1) environmental; 2)soil Targeted loci environmental

Microorganisms are key players in biogeochemical cycles, organic matter decomposition and nutrient breakdown. Hydrocarbons are worldwide distributed pollutants that disturb various ecosystems. Bioremediation, either by nutrient management or inoculation with degrading microorganisms, is a cost-effective environmentally-sustainable technology. Nonetheless, the effects of bioremediation on microbial community are not well known. The aim of this study was to characterize the short-lapse dynamics of soil microbial communities in response to hydrocarbon pollution during different bioremediation treatments. Diesel-spiked soil microcosms were inoculated with either a defined bacterial consortium or a hydrocarbonoclastic bacterial enrichment and incubated during 12 weeks. Microbial community dynamics was followed weekly by Illumina 16S rRNA gene sequencing. Both consortium and enrichment enhanced hydrocarbon degradation in diesel-polluted microcosms without a significant effect on the soil microbial community. A pronounced and rapid bloom of a native Gammaproteobacteria belonging to the Moraxellaceae family, was observed in all diesel-polluted soils. The bloom was the response of a unique operational taxonomic unit (OTU) related to the Alkanindiges genus. This OTU represented ~0.1% of the sequences in the original community, but surprisingly bloomed to >27% two weeks after pollution and reached >60% after six weeks. Despite this Alkanindiges-related bloom, inoculated strains were maintained in the community and may explain the differences in hydrocarbon degradation rates. This study shows a detailed dynamics of an unprecedented soil bacterial bloom in response to hydrocarbon pollution, resembling microbial blooms observed in marine environments. This rare-to-dominant bacterial shift illustrates the potential role of rare microbiome facing drastic environmental disturbances.Pollution is a major disturbance detrimental or beneficial for microbial community members depending on their ability to handle this selective pressure. High-diversity environments such as soils are composed by multiple underrepresented “rare” community members that presumably act as a reservoir of ecological functions and may increase in population after a disturbance. In this study, an explosive bloom of a rare soil community member in response to hydrocarbon pollution is reported. This marked population shift supports the concept of “conditionally rare taxa”, in which rareness is only a temporary state conditioned by environmental constraints.