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.

Fundamental drivers for endolithic microbial community assemblies in the hyperarid Atacama Desert

“…In all cases, while the hypolithic communities were recruited from the surrounding soils, habitat filtering shaped the assembly of the rock communities (Pointing et al ., ; Makhalanyane et al ., ; Lee et al ., ). Community assembly controlled by the substrate was also observed for the hypolithic colonization of meteorites found in Nullarbor Plain, Australia (Tait et al ., ). While not from deserts, partitioning of Cyanobacteria was reported for microbial communities in dolomite and limestone substrates, demonstrating mineral preferences of euendolithic communities at the OTU phylogenetic level (Couradeau et al ., ).…”

Fine scale diversification of endolithic microbial communities in the hyper-arid Atacama Desert

“…In all cases, while the hypolithic communities were recruited from the surrounding soils, habitat filtering shaped the assembly of the rock communities (Pointing et al , 2009; Makhalanyane et al , 2013; Lee et al , 2016). Community assembly controlled by the substrate was also observed for the hypolithic colonization of meteorites found in Nullarbor Plain, Australia (Tait et al , 2017). While not in desert, partitioning of Cyanobacteria was reported for microbial communities in dolomite and limestone substrates, demonstrating mineral preferences of euendolithic communities at the OTU phylogenetic level (Couradeau et al , 2017).…”

Evaluation of meteorites as habitats for terrestrial microorganisms: Results from the Nullarbor Plain, Australia, a Mars analogue site

“…Indeed, we found a surprisingly high level of microbial colonization and diversity in meteorite fractures: 10-20 µm thick biofilms, inhabited by a range of morphologically distinct terrestrial microorganisms, commonly cover extensive regions of Mg-calcite veins ( Figure 6A). Microbial community composition/structure of Nullarbor ordinary chondrites is described in detail in (Tait et al, 2017). Additionally, in the course of our sampling we glutaraldehyde-fixed cells in the process of -or having recently completed -binary cell division (e.g., diplobacilli and diplococci, Figures 6A and 7B,C).…”

“…A subset of meteorite samples were glutaraldehyde-fixed in the field for later imaging of microorganisms endemic to the Nullarbor Plain using a Field Emission Gun Scanning Electron Microscope (FEG-SEM). Another subset of samples were collected for an independent 16S rRNA gene phylogenetics study of microbial populations, the results of which can be found in Tait et al (2017). Secondary minerals were identified, and their abundances quantified, using powder XRay Diffraction (XRD).…”