Diversity and geochemical structuring of bacterial communities along a salinity gradient in a carbonate aquifer subject to seawater intrusion.

In aquifers subject to saline water intrusion, the mixing zone between freshwater and saltwater displays strong physico-chemical gradients. Although the microbial component of these specific environments has been largely disregarded, the contribution of micro-organisms to biogeochemical reactions impacting water geochemistry has previously been conjectured. The objective of this study was to characterize and compare bacterial community diversity and composition along a vertical saline gradient in a carbonate coastal aquifer using high throughput sequencing of 16S rRNA genes. At different depths of the mixing zone, stable geochemical and hydrological conditions were associated with autochthonous bacterial communities harboring clearly distinct structures. Diversity pattern did not follow the salinity gradient, although multivariate analysis indicated that salinity was one of the major drivers of bacterial community composition, with organic carbon, pH and CO2 partial pressure. Correlation analyses between the relative abundance of bacterial taxa and geochemical parameters suggested that rare taxa may contribute to biogeochemical processes taking place at the interface between freshwater and saltwater. Bacterial respiration or alternative metabolisms such as sulfide oxidation or organic acids production may be responsible for the acidification and the resulting induced calcite dissolution observed at a specific depth of the mixing zone.

Diversity and spatiotemporal dynamics of bacterial communities: physicochemical and other drivers along an acid mine drainage.

Deciphering the biotic and abiotic factors that control microbial community structure over time and along an environmental gradient is a pivotal question in microbial ecology. Carnoulès mine (France), which is characterized by acid waters and very high concentrations of arsenic, iron, and sulfate, provides an excellent opportunity to study these factors along the pollution gradient of Reigous Creek. To this end, biodiversity and spatiotemporal distribution of bacterial communities were characterized using T-RFLP fingerprinting and high-throughput sequencing. Patterns of spatial and temporal variations in bacterial community composition linked to changes in the physicochemical conditions suggested that species-sorting processes were at work in the acid mine drainage. Arsenic, temperature, and sulfate appeared to be the most important factors that drove the composition of bacterial communities along this continuum. Time series investigation along the pollution gradient also highlighted habitat specialization for some major members of the community (Acidithiobacillus and Thiomonas), dispersal for Acidithiobacillus, and evidence of extinction/re-thriving processes for Gallionella. Finally, pyrosequencing revealed a broader phylogenetic range of taxa than previous clone library-based diversity. Overall, our findings suggest that in addition to environmental filtering processes, additional forces (dispersal, birth/death events) could operate in AMD community.

Fate of Sb(V) and Sb(III) species along a gradient of pH and oxygen concentration in the Carnoulès mine waters (Southern France).

The speciation and behaviour of antimony were investigated in surface waters downstream from the abandoned Pb-Zn Carnoulès mine (Gard, France). These waters exhibit a permanent gradient of oxygen concentration and pH, ranging from acid suboxic in Reigous Creek at the outlet of sulfide tailings impoundment, to near neutral oxygenated at downstream sites along the rivers Amous and Gardon. The concentration of total dissolved (<0.22 µm) antimony, acquired through a seven-year monitoring, decreased from 7.7-409.9 µg L(-1) at the source of Reigous Creek to 0.22-0.45 µg L(-1) in the Gardon River, showing natural Sb attenuation. Speciation analysis carried out during three surveys indicated that Sb(III) represented up to 70% of the total dissolved Sb concentration at the source of Reigous Creek, while Sb(V) represented less than 50%. Field characterization showed that Sb(III) and Sb(V) species were attenuated through dilution and were also removed from the dissolved phase during downstream transport. Speciation analysis in suspended particulate matter extracts gave a distribution of particulate Sb into 70 to 100% of Sb(III) and less than 30% of Sb(V). The removal of Sb(III) and Sb(V) species from the dissolved phase was concordant with the oversaturation of Reigous Creek water relative to Sb(III)- and Sb(V)-oxides and Sb(III)- and Sb(V)-Fe oxides. Sb(III) was more efficiently removed than Sb(V) or As(III) and it was no more detectable in the dissolved phase at downstream sites in the rivers Amous and Gardon. Conversely, the concentration of Sb(V) in the rivers Amous and Gardon still denoted contamination arising from the Carnoulès mine. The range of log Kd values, from 2.4 L kg(-1) to 4.9 L kg(-1), indicated that Sb was mainly transported in the dissolved phase downstream the Reigous Creek input. Altogether, these results give a better understanding of the fate of Sb downstream from sulfide-rich mining wastes.

Characterization of the active bacterial community involved in natural attenuation processes in arsenic-rich creek sediments.

Acid mine drainage of the Carnoulès mine (France) is characterized by acid waters containing high concentrations of arsenic and iron. In the first 30 m along the Reigous, a small creek draining the site, more than 38% of the dissolved arsenic was removed by co-precipitation with Fe(III), in agreement with previous studies, which suggest a role of microbial activities in the co-precipitation of As(III) and As(V) with Fe(III) and sulfate. To investigate how this particular ecosystem functions, the bacterial community was characterized in water and sediments by 16S rRNA encoding gene library analysis. Based on the results obtained using a metaproteomic approach on sediments combined with high-sensitivity HPLC-chip spectrometry, several GroEL orthologs expressed by the community were characterized, and the active members of the prokaryotic community inhabiting the creek sediments were identified. Many of these bacteria are ß-proteobacteria such as Gallionella and Thiomonas, but γ-proteobacteria such as Acidithiobacillus ferrooxidans and α-proteobacteria such as Acidiphilium, Actinobacteria, and Firmicutes were also detected.