Exposure of phototrophs to 548 days in low Earth orbit: microbial selection pressures in outer space and on early earth

Abstract

An epilithic microbial community was launched into low Earth orbit, and exposed to conditions in outer space for 548 days on the European Space Agency EXPOSE-E facility outside the International Space Station. The natural phototroph biofilm was augmented with akinetes of Anabaena cylindrica and vegetative cells of Nostoc commune and Chroococcidiopsis. In space-exposed dark controls, two algae (Chlorella and Rosenvingiella spp.), a cyanobacterium (Gloeocapsa sp.) and two bacteria associated with the natural community survived. Of the augmented organisms, cells of A. cylindrica and Chroococcidiopsis survived, but no cells of N. commune. Only cells of Chroococcidiopsis were cultured from samples exposed to the unattenuated extraterrestrial ultraviolet (UV) spectrum (>110 nm or 200 nm). Raman spectroscopy and bright-field microscopy showed that under these conditions the surface cells were bleached and their carotenoids were destroyed, although cell morphology was preserved. These experiments demonstrate that outer space can act as a selection pressure on the composition of microbial communities. The results obtained from samples exposed to >200 nm UV (simulating the putative worst-case UV exposure on the early Earth) demonstrate the potential for epilithic colonization of land masses during that time, but that UV radiation on anoxic planets can act as a strong selection pressure on surface-dwelling organisms. Finally, these experiments have yielded new phototrophic organisms of potential use in biomass and oxygen production in space exploration.

Figure 1

Photographs of the rock samples after spaceflight in low Earth orbit showing bleaching of photopigments in surface biofilm under 100% UV radiation (>110 nm and >200 nm) and lack of bleaching in dark control samples and those exposed to 0.1% UV radiation (scale bar 1.5 mm).

Figure 2

Micrographs of space flight dark control cells and cells exposed to UV radiation >200 nm (CO₂ atmosphere). (a) Bright-field micrograph of surface biofilm of dark control sample (scale bar 20 μm). (b) Bright-field micrograph of surface biofilm of UV-exposed sample. Brown cells in upper section of micrograph correspond to exposed surface layer, green cells below them are cells protected under the surface layer (scale bar 20 μm). (c) Secondary scanning electron microscopy image of surface dark control sample showing biofilm (detachment of biofilm was observed in all samples and is a result of desiccation) (scale bar 25 μm). Cells are bound together by polysaccharide (d) scanning electron microscopy of sample exposed to 100% UV radiation >200 nm showing detached biofilm (scale bar 25 μm).

Figure 3

Raman spectra of the surface of epilithic community exposed to conditions in low Earth orbit.

Figure 4

Phototroph phylogenetic trees. (a) Neighbour-joining tree of the algal community of the limestone rock using the plastid 23S domain V plastid rDNA sequence data (clone library sequences in bold). The clones are represented as phylotypes (defined as 97% sequence similarity). The percentage of clones in each phylotype is shown in parentheses. Isolates that survived in the space flight samples are underlined. The scale bar corresponds to 0.02 changes per nucleotide. The percentage of bootstrap replicates (1000 replicates) resulting in the same cluster is given near the respective nodes for bootstrap values higher than 80%. Phormidium articulatum was used as an outgroup. (b) 16S rDNA phylogenetic tree of the isolates from the cyanobacterial community of the limestone rocks (for the full community analysis see Olsson-Francis et al., 2010). The neighbour-joining tree was constructed using nucleotide positions 112–770 (E. coli numbering). The scale bar corresponds to 0.02 changes per nucleotide. The percentage of bootstrap replicates (1000 replicates) resulting in the same cluster is given near the respective nodes for bootstrap values higher than 80%. The isolate that survived in the space flight sample is underlined. E. coli was used as an outgroup.

Figure 5

The non-cyanobacterial bacterial phylogenetic tree (cyanobacterial wedge shown in Figure 4b). The neighbour-joining tree was constructed using nucleotide positions 30–902 (E. coli numbering). Phylotypes obtained from the rock clone library are shown in bold. The two isolates examined in this study are underlined. The scale bar corresponds to 0.02 changes per nucleotide. The percentage of bootstrap replicates (1000 replicates) resulting in the same cluster is given near the respective nodes for bootstrap values higher than 80%. Aquifex sp. was used as an outgroup.