Space microbiology

Abstract

The responses of microorganisms (viruses, bacterial cells, bacterial and fungal spores, and lichens) to selected factors of space (microgravity, galactic cosmic radiation, solar UV radiation, and space vacuum) were determined in space and laboratory simulation experiments. In general, microorganisms tend to thrive in the space flight environment in terms of enhanced growth parameters and a demonstrated ability to proliferate in the presence of normally inhibitory levels of antibiotics. The mechanisms responsible for the observed biological responses, however, are not yet fully understood. A hypothesized interaction of microgravity with radiation-induced DNA repair processes was experimentally refuted. The survival of microorganisms in outer space was investigated to tackle questions on the upper boundary of the biosphere and on the likelihood of interplanetary transport of microorganisms. It was found that extraterrestrial solar UV radiation was the most deleterious factor of space. Among all organisms tested, only lichens (Rhizocarpon geographicum and Xanthoria elegans) maintained full viability after 2 weeks in outer space, whereas all other test systems were inactivated by orders of magnitude. Using optical filters and spores of Bacillus subtilis as a biological UV dosimeter, it was found that the current ozone layer reduces the biological effectiveness of solar UV by 3 orders of magnitude. If shielded against solar UV, spores of B. subtilis were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called lithopanspermia hypothesis.

FIG. 1.

Solar terrestrial (A) and extraterrestrial (B) UV irradiance spectra, action spectra for DNA damage as an example of biological sensitivity (dashed lines), and biological effectiveness spectra (bold red lines) for terrestrial and extraterrestrial conditions. (Modified from Fig. 1 in reference with kind permission of Springer Science and Business Media.)

FIG. 2.

Altitude profile of Earth's atmospheric components and pressure. (Modified from reference with permission of the publisher.)

FIG. 3.

Absorption spectra of Earth's atmosphere at the surface and at altitude. The solar radiation spectra are given for the top of the atmosphere (orange) and at sea level (blue).

FIG. 4.

Radiobiological chain of events that starts in a microbial cell after exposure to ionizing radiation, with two alternative pathways of interaction, resulting in either direct or indirect radiation damage. (Modified from Fig. 7-05 in reference with kind permission of Springer Science and Business Media.)

FIG. 5.

Integral net fraction of inactivated spores of B. subtilis as a function of the impact parameter, i.e., the radial distance from the HZE particle trajectory, and integral fraction of all spores investigated in that area. Results are from Biostack III on the Apollo Soyuz test project (ASTP). (Modified from reference with permission from Elsevier.)

FIG. 6.

Inactivation cross section, σ_i, of B. subtilis HA 101 spores as a function of the LET and atomic number Z, determined from fluence inactivation curves at heavy ion accelerators (Lawrence Berkeley Laboratory, Berkeley, CA [A] and Gesellschaft für Strahlenforschung, Darmstadt, Germany [B]) and from Biostack experiments in space (C). (Modified from Fig. 12 in reference with kind permission of Springer Science and Business Media.)

FIG. 7.

(A) UV-irradiation conditions (short-wavelength cutoff by use of a filtering system, with corresponding simulated ozone column thickness) in the RD-UVRAD experiment during the German D2 mission. (B) Calculated biological effectiveness spectra for the different experimental conditions according to the sensitivity curve of the biofilm dosimeter. (C) Biological effectiveness of radiation, determined experimentally by use of the biofilm dosimeter (blue circles) and calculated by integrating the biological effectiveness spectra (B) over wavelengths (red circles). DU, Dobson unit, which measures the stratospheric ozone. 1 DU refers to a layer of ozone of 10 μm in thickness under standard temperature and pressure. GC, ground control data measured at noon in summer on the roof of the DLR in Cologne, Germany. (Modified from reference with permission from Elsevier.)

FIG. 8.

Repair kinetics of radiation-induced DNA damage under microgravity conditions. (A) Rejoining of DNA strand breaks in X-irradiated cells of E. coli B/r. (B) Induction of SOS response in X-irradiated cells of E. coli PQ37. (C) Survival of spores of B. subtilis HA 101 after UV irradiation. (Modified from reference with permission of the publisher.)

FIG. 9.

MEED exposure facility mounted on the camera beam of the lunar orbiter of the Apollo 16 mission (A) and sample cuvette for exposing dry layers of microorganisms to solar UV radiation and space vacuum (B). (Reprinted from reference .)

FIG. 10.

Exposure tray of the ES029 experiment (A), which was mounted (arrow) on a pallet inside the cargo bay of SL1 (B), and exposure tray (C) of the ERA facility (D) on board the EURECA satellite. (Courtesy of DLR [A], NASA [B], and ESA [C and D].)

FIG. 11.

EXPOSE-E facility mounted on the EuTeF platform of the European Columbus module of the ISS. The picture was taken by the crew of STS 122, when leaving the ISS. (Courtesy of ESA and NASA.)

FIG. 12.

Scheme of the exposure conditions in the Exostack experiment (A) on board the LDEF (B) (arrow). (Panel A modified from reference with permission from Elsevier, panel B courtesy of NASA.)

FIG. 13.

Biopan facility open as in flight (A) and mounted on the Foton reentry capsule (B). (Courtesy of ESA.)

FIG. 14.

Survival of microorganisms for extended periods in space vacuum (i.e., shielded against solar UV radiation) (filled symbols) or exposed to full outer space conditions (i.e., space vacuum, solar UV radiation, and cosmic radiation) (open symbols). The microbes examined were B. subtilis spores in monolayers, in multilayers mixed with glucose, mixed with clay or meteorite powder, or embedded in artificial meteorites of 1 cm in diameter; cells of Haloarcula; and the lichens Rhizocarpon geographicum and Xanthoria elegans. Data are from experiments on the Foton Biopan, MIR, EURECA, and LDEF.

FIG. 15.

Action spectra of inactivation of spores of B. subtilis HA 101 by extraterrestrial solar UV radiation, exposed to space vacuum (blue circles) or to 10⁵ Pa in air or argon (red circles). The data are mean values from experiments on SL1, D2, and EURECA. The spectra are normalized to 1 at λ = 260 nm at 10⁵ Pa. For comparison, the action spectrum of DNA damage is shown (dashed line). (Modified from reference with permission from Elsevier.)

FIG. 16.

Survival as a function of applied shock pressure during shock recovery experiments with spores of B. subtilis TKL 6312 and cells of Chroococcidiopsis sp. (A) and vitality of Xanthoria elegans lichen mycobionts and photobionts encased in gabbro rock plates (B). Open circles indicate survival below the threshold of detection. (Modified from reference with permission of the publisher.)

FIG. 17.

STONE facility mounted on the stagnation point of the Foton reentry capsule, before launch (A) and after reeentry and landing (B). (Courtesy of ESA.)