sequence coverage., (© 2013 John Wiley & Sons Ltd.)

Background: The development of Tuber melanosporum mycorrhizal symbiosis is associated with the production of an area devoid of vegetation (commonly referred to by the French word 'brûlé') around the symbiotic plants and where the fruiting bodies of T. melanosporum are usually collected. The extent of the ecological impact of such an area is still being discovered. While the relationship between T. melanosporum and the other fungi present in the brûlé has been assessed, no data are available on the relationship between this fungus and the bacteria inhabiting the brûlé., Methodology/principal Findings: We used DGGE and DNA microarrays of 16S rRNA gene fragments to compare the bacterial and archaeal communities inside and outside of truffle brûlés. Soil samples were collected in 2008 from four productive T. melanosporum/Quercus pubescens truffle-grounds located in Cahors, France, showing characteristic truffle brûlé. All the samples were analyzed by DGGE and one truffle-ground was analyzed also using phylogenetic microarrays. DGGE profiles showed differences in the bacterial community composition, and the microarrays revealed a few differences in relative richness between the brûlé interior and exterior zones, as well as differences in the relative abundance of several taxa., Conclusions/significance: The different signal intensities we have measured for members of bacteria and archaea inside versus outside the brûlé are the first demonstration, to our knowledge, that not only fungal communities, but also other microorganisms are affected by T. melanosporum. Firmicutes (e.g., Bacillus), several genera of Actinobacteria, and a few Cyanobacteria had greater representation inside the brûlé compared with outside, whereas Pseudomonas and several genera within the class Flavobacteriaceae had higher relative abundances outside the brûlé. The findings from this study may contribute to future searches for microbial bio-indicators of brûlés.

Background: Acid Mine Drainages (AMDs) are extreme environments characterized by very acid conditions and heavy metal contaminations. In these ecosystems, the bacterial diversity is considered to be low. Previous culture-independent approaches performed in the AMD of Carnoulès (France) confirmed this low species richness. However, very little is known about the cultured bacteria in this ecosystem. The aims of the study were firstly to apply novel culture methods in order to access to the largest cultured bacterial diversity, and secondly to better define the robustness of the community for 3 important functions: As(III) oxidation, cellulose degradation and cobalamine biosynthesis., Results: Despite the oligotrophic and acidic conditions found in AMDs, the newly designed media covered a large range of nutrient concentrations and a pH range from 3.5 to 9.8, in order to target also non-acidophilic bacteria. These approaches generated 49 isolates representing 19 genera belonging to 4 different phyla. Importantly, overall diversity gained 16 extra genera never detected in Carnoulès. Among the 19 genera, 3 were previously uncultured, one of them being novel in databases. This strategy increased the overall diversity in the Carnoulès sediment by 70% when compared with previous culture-independent approaches, as specific phylogenetic groups (e.g. the subclass Actinobacteridae or the order Rhizobiales) were only detected by culture. Cobalamin auxotrophy, cellulose degradation and As(III)-oxidation are 3 crucial functions in this ecosystem, and a previous meta- and proteo-genomic work attributed each function to only one taxon. Here, we demonstrate that other members of this community can also assume these functions, thus increasing the overall community robustness., Conclusions: This work highlights that bacterial diversity in AMDs is much higher than previously envisaged, thus pointing out that the AMD system is functionally more robust than expected. The isolated bacteria may be part of the rare biosphere which remained previously undetected due to molecular biases. No matter their current ecological relevance, the exploration of the full diversity remains crucial to decipher the function and dynamic of any community. This work also underlines the importance to associate culture-dependent and -independent approaches to gain an integrative view of the community function.

The Seine River is strongly affected by the effluents from the Acheres wastewater treatment plant (WWTP) downstream of the city of Paris. We have shown that the effluents introduce large amounts of ammonia and inoculate the receiving medium with nitrifying bacteria. The aim of the present study was to investigate the diversity of the ammonia-oxidizing bacterial population by identifying autochthonous bacteria from upstream and/or allochthonous ammonia-oxidizing bacteria from the WWTP effluents. Measurements of potential nitrifying activity, competitive PCR, and denaturing gradient gel electrophoresis (DGGE) of 16S ribosomal DNA fragments specific to ammonia-oxidizing bacteria (AOB) were used to explore the succession and shifts of the ammonia-oxidizing community in the lower Seine River and to analyze the temporal and spatial functioning of the system at several different sampling dates. A major revelation was the stability of the patterns. The CTO primers used in this study (G. A. Kowalchuk, J. R. Stephen, W. D. Boer, J. I. Prosser, T. M. Embley, and J. W. Woldendorp, Appl. Environ. Microbiol. 63:1489-1497, 1997) were shown not to be completely specific to AOB of the beta subclass of Proteobacteria. We further demonstrated that when DGGE patterns are interpreted, all the different bands must be sequenced, as one major DGGE band proved to be affiliated with a group of non-AOB in the beta subclass of Proteobacteria. The majority of AOB (75 to 90%) present in the lower Seine river downstream of the effluent output belong to lineage 6a, represented by Nitrosomonas oligotropha- and Nitrosomonas ureae-like bacteria. This dominant lineage was represented by three bands on the DGGE gel. The major lineage-6a AOB species, introduced by the WWTP effluents, survived and might have grown in the receiving medium far downstream, in the estuary; it represented about 40% of the whole AOB population. The other two species belonging to lineage 6a seem to be autochthonous bacteria. One of them developed a few kilometers downstream of the WWTP effluent input in an ammonia-enriched environment, and the other appeared in the freshwater part of the estuary and was apparently more adapted to estuarine conditions, i.e., an increase in the amount of suspended matter, a low ammonia concentration, and high turnover of organic matter. The rest of the AOB population was represented in equal proportions by Nitrosospira- and Nitrosococcus mobilis-like species.

The Achères wastewater treatment plant, located just downstream of Paris, discharges its effluents into the lower Seine River. The effluents contain large numbers of heterotrophic bacteria, organic matter, and ammonium and are a source of nitrifying bacteria. As a result, degradation of organic matter by heterotrophic bacteria and subsequent oxygen depletion occur immediately downstream of the effluent outlet, whereas nitrifying bacteria apparently need to build up a significant biomass before ammonium oxidation significantly depletes the oxygen. We quantified the potential total nitrifying activity and the potential activities of the ammonia- and nitrite-oxidizing communities along the Seine River. In the summer, the maximum nitrifying activity occurs in the upper freshwater estuary, approximately 200 km downstream of Achères. The quantities of nitrifying bacteria, based on amoA gene copy numbers, and of Nitrobacter organisms, based on 16S rRNA gene copy numbers, were correlated with the potential nitrifying activities. The species composition of ammonia-oxidizing bacteria was investigated at two sites: the Triel station just downstream from Achères (km 84) and the Seine freshwater estuary at the Duclair station (km 278). By means of PCR primers targeting the amoA gene, a gene library was created. Phylogenetic analysis revealed that the majority of the analyzed clones at both sites were affiliated with the genus NITROSOMONAS: The Nitrosomonas oligotropha- and Nitrosomonas urea-related clones represented nearly 81% of the community of ammonia-oxidizing bacteria at Triel and 60% at Duclair. Two other ammonia-oxidizing clusters of the beta subclass of the Proteobacteria, i.e., Nitrosomonas europaea- and Nitrosospira-like bacteria, were found in smaller numbers. The major change in the ammonia-oxidizing community between the two stations along the Seine River-upper estuary continuum was the replacement of the N. oligotropha- and N. urea-related bacteria by the Nitrosospira-affiliated bacteria. Although the diversities of the ammonia oxidizers appear to be similar for the two sites, only half of the restriction patterns are common to both sites, which could be explained by the differences in ammonium concentrations, which are much lower in the upper estuary than in the river at the effluent outlet. These results imply a significant immigration and/or selection of the ammonia-oxidizing bacterial population along the continuum of the Seine River from Paris to the estuary.