Effect of shadowing on survival of bacteria under conditions simulating the Martian atmosphere and UV radiation

Abstract

Spacecraft-associated spores and four non-spore-forming bacterial isolates were prepared in Atacama Desert soil suspensions and tested both in solution and in a desiccated state to elucidate the shadowing effect of soil particulates on bacterial survival under simulated Martian atmospheric and UV irradiation conditions. All non-spore-forming cells that were prepared in nutrient-depleted, 0.2-microm-filtered desert soil (DSE) microcosms and desiccated for 75 days on aluminum died, whereas cells prepared similarly in 60-microm-filtered desert soil (DS) microcosms survived such conditions. Among the bacterial cells tested, Microbacterium schleiferi and Arthrobacter sp. exhibited elevated resistance to 254-nm UV irradiation (low-pressure Hg lamp), and their survival indices were comparable to those of DS- and DSE-associated Bacillus pumilus spores. Desiccated DSE-associated spores survived exposure to full Martian UV irradiation (200 to 400 nm) for 5 min and were only slightly affected by Martian atmospheric conditions in the absence of UV irradiation. Although prolonged UV irradiation (5 min to 12 h) killed substantial portions of the spores in DSE microcosms (approximately 5- to 6-log reduction with Martian UV irradiation), dramatic survival of spores was apparent in DS-spore microcosms. The survival of soil-associated wild-type spores under Martian conditions could have repercussions for forward contamination of extraterrestrial environments, especially Mars.

FIG. 1.

UV spectra (200 to 400 nm) of the solar simulator (solid line) and the deuterium lamp (dotted line) employed in this study. The lighting spectrum of Mars modeled for near-equator regions at noon and in a dust-free atmosphere by Patel et al. (37) is indicated by the dashed line. The output of the solar simulator was measured and adjusted to match the spectrum of the Mars model. The integrated irradiance over the wavelength range from 200 to 400 nm was 30 W m⁻² for the solar simulator, 32 W m⁻² for the Mars model, and 3.4 W m⁻² for the deuterium lamp.

FIG. 2.

Survival of hydrated spores or cells in (A) DSE and (B) DS after several cumulative doses of UV₂₅₄ irradiation. N and N⁰ represent cfu of the samples without and with radiation, respectively.

FIG. 3.

Environmental scanning electron micrographs of several bacteria and spores prepared in DSE and DS and desiccated on spacecraft-qualified aluminum 6061. The arrows indicate where spores or bacterial cells are present. Some spores or bacterial cells were exposed to the UV, and some were hidden under soil particles.

FIG. 4.

Environmental scanning electron micrographs of several bacteria and spores prepared in DSE and DS and desiccated on spacecraft-qualified aluminum 6061. The arrows indicate where bacterial cells are present. Some cells were exposed to the UV irradiation, and some were hidden under soil particles.

FIG. 5.

Survival of desiccated spores or cells on spacecraft-qualified aluminum 6061 after several cumulative doses of UV₂₅₄ irradiation. (A) DSE. (B) DS. N and N⁰ represent cfu of the samples without and with radiation, respectively.

FIG. 6.

Survival of desiccated B. pumilus SAFR-032 spores on spacecraft-qualified aluminum 6061 under various simulated Mars conditions. MUVM, incubation in the Mars atmosphere without UV irradiation for 12 h at room temperature, followed by 12 h at −60°C; MUV-5, incubation with Mars UV irradiation in air for 5 min; MUV-30, incubation with Mars UV irradiation in air for 30 min; MUV-1440, incubation with Mars UV irradiation in air for 12 h; MUVP-5, incubation with Mars UV irradiation in the Mars atmosphere for 5 min; MUVP-30, incubation with Mars UV irradiation in the Mars atmosphere for 30 min; MUVP-1440, incubation in the Mars atmosphere for 12 h with Mars UV irradiation at room temperature, followed by 12 h with no UV irradiation at −60°C; +, not detected as CFU but detected by incubating an exposed coupon in liquid medium; −, not detected on solid medium or in liquid medium. All experiments were performed at room temperature, unless otherwise noted. Mars UV irradiation was provided by the solar simulator, and the Mars atmosphere used contained 7 × 10² Pa CO₂. N and N⁰ represent cfu of the samples without and with radiation, respectively.