title: Diversity and geochemical structuring of bacterial communities along a salinity gradient in a carbonate aquifer subject to seawater intrusion.
abstract: 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.
authors.affiliation: Laboratoire HydroSciences Montpellier, HSM, UMR 5569 (IRD, CNRS, Universités Montpellier 1 et 2), Université Montpellier 2, Montpellier, France.
authors.forename: Marina
authors.initials: M
authors.lastname: Héry
authors.forename: Aurélie
authors.initials: A
authors.lastname: Volant
authors.forename: Charlotte
authors.initials: C
authors.lastname: Garing
authors.forename: Linda
authors.initials: L
authors.lastname: Luquot
authors.forename: Françoise
authors.initials: F
authors.lastname: Elbaz Poulichet
authors.forename: Philippe
authors.initials: P
authors.lastname: Gouze
mesh_terms: D001419:Bacteria / Q000145:classification* / Q000235:genetics*; D001483:Base Sequence; D044822:Biodiversity; D002254:Carbonates / Q000378:metabolism; D005618:Fresh Water / Q000382:microbiology*; D020459:Genes, rRNA; D060587:Groundwater / Q000382:microbiology*; D059014:High-Throughput Nucleotide Sequencing; D059013:Microbial Consortia; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D054712:Salinity; D012623:Seawater / Q000382:microbiology*; D017422:Sequence Analysis, DNA; D014871:Water Microbiology
chemical_list: D002254:Carbonates; D012336:RNA, Ribosomal, 16S
keywords: 16S rRNA gene; bacterial diversity; freshwater-saltwater mixing zone; pyrosequencing; subterranean estuary