Survivability of Soil and Permafrost Microbial Communities after Irradiation with Accelerated Electrons under Simulated Martian and Open Space Conditions

Exobiology of the Venusian Clouds: New Insights into Habitability through Terrestrial Models and Methods of Detection

The search for life beyond Earth has focused on Mars and the icy moons Europa and Enceladus, all of which are considered a safe haven for life due to evidence of current or past water. The surface of Venus, on the other hand, has extreme conditions that make it a nonhabitable environment to life as we know it. This is in contrast, however, to its cloud layer, which, while still an extreme environment, may prove to be a safe haven for some extreme forms of life similar to extremophiles on Earth. We consider the venusian clouds a habitable environment based on the presence of (1) a solvent for biochemical reactions, (2) appropriate physicochemical conditions, (3) available energy, and (4) biologically relevant elements. The diversity of extreme microbial ecosystems on Earth has allowed us to identify terrestrial chemolithoautotrophic microorganisms that may be analogs to putative venusian organisms. Here, we hypothesize and describe biological processes that may be performed by such organisms in the venusian clouds. To detect putative venusian organisms, we describe potential biosignature detection methods, which include metal-microbial interactions and optical methods. Finally, we describe currently available technology that can potentially be used for modeling and simulation experiments.

The bacterial and fungal nest microbiomes in populations of the social spider Stegodyphus dumicola

Social spiders of the species Stegodyphus dumicola live in communal nests with hundreds of individuals and are characterized by extremely low species-wide genetic diversity. The lack of genetic diversity in combination with group living imposes a potential threat for infection by pathogens. We therefore proposed that specific microbial symbionts inhabiting the spider nests may provide antimicrobial defense. To compare the bacterial and fungal diversity in 17 nests from three different locations in Namibia, we used 16S rRNA gene and internal transcribed spacer (ITS2) sequencing. The nest microbiomes differed between geographically distinct spider populations and appeared largely determined by the local environment. Nevertheless, we identified a core microbiome consisting of four bacterial genera (Curtobacterium, Modestobacter, Sphingomonas, Massilia) and four fungal genera (Aureobasidium, Didymella, Alternaria, Ascochyta), which likely are selected from surrounding soil and plants by the nest environment. We did not find indications for a strain- or species-specific symbiosis in the nests. Isolation of bacteria and fungi from nest material retrieved a few bacterial strains with antimicrobial activity but a number of antimicrobial fungi, including members of the fungal core microbiome. The significance of antimicrobial taxa in the nest microbiome for host protection remains to be shown.

Effects of Radiation Intensity, Mineral Matrix, and Pre-Irradiation on the Bacterial Resistance to Gamma Irradiation under Low Temperature Conditions

Ionizing radiation is one of the main factors limiting the survival of microorganisms in extraterrestrial conditions. The survivability of microorganisms under irradiation depends significantly on the conditions, in which the irradiation occurs. In particular, temperature, pressure, oxygen and water concentrations are of great influence. However, the influence of factors such as the radiation intensity (in low-temperature conditions) and the type of mineral matrix, in which microorganisms are located, has been practically unstudied. It has been shown that the radioresistance of bacteria can increase after their exposure to sublethal doses and subsequent repair of damage under favorable conditions, however, such studies are also few and the influence of other factors of extraterrestrial space (temperature, pressure) was not studied in them. The viability of bacteria Arthrobacter polychromogenes, Kocuria rosea and Xanthomonas sp. after irradiation with gamma radiation at a dose of 1 kGy under conditions of low pressure (1 Torr) and low temperature (-50 °C) at different radiation intensities (4 vs. 0.8 kGy/h) with immobilization of bacteria on various mineral matrices (montmorillonite vs. analogue of lunar dust) has been studied. Native, previously non-irradiated strains, and strains that were previously irradiated with gamma radiation and subjected to 10 passages of cultivation on solid media were irradiated. The number of survived cells was determined by culturing on a solid medium. It has been shown that the radioresistance of bacteria depends significantly on the type of mineral matrix, on which they are immobilized, wherein montmorillonite contributes to an increased survivability in comparison with a silicate matrix. Survivability of the studied bacteria was found to increase with decreasing radiation intensity, despite the impossibility of active reparation processes under experimental conditions. Considering the low intensity of radiation on various space objects in comparison with radiobiological experiments, this suggests a longer preservation of the viable microorganisms outside the Earth than is commonly believed. An increase in bacterial radioresistance was revealed even after one cycle of irradiation of the strains and their subsequent cultivation under favourable conditions. This indicates the possibility of hypothetical microorganisms on Mars increasing their radioresistance.

The distinct microbial community in Aurelia coerulea polyps versus medusae and its dynamics after exposure to 60Co-γ radiation

Radiation (e.g., nuclear leakage) is a common harmful factor in the ocean that potentially affects the microbial community in nearby benthic hosts such as jellyfish polyps, which is essential for the maintenance of jellyfish populations and high-quality medusae. After comparison with the microbial community of medusae, the effect of ⁶⁰Co-γ on the microbial community in Aurelia coerulea polyps was dynamically tested using 16S rRNA gene sequencing. Our results suggested that Proteobacteria (76.19 ± 3.24%), Tenericutes (12.93 ± 3.20%) and Firmicutes (8.33 ± 1.06%) are most abundant in medusae, while Proteobacteria (29.49 ± 2.29%), Firmicutes (46.25 ± 5.59%), and Bacteroidetes (20.16 ± 2.65%) are the top three phyla in polyps. After ⁶⁰Co-γ radiation, the proportion of Proteobacteria increased from 29.49 ± 2.29% to 59.40 ± 3.09% over 5 days, while that of Firmicutes decreased from 46.25 ± 5.59% to 13.58 ± 3.74%. At the class level, Gammaproteobacteria continually increased during the 5 days after radiation exposure, whereas Bacilli declined, followed by partial recovery, and Alphaproteobacteria and Flavobacteriia remained almost unchanged. Intriguingly, Staphylococcus from Firmicutes and three other genera, Rhodobacter, Vibrio, and Methylophaga, from Proteobacteria greatly overlapped according to their KEGG functions. It is concluded that the microbial community in A. coerulea polyps is distinct from that in the medusae and is greatly affected by ⁶⁰Co-γ exposure, with a growth (0-3 d) period and a redistribution (3-5 d) period. The dynamic change in the microbial community is probably an important self-defense process in response to external interference that is regulated by the host's physiological characteristics and the intense interspecific competition among symbiotic microbes with similar functions and functional redundancies.

Stress-Tolerance and Taxonomy of Culturable Bacterial Communities Isolated from a Central Mojave Desert Soil Sample

The arid Mojave Desert is one of the most significant terrestrial analogue objects for astrobiological research due to its genesis, mineralogy, and climate. However, the knowledge of culturable bacterial communities found in this extreme ecotope’s soil is yet insufficient. Therefore, our research has been aimed to fulfil this lack of knowledge and improve the understanding of functioning of edaphic bacterial communities of the Central Mojave Desert soil. We characterized aerobic heterotrophic soil bacterial communities of the central region of the Mojave Desert. A high total number of prokaryotic cells and a high proportion of culturable forms in the soil studied were observed. Prevalence of Actinobacteria, Proteobacteria, and Firmicutes was discovered. The dominance of pigmented strains in culturable communities and high proportion of thermotolerant and pH-tolerant bacteria were detected. Resistance to a number of salts, including the ones found in Martian regolith, as well as antibiotic resistance, were also estimated.

Survival of Radioresistant Bacteria on Europa’s Surface after Pulse Ejection of Subsurface Ocean Water

We briefly present preliminary results of our study of the radioresistant bacteria in a low temperature and pressure and high-radiation environment and hypothesize the ability of microorganisms to survive extraterrestrial high-radiation environments, such as the icy surface of Jupiter’s moon, Europa. In this study, samples containing a strain of Deinococcus radiodurans VKM B-1422T embedded into a simulated version of Europa’s ice were put under extreme environmental (−130 °C, 0.01 mbar) and radiation conditions using a specially designed experimental vacuum chamber. The samples were irradiated with 5, 10, 50, and 100 kGy doses and subsequently studied for residual viable cells. We estimate the limit of the accumulated dose that viable cells in those conditions could withstand at 50 kGy. Combining our numerical modelling of the accumulated dose in ice with observations of water eruption events on Europa, we hypothesize that in the case of such events, it is possible that putative extraterrestrial organisms might retain viability in a dormant state for up to 10,000 years, and could be sampled and studied by future probe missions.

Soil bacterial communities of Sahara and Gibson deserts: Physiological and taxonomical characteristics

The purpose of this research was to investigate the structure of soil bacteria communities present in the Gibson (Australia) and the Sahara (Egypt) deserts, as well as to estimate strain survivability under different environmental factors. It should be noticed that the screening of bacterial resistance to wide spectra of principally different stress conditions was performed for the first time. Experiments were conducted with culturable bacterial communities. Strains were identified using 16S rRNA sequencing, and stress-tolerance was estimated by growing strains in various nutrient media. In order to characterize the community the epifluorescent microscopy and multisubstrate testing were also performed. High bacterial abundance in the desert soils was detected, and there was seen a significant proportion of culturable cells. The close numbers of psychotropic and mesophilic bacteria in arid ecosystems were revealed. The representatives of the Actinobacteria phylum were dominant in the microbial communities, and Firmicutes, Proteobacteria, and Bacteroidetes phyla representatives were also identified. Tolerance of the axenic bacterial cultures, isolated from arid desert ecotopes, to temperature, pH, salts (KCl, NaCl, MgSO4, NaHCO3), strong oxidizers (Mg(ClO4)2), and antibiotics (ampicillin, cephalexin, chloramphenicol, tetracycline, doxycycline, kanamycin, rifampicin) was studied. The bacterial isolates were characterized by polyextremotolerance and by the ability to maintain metabolic activity in vitro while influenced by a wide range of physicochemical and biotic factors.