PhD Defense | Valérie Van Eesbeeck | Microbial dynamics in the aquatic environments of a nuclear reactor | UCLouvain

Microbial dynamics in the aquatic environments of a nuclear reactor

Nuclear reactors contain various watery environments, such as spent nuclear fuel pools for the intermediate storage of spent nuclear fuel underwater, the primary and secondary cooling circuits for the cooling of nuclear fuel in the reactor vessel and different ultrapure water tanks for the replenishing of evaporated water. These water systems are maintained at high purity levels through constant filtration and deionization in purification circuits, resulting in ultrapure waters with low conductivities and nutrient levels, occasionally exposed to high levels of radiation. Despite the extremely challenging conditions in these waters, microorganisms such as bacteria, fungi and microalgae have been previously detected. While most of the previous studies were performed on spent nuclear fuel pools, the aim of this work was to investigate the bacterial communities in different watery environments of the BR2 nuclear research reactor at SCK CEN in Mol, Belgium, with a particular focus on an open basin surrounding the reactor vessel.

In a first study, we investigated the viable microbial population in a range of interlinked watery environments using a cultivation-based approach. This yielded an extensive strain collection of 33 distinct bacterial species, which is the largest catalogue of isolates described so far in a single study. Furthermore, we attempted to characterize the radiation susceptibility of some of the isolated strains, which resulted in the identification of Sphingomonas melonis as the most radio-resistant species, as it survived an acute irradiation dose of 2.1 kGy.

As the BR2 reactor runs in successive cycles of operation and shutdown, this generates highly dynamic conditions in the basin surrounding the reactor core, with periodically shifting physico-chemical parameters such as temperature, radiation and flow rate. In the second part of our work, we characterized the long-term microbial community dynamics in this basin through 16S rRNA amplicon sequencing. Two sampling campaigns spanning several months were performed, which resulted in the characterization of a diverse bacterial population displaying clear shifts in community profiles: cycles were mostly dominated by an unclassified Gammaproteobacterium and Pelomonas, whereas Methylobacterium prevailed during shutdowns.

Finally, in order to dig deeper into the taxonomic and functional characteristics of the microbial community in the basin and characterize its dynamics more in depth, we adopted a shotgun metagenomics sequencing approach. To this aim, we designed a specialized filtration system in order to be able to collect a sufficient amount of cell material. With regard to the functional characterization of the community, several pathways believed to play a role in cell function recovery after irradiation were more highly represented during shutdowns. Furthermore, we managed to almost entirely reconstruct two MAGs from the metagenome, corresponding to Bradyrhizobium sp. BTAi1 and Methylobacterium sp. UNC378MF. These strains harbored significant adaptations in their genome allowing them to cope with the extremely challenging conditions prevailing in the basin.

In conclusion, we managed to uncover a large microbial diversity in the various watery environments of the BR2, which were shown to be mostly dominated by bacteria. Members of the community were believed to harbor significant evolutionary adaptations allowing them to survive in these extremely challenging environments.

Promotor:

• Jacques Mahillon (UCLouvain)

SCK CEN mentors:

• Natalie Leys (SCK CEN)

Click here for a list of obtained PhD degrees.