The Effects of HZE Particles, γ and X-ray Radiation on the Survival and Genetic Integrity of Halobacterium salinarum NRC-1, Halococcus hamelinensis, and Halococcus morrhuae

Exploring Andean High-Altitude Lake Extremophiles through Advanced Proteotyping

Quickly identifying and characterizing isolates from extreme environments is currently challenging while very important to explore the Earth's biodiversity. As these isolates may, in principle, be distantly related to known species, techniques are needed to reliably identify the branch of life to which they belong. Proteotyping these environmental isolates by tandem mass spectrometry offers a rapid and cost-effective option for their identification using their peptide profiles. In this study, we document the first high-throughput proteotyping approach for environmental extremophilic and halophilic isolates. Microorganisms were isolated from samples originating from high-altitude Andean lakes (3700-4300 m a.s.l.) in the Chilean Altiplano, which represent environments on Earth that resemble conditions on other planets. A total of 66 microorganisms were cultivated and identified by proteotyping and 16S rRNA gene amplicon sequencing. Both the approaches revealed the same genus identification for all isolates except for three isolates possibly representing not yet taxonomically characterized organisms based on their peptidomes. Proteotyping was able to indicate the presence of two potentially new genera from the families of Paracoccaceae and Chromatiaceae/Alteromonadaceae, which have been overlooked by 16S rRNA amplicon sequencing approach only. The paper highlights that proteotyping has the potential to discover undescribed microorganisms from extreme environments.

The archaeal class Halobacteria and astrobiology: Knowledge gaps and research opportunities

Water bodies on Mars and the icy moons of the outer solar system are now recognized as likely being associated with high levels of salt. Therefore, the study of high salinity environments and their inhabitants has become increasingly relevant for Astrobiology. Members of the archaeal class Halobacteria are the most successful microbial group living in hypersaline conditions and are recognized as key model organisms for exposure experiments. Despite this, data for the class is uneven across taxa and widely dispersed across the literature, which has made it difficult to properly assess the potential for species of Halobacteria to survive under the polyextreme conditions found beyond Earth. Here we provide an overview of published data on astrobiology-linked exposure experiments performed with members of the Halobacteria, identifying clear knowledge gaps and research opportunities.

Insights into the Survival Capabilities of Cryomyces antarcticus Hydrated Colonies after Exposure to Fe Particle Radiation

The modern concept of the evolution of Mars assumes that life could potentially have originated on the planet Mars, possibly during the end of the late heavy bombardment, and could then be transferred to other planets. Since then, physical and chemical conditions on Mars changed and now strongly limit the presence of terrestrial-like life forms. These adverse conditions include scarcity of liquid water (although brine solutions may exist), low temperature and atmospheric pressure, and cosmic radiation. Ionizing radiation is very important among these life-constraining factors because it damages DNA and other cellular components, particularly in liquid conditions where radiation-induced reactive oxidants diffuse freely. Here, we investigated the impact of high doses (up to 2 kGy) of densely-ionizing (197.6 keV/µm), space-relevant iron ions (corresponding on the irradiation that reach the uppermost layer of the Mars subsurface) on the survival of an extremophilic terrestrial organism-Cryomyces antarcticus-in liquid medium and under atmospheric conditions, through different techniques. Results showed that it survived in a metabolically active state when subjected to high doses of Fe ions and was able to repair eventual DNA damages. It implies that some terrestrial life forms can withstand prolonged exposure to space-relevant ion radiation.

Questioning the radiation limits of life: Ignicoccus hospitalis between replication and VBNC

Radiation of ionizing or non-ionizing nature has harmful effects on cellular components like DNA as radiation can compromise its proper integrity. To cope with damages caused by external stimuli including radiation, within living cells, several fast and efficient repair mechanisms have evolved. Previous studies addressing organismic radiation tolerance have shown that radiotolerance is a predominant property among extremophilic microorganisms including (hyper-) thermophilic archaea. The analysis of the ionizing radiation tolerance of the chemolithoautotrophic, obligate anaerobic, hyperthermophilic Crenarchaeon Ignicoccus hospitalis showed a D₁₀-value of 4.7 kGy, fourfold exceeding the doses previously determined for other extremophilic archaea. The genome integrity of I. hospitalis after γ-ray exposure in relation to its survival was visualized by RAPD and qPCR. Furthermore, the discrimination between reproduction, and ongoing metabolic activity was possible for the first time indicating that a potential viable but non-culturable (VBNC) state may also account for I. hospitalis.

Iron Ladies - How Desiccated Asexual Rotifer Adineta vaga Deal With X-Rays and Heavy Ions?

Space exposure experiments from the last 15 years have unexpectedly shown that several terrestrial organisms, including some multi-cellular species, are able to survive in open space without protection. The robustness of bdelloid rotifers suggests that these tiny creatures can possibly be added to the still restricted list of animals that can deal with the exposure to harsh condition of space. Bdelloids are one of the smallest animals on Earth. Living all over the world, mostly in semi-terrestrial environments, they appear to be extremely stress tolerant. Their desiccation tolerance at any stage of their life cycle is known to confer tolerance to a variety of stresses including high doses of radiation and freezing. In addition, they constitute a major scandal in evolutionary biology due to the putative absence of sexual reproduction for at least 60 million years. Adineta vaga, with its unique characteristics and a draft genome available, was selected by ESA (European Space Agency) as a model system to study extreme resistance of organisms exposed to space environment. In this manuscript, we documented the resistance of desiccated A. vaga individuals exposed to increasing doses of X-ray, protons and Fe ions. Consequences of exposure to different sources of radiation were investigated in regard to the cellular type including somatic (survival assay) and germinal cells (fertility assay). Then, the capacity of A. vaga individuals to repair DNA DSB induced by different source of radiation was investigated. Bdelloid rotifers represent a promising model in order to investigate damage induced by high or low LET radiation. The possibility of exposure both on hydrated or desiccated specimens may help to decipher contribution of direct and indirect radiation damage on biological processes. Results achieved through this study consolidate our knowledge about the radioresistance of A. vaga and improve our capacity to compare extreme resistance against radiation among living organisms including metazoan.

Polyploidy in halophilic archaea: regulation, evolutionary advantages, and gene conversion

All analyzed haloarachea are polyploid. In addition, haloarchaea contain more than one type of chromosome, and thus the gene dosage can be regulated independently on different replicons. Haloarchaea and several additional archaea have more than one replication origin on their major chromosome, in stark contrast with bacteria, which have a single replication origin. Two of these replication origins of Haloferax volcanii have been studied in detail and turned out to have very different properties. The chromosome copy number appears to be regulated in response to growth phases and environmental factors. Archaea typically contain about two Origin Recognition Complex (ORC) proteins, which are homologous to eukaryotic ORC proteins. However, haloarchaea are the only archaeal group that contains a multitude of ORC proteins. All 16 ORC protein paralogs from H. volcanii are involved in chromosome copy number regulation. Polyploidy has many evolutionary advantages for haloarchaea, e.g. a high resistance to desiccation, survival over geological times, and the relaxation of cell cycle-specific replication control. A further advantage is the ability to grow in the absence of external phosphate while using the many genome copies as internal phosphate storage polymers. Very efficient gene conversion operates in haloarchaea and results in the unification of genome copies. Taken together, haloarchaea are excellent models to study many aspects of genome biology in prokaryotes, exhibiting properties that have not been found in bacteria.

Review of microbial resistance to chronic ionizing radiation exposure under environmental conditions

Ionizing radiation (IR) produces multiple types of damage to nucleic acids, proteins and other crucial cellular components. Nevertheless, various microorganisms from phylogenetically distant taxa (bacteria, archaea, fungi) can resist IR levels many orders of magnitude above natural background. This intriguing phenomenon of radioresistance probably arose independently many times throughout evolution as a byproduct of selective pressures from other stresses (e.g. desiccation, UV radiation, chemical oxidants). Most of the literature on microbial radioresistance is based on acute γ-irradiation experiments performed in the laboratory, typically involving pure cultures grown under near-optimal conditions. There is much less information about the upper limits of radioresistance in the field, such as in radioactively-contaminated areas, where several radiation types (e.g. α and β, as well as γ) and other stressors (e.g. non-optimal temperature and nutrient levels, toxic chemicals, interspecific competition) act over multiple generations. Here we discuss several examples of radioresistant microbes isolated from extremely radioactive locations (e.g. Chernobyl and Mayak nuclear plant sites) and estimate the radiation dose rates they were able to tolerate. Some of these organisms (e.g. the fungus Cladosporium cladosporioides, the cyanobacterium Geitlerinema amphibium) are widely-distributed and colonize a variety of habitats. These examples suggest that resistance to chronic IR and chemical contamination is not limited to rare specialized strains from extreme environments, but can occur among common microbial taxa, perhaps due to overlap between mechanisms of resistance to IR and other stressors.

Lack of correlation of desiccation and radiation tolerance in microorganisms from diverse extreme environments tested under anoxic conditions

Four facultative anaerobic and two obligate anaerobic bacteria were isolated from extreme environments (deep subsurface halite mine, sulfidic anoxic spring, mineral-rich river) in the frame MASE (Mars Analogues for Space Exploration) project. The isolates were investigated under anoxic conditions for their survivability after desiccation up to 6 months and their tolerance to ionizing radiation up to 3000 Gy. The results indicated that tolerances to both stresses are strain-specific features. Yersinia intermedia MASE-LG-1 showed a high desiccation tolerance but its radiation tolerance was very low. The most radiation-tolerant strains were Buttiauxella sp. MASE-IM-9 and Halanaerobium sp. MASE-BB-1. In both cases, cultivable cells were detectable after an exposure to 3 kGy of ionizing radiation, but cells only survived desiccation for 90 and 30 days, respectively. Although a correlation between desiccation and ionizing radiation resistance has been hypothesized for some aerobic microorganisms, our data showed that there was no correlation between tolerance to desiccation and ionizing radiation, suggesting that the physiological basis of both forms of tolerances is not necessarily linked. In addition, these results indicated that facultative and obligate anaerobic organisms living in extreme environments possess varied species-specific tolerances to extremes.

Cryptoendolithic Antarctic Black Fungus Cryomyces antarcticus Irradiated with Accelerated Helium Ions: Survival and Metabolic Activity, DNA and Ultrastructural Damage

Space represents an extremely harmful environment for life and survival of terrestrial organisms. In the last decades, a considerable deal of attention was paid to characterize the effects of spaceflight relevant radiation on various model organisms. The aim of this study was to test the survival capacity of the cryptoendolithic black fungus Cryomyces antarcticus CCFEE 515 to space relevant radiation, to outline its endurance to space conditions. In the frame of an international radiation campaign, dried fungal colonies were irradiated with accelerated Helium ion (150 MeV/n, LET 2.2 keV/μm), up to a final dose of 1,000 Gy, as one of the space-relevant ionizing radiation. Results showed that the fungus maintained high survival and metabolic activity with no detectable DNA and ultrastructural damage, even after the highest dose irradiation. These data give clues on the resistance of life toward space ionizing radiation in general and on the resistance and responses of eukaryotic cells in particular.

Evaluation of the Resistance of Chroococcidiopsis spp. to Sparsely and Densely Ionizing Irradiation

Studying the resistance of cyanobacteria to ionizing radiation provides relevant information regarding astrobiology-related topics including the search for life on Mars, lithopanspermia, and biological life-support systems. Here, we report on the resistance of desert cyanobacteria of the genus Chroococcidiopsis, which were exposed (as part of the STARLIFE series of experiments) in both hydrated and dried states to ionizing radiation with different linear energy transfer values (0.2 to 200 keV/μm). Irradiation with up to 1 kGy of He or Si ions, 2 kGy of Fe ions, 5 kGy of X-rays, or 11.59 kGy of γ rays (⁶⁰Co) did not eradicate Chroococcidiopsis populations, nor did it induce detectable damage to DNA or plasma membranes. The relevance of these results for astrobiology is briefly discussed. Key Words: Ionizing radiation-Linear energy transfer-Lithopanspermia-Cyanobacterial radioresistance-Chroococcidiopsis-Mars. Astrobiology 17, 118-125.

STARLIFE-An International Campaign to Study the Role of Galactic Cosmic Radiation in Astrobiological Model Systems

In-depth knowledge regarding the biological effects of the radiation field in space is required for assessing the radiation risks in space. To obtain this knowledge, a set of different astrobiological model systems has been studied within the STARLIFE radiation campaign during six irradiation campaigns (2013-2015). The STARLIFE group is an international consortium with the aim to investigate the responses of different astrobiological model systems to the different types of ionizing radiation (X-rays, γ rays, heavy ions) representing major parts of the galactic cosmic radiation spectrum. Low- and high-energy charged particle radiation experiments have been conducted at the Heavy Ion Medical Accelerator in Chiba (HIMAC) facility at the National Institute of Radiological Sciences (NIRS) in Chiba, Japan. X-rays or γ rays were used as reference radiation at the German Aerospace Center (DLR, Cologne, Germany) or Beta-Gamma-Service GmbH (BGS, Wiehl, Germany) to derive the biological efficiency of different radiation qualities. All samples were exposed under identical conditions to the same dose and qualities of ionizing radiation (i) allowing a direct comparison between the tested specimens and (ii) providing information on the impact of the space radiation environment on currently used astrobiological model organisms. Key Words: Space radiation environment-Sparsely ionizing radiation-Densely ionizing radiation-Heavy ions-Gamma radiation-Astrobiological model systems. Astrobiology 17, 101-109.