Lichen survival in space vacuum

Yes—but the precise answer is: one particular lichen species survived, in a controlled exterior-ISS exposure apparatus, with substantial damage. Verified Answer #1

In ESA’s LIFE experiment, desiccated thalli of Xanthoria elegans were mounted in the EXPOSE-E facility outside the ISS’s Columbus module. Verified Answer #1

The space-vacuum tray was vented to low-Earth orbit and maintained approximately 10⁻⁴ to 10⁻⁷ Pa for 559 days, or about 18.4 months. Verified Answer #1

Separate samples experienced darkness, 0.1% solar transmission, or the experiment’s “full insolation” through a magnesium-fluoride optical window. Verified Answer #1

After return to Earth and rehydration, every vacuum-treatment group contained metabolically active cells from both partners of the lichen symbiosis—the photosynthetic alga, or photobiont, and the fungus, or mycobiont. (astroseti.org) Verified Answer #1

Vacuum-only results Verified Answer #1

The reported mean percentages of metabolically active cells after the three actual space-vacuum treatments were: Verified Answer #1

| Solar treatment during vacuum exposure | Algal photobiont | Fungal mycobiont | Biological replicates | |---|---:|---:|---:| | Dark | 60.8% | 75.7% | 4 | | 0.1% insolation | 61.2% | 82.7% | 2 | | Full experimental insolation | 83.1% | 86.6% | 2 | Verified Answer #1

These percentages came from FUN-1 live/dead staining and indicate the fraction of examined cells that remained metabolically responsive after rehydration; they are not percentages of complete lichen thalli that survived. Verified Answer #1

The small number of biological replicates—two to four per treatment—also limits how precisely the rates can be generalized. (astroseti.org) Verified Answer #1

A subsequent, stronger viability test isolated the algal partner and cultured it for about 200 days. Verified Answer #1

Algae recovered from all three vacuum treatments formed growing, proliferating colonies, demonstrating survival rather than merely preserved cell structure or residual fluorescence. Verified Answer #1

However, only 13–50% of inoculated algal clusters from the vacuum groups formed colonies, compared with 100% of the control clusters. (elib.dlr.de) Verified Answer #1

Important correction to a commonly repeated statistic Verified Answer #1

The frequently quoted figures of 71% photobiont viability and 84% mycobiont viability were averages across all six ISS treatments—three genuine space-vacuum treatments plus three treatments under a simulated Martian atmosphere of about 10³ Pa. Verified Answer #1

They should therefore not be described as vacuum-only survival rates. Verified Answer #1

The vacuum-specific results are those in the table above. (astroseti.org) Verified Answer #1

How did it survive? Verified Answer #1

The lichen did not remain hydrated and actively growing in space. Verified Answer #1

It remained in anhydrobiosis: a severely desiccated, metabolically dormant state. Verified Answer #1

Deactivation of photosynthetic machinery while dry reduces damaging reactive-oxygen production, and metabolic and photosynthetic activity can restart after water becomes available. Verified Answer #1

The lichen’s layered fungal cortex also physically protects interior algal cells, while its orange pigment parietin absorbs substantial ultraviolet and blue light. Verified Answer #1

The experiment found that protected clusters of algal cells deeper in the thallus survived better than exposed cells. (astroseti.org) Verified Answer #1

“On the hull” and “direct” need qualification Verified Answer #1

The samples were not loose lichens pasted directly onto the bare pressure hull. Verified Answer #1

They were secured in sample slots in EXPOSE-E, an exposure platform mounted outside Columbus. Verified Answer #1

The vacuum tray was genuinely vented to space, but the samples were housed behind magnesium-fluoride windows that transmitted radiation above approximately 110 nm. Verified Answer #1

Thus this was direct exposure to low-Earth-orbit vacuum, but not completely unmediated exposure to every aspect of the external spacecraft environment. (astroseti.org) Verified Answer #1

Nor were the lichens unharmed. Verified Answer #1

Vacuum-exposed algal cells showed major ultrastructural injury: microscopy found collapsed protoplasts in 61–97% of examined cells, depending on the irradiation treatment. Verified Answer #1

The fully illuminated vacuum sample recovered only about 45–50% of its preflight photosystem-II performance, despite many cells still staining as metabolically active. Verified Answer #1

Different assays therefore agree on survival but show that survival was partial and physiologically costly. (astroseti.org) Verified Answer #1

Bottom line Verified Answer #1

Yes. Xanthoria elegans demonstrably survived approximately 18 months in near-space vacuum outside the ISS, and viable algal cells subsequently resumed growth. But this does not mean that every lichen species—or every cell in the tested lichen—would survive. Verified Answer #1

The result applies to a highly desiccation-tolerant species, in a small controlled experiment, inside an exterior exposure carrier; it demonstrates partial survival and recovery, not normal growth during vacuum exposure or indefinite resistance. Verified Answer #1

Primary references Verified Answer #1

Brandt, A., de Vera, J.-P., Onofri, S., & Ott, S. (2015). Verified Answer #1

Viability of the lichen Xanthoria elegans and its symbionts after 18 months of space exposure and simulated Mars conditions on the ISS. International Journal of Astrobiology, 14(3), 411–425. Verified Answer #1

DOI: 10.1017/S1473550414000214. (elib.dlr.de) Verified Answer #1

Brandt, A., Posthoff, E., de Vera, J.-P., Onofri, S., & Ott, S. (2016). Verified Answer #1

Characterisation of growth and ultrastructural effects of the Xanthoria elegans photobiont after 1.5 years of space exposure on the International Space Station. Origins of Life and Evolution of Biospheres, 46(2–3), 311–321. Verified Answer #1

DOI: 10.1007/s11084-015-9470-1. (elib.dlr.de) Verified Answer #1

Rabbow, E., et al. (2012). Verified Answer #1

EXPOSE-E: An ESA astrobiology mission 1.5 years in space. Astrobiology, 12(5), 374–386. Verified Answer #1

DOI: 10.1089/ast.2011.0760. (researchgate.net) Verified Answer #1

Sources: [1] IJA1400021 1..15 [2] 11084_2015_9470_Article 1..11 [3] electronic library - Viability of the lichen Xanthoria elegans and its symbionts after 18 months of space exposure and simulated Mars conditions on the ISS [4] (PDF) EXPOSE-E: an ESA astrobiology mission 1.5 years in space Verified Answer #1