From rare to dominant: A fine-tuned soil bacterial bloom during petroleum hydrocarbon bioremediation

Hydrocarbons are worldwide-distributed pollutants that disturb various ecosystems. The aim of this study was to characterize the short-lapse dynamics of soil microbial communities in response to hydrocarbon pollution and different bioremediation treatments. Replicate diesel-spiked soil microcosms were inoculated with either a defined bacterial consortium or a hydrocarbonoclastic bacterial enrichment and incubated for 12 weeks. The microbial community dynamics was followed weekly in microcosms using Illumina 16S rRNA gene sequencing. Both the bacterial consortium and enrichment enhanced hydrocarbon degradation in diesel-polluted soils. A pronounced and rapid bloom of a native gammaproteobacterium was observed in all dieselpolluted soils. A unique operational taxonomic unit (OTU) related to the Alkanindiges genus represented ~0.1% of the sequences in the original community but surprisingly reached > 60% after 6 weeks. Despite this Alkanindiges-related bloom, inoculated strains were maintained in the community and may explain the differences in hydrocarbon degradation. This study shows the detailed dynamics of a soil bacterial bloom in response to hydrocarbon pollution, resembling microbial blooms observed in marine environments. Rare community members presumably act as a reservoir of ecological functions in high-diversity environments, such as soils. This rare-to-dominant bacterial shift illustrates the potential role of a rare biosphere facing drastic environmental disturbances. Additionally, it supports the concept of "conditionally rare taxa," in which rareness is a temporary state conditioned by environmental constraints.