Analysis of the microbial community inhabiting the basin water surrounding the core vessel of the BR2 reactor at SCK CEN, Mol, Belgium, using a shotgun metagenomics approach.

This work describes, for the first time, the microbial dynamics in the water basin of a nuclear research reactor in operation, using a shotgun metagenomics approach. We aimed to characterize the dynamics of the microbial population living in this extreme man-made environment, i.e. an ultrapure deionized water basin, containing a range of radionuclides and exposed to high doses of radiation. Samples were taken at different time points across shutdowns and cycles of operation using a custom designed filtration system, which to our knowledge is the first study using this type of research design. A shotgun metagenomics approach was adopted, in combination with 16S rDNA amplicon sequencing, to characterize the microbial population both on the taxonomic and functional level.The periodic shifts in physico-chemical parameters linked with the operational status of the reactor were shown to significantly affect the microbial community in the basin water. Methylobacterium seems to be a key player in the community, as it is the most abundant taxon overall. Moreover, it was significantly more abundant during shutdown, whereas an unclassified Gammaproteobacterium had a higher relative abundance during cycles. On the functional level, some pathways were more highly represented during shutdowns, namely pyruvate, glycerophospholipid and purine metabolisms, as well as biosynthesis of valine, leucine and isoleucine. These pathways indicate a role in cell function recovery after irradiation.In addition, two genomes were almost entirely reconstructed from the metagenome. They corresponded to Bradyrhizobium sp. BTAi1 and Methylobacterium sp. UNC378MF, with a genome completeness of 97% and 84%, respectively. These genomes displayed significant adaptations that allowed them to thrive under the harsh conditions found in our basin water system.