Microbial Populations of Stony Meteorites: Substrate Controls on First Colonizers

Abstract

Finding fresh, sterilized rocks provides ecologists with a clean slate to test ideas about first colonization and the evolution of soils de novo. Lava has been used previously in first colonizer studies due to the sterilizing heat required for its formation. However, fresh lava typically falls upon older volcanic successions of similar chemistry and modal mineral abundance. Given enough time, this results in the development of similar microbial communities in the newly erupted lava due to a lack of contrast between the new and old substrates. Meteorites, which are sterile when they fall to Earth, provide such contrast because their reduced and mafic chemistry commonly differs to the surfaces on which they land; thus allowing investigation of how community membership and structure respond to this new substrate over time. We conducted 16S rRNA gene analysis on meteorites and soil from the Nullarbor Plain, Australia. We found that the meteorites have low species richness and evenness compared to soil sampled from directly beneath each meteorite. Despite the meteorites being found kilometers apart, the community structure of each meteorite bore more similarity to those of other meteorites (of similar composition) than to the community structure of the soil on which it resided. Meteorites were dominated by sequences that affiliated with the Actinobacteria with the major Operational Taxonomic Unit (OTU) classified as Rubrobacter radiotolerans. Proteobacteria and Bacteroidetes were the next most abundant phyla. The soils were also dominated by Actinobacteria but to a lesser extent than the meteorites. We also found OTUs affiliated with iron/sulfur cycling organisms Geobacter spp. and Desulfovibrio spp. This is an important finding as meteorites contain abundant metal and sulfur for use as energy sources. These ecological findings demonstrate that the structure of the microbial community in these meteorites is controlled by the substrate, and will not reach homeostasis with the Nullarbor community, even after ca. 35,000 years. Our findings show that meteorites provide a unique, sterile substrate with which to test ideas relating to first-colonizers. Although meteorites are colonized by microorganisms, the microbial population is unlikely to match the community of the surrounding soil on which they fall.

Keywords: 16S rRNA gene; Nullarbor Plain; arid soils; astrobiology; geomicrobiology; mars analog site; meteorites.

FIGURE 1

Nullarbor Plain Map. This figure shows the Watson location in the Nullarbor Plain, Australia. Circles indicate the locations from which the four meteorites and associated soils were collected.

FIGURE 2

Meteorite Samples. This figure shows two meteorites used in the experiment and depicts field-based sub-sectioning methods. (A) Sample Watson 021 in situ. (B) Sample Watson 019 in situ after being flipped over during collection. (C) Sample Watson 019 being sub-sectioned over autoclaved aluminum foil.

FIGURE 3

Heatmap of Operational Taxonomic Unit Abundance. Heatmap analysis of OTU abundance in meteorites and soil samples. Analysis performed for OTUs at a distance of ≤0.03. The scale bar represents the fractional abundance of each OTU within each sample. The identity score, accession number, and the name of the nearest named isolate according to NCBI BLAST are indicated beside the heatmap. OTUs that were unable to be classified beyond the level of the domain Bacteria in the Silva database are also noted.

FIGURE 4

Phyla and Actinobacteria Abundance of Meteorites and Soil. This figure shows the relative abundance of different phyla and classes. (A) The major phyla classified according to the Silva taxonomy identification. ‘Other’ phyla include all phyla present at abundances less than 3%. ‘Bacteria Unclassified’ were OTUs that could not be classified below the domain Bacteria. (B) Taxonomic composition of the important soil phylum, Actinobacteria.

FIGURE 5

Rarefaction Curve. Rarefaction analysis of all samples at a clustering distance of ≤0.03. Warm colors are soil samples, cool colors are meteorites.

FIGURE 6

Community Structure Dissimilarity. This calculation was made using Yue and Clayton (2005) indices at a clustering distance of ≤0.03. The colors are scaled to the highest level of similarity between any two samples (red) and the lowest level of similarity (black). The white outline represents the direct comparison between the meteorite and its underlying soil.

FIGURE 7

Non-metric Multidimensional Scaling. NMDS plot for Nullarbor meteorite (MA, MB, MC, MD) and soil (SB, SC SD) samples based on the Yue and Clayton (2005) community structure. The stress value is 0.146 [Stress values <0.2 indicate that an NMDS ordination plot has good spatial representation of differences between communities (Levshina, 2015)].