Niche partitioning of microbial communities at an ancient vitrified hillfort: implications for vitrified radioactive waste disposal

Abstract

Because microbes cannot be eliminated from radioactive waste disposal facilities, the consequences of bio-colonization must be understood. At a pre-Viking era vitrified hillfort, Broborg, Sweden, anthropogenic glass has been subjected to bio-colonization for over 1,500 years. Broborg is used as a habitat analogue for disposed radioactive waste glass to inform how microbial processes might influence long-term glass durability. Electron microscopy and DNA sequencing of surficial material from the Broborg vitrified wall, adjacent soil, and general topsoil show that the ancient glass supports a niche microbial community of bacteria, fungi, and protists potentially involved in glass alteration. Communities associated with the vitrified wall are distinct and less diverse than soil communities. The vitrified niche of the wall and adjacent soil are dominated by lichens, lichen-associated microbes, and other epilithic, endolithic, and epigeic organisms. These organisms exhibit potential bio-corrosive properties, including silicate dissolution, extraction of essential elements, and secretion of geochemically reactive organic acids, that could be detrimental to glass durability. However, long-term biofilms can also possess a homeostatic function that could limit glass alteration. This study demonstrates potential impacts that microbial colonization and niche partitioning can have on glass alteration, and subsequent release of radionuclides from a disposal facility for vitrified radioactive waste.

Keywords: Habitat analogue; community structure; microbes and surfaces; molecular ecology; near surface radioactive waste disposal.

Figure 1.

Sampling locations in relation to the vitrified wall. The location of the vitrified wall is based on ocular observations in the field (mapped with RTK_GPS) and corresponds well with earlier magnetometry measurements (Kresten et al. 1993; Englund et al. 2018). Samples 9A, 9C, 13, and 14 refer to rock sampling sites (no soil analysis).

Figure 2.

Scanning electron microscopy images from Broborg vitrified material, with presumptive identification: A) Mortierellomycetes (Jiang et al. 2010); B) Agaricales spore (Jeppson et al. 2017); C) Diatom (Foged 1979); </P/>D) Yeast spores (Rozali et al. 2017); E) Actinomycetes (Selvameenal et al. 2009); F) Ascospores of Talaromyces trachyspermu (Tranchida et al. 2014); </P/>G) Yeast cells (Gorbushina 2007); H) Endolithic fungal hyphae (Lecanora) (Gehrmann et al. 1988); I and J) Testate amoebae (Lamentowicz et al. 2015); K) Trebouxiophyceae (Honegger 2009); L) Biomineralization of fungal hyphae by Basidomycota (Kolo and Claeys 2005).

Figure 3.

Geospatial analysis (kriging) of selected soil chemical and biological properties: A) Boundary map for kriging plots shown; B) Kriging plots of pyroxene, maghemite, amphibole, MnO, P₂O₅, SiO₂, Ascomycota, and Chlorophyta. Relative abundances (%) are interpolated with respect to GPS coordinates, easting (x axis) and northing (y axis).

Figure 3.

Geospatial analysis (kriging) of selected soil chemical and biological properties: A) Boundary map for kriging plots shown; B) Kriging plots of pyroxene, maghemite, amphibole, MnO, P₂O₅, SiO₂, Ascomycota, and Chlorophyta. Relative abundances (%) are interpolated with respect to GPS coordinates, easting (x axis) and northing (y axis).

Figure 4.

Most abundant class-level taxa directly associated with the vitrified wall, in soil adjacent to the vitrified wall, and in the general topsoil: A) Most abundant 18S rRNA genes at the class level; B) Most abundant ITS region at the class level; C) Most abundant 16S rRNA genes at the class level. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.

Figure 4.

Most abundant class-level taxa directly associated with the vitrified wall, in soil adjacent to the vitrified wall, and in the general topsoil: A) Most abundant 18S rRNA genes at the class level; B) Most abundant ITS region at the class level; C) Most abundant 16S rRNA genes at the class level. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.

Figure 4.

Most abundant class-level taxa directly associated with the vitrified wall, in soil adjacent to the vitrified wall, and in the general topsoil: A) Most abundant 18S rRNA genes at the class level; B) Most abundant ITS region at the class level; C) Most abundant 16S rRNA genes at the class level. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.

Figure 5.

Alpha diversity for 16S rRNA gene, 18S rRNA gene, and ITS region amplicons measured by observed OTUs

Figure 6.

Taxa enriched in vitrified niche (top) or general topsoil (bottom): A) 18S rRNA gene enrichments; B) ITS region enrichments; C) 16S rRNA gene enrichments. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.

Figure 6.

Taxa enriched in vitrified niche (top) or general topsoil (bottom): A) 18S rRNA gene enrichments; B) ITS region enrichments; C) 16S rRNA gene enrichments. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.

Figure 6.

Taxa enriched in vitrified niche (top) or general topsoil (bottom): A) 18S rRNA gene enrichments; B) ITS region enrichments; C) 16S rRNA gene enrichments. Taxa labeled as “_?” are either unclassified at that level during taxonomy assignment or unclassified in the database.