Model organisms for study of Astrobiology
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Why do we need to study organisms to study astrobiology?
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Astrobiology is defined as "an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and if it does, how humans can detect it."
Astrobiologists study organisms to study evolution and the origins of life. The panspermia hypothesis suggests that life on Earth came from outer space. For organisms to travel such a long distance and arrive on planet Earth, they need to be extremely resilient to harsh conditions. Such organisms which are resilient towards intense heat, cold, pH changes and extreme pressure, are called extremophiles. Extremophiles are hence used as model organisms to study astrobiology.
One of the most famous extremophiles which are model organisms for astrobiological studies are Tardigrades.
Tardigrades:
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Tardigrades known colloquially as water bears or moss piglets are a phylum of water-dwelling eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them little water bears. In 1777, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means "slow steppers".
They have been found everywhere, from mountaintops to the deep sea and mud volcanoes, and from tropical rainforests to the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation that would quickly kill most other known forms of life, which is why they are a subject of study for astrobiology.
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Tardigrades have survived exposure to outer space. About 1,300 known species form the phylum Tardigrada, a part of the superphylum Ecdysozoa. The earliest known true members of the group are known from Cretaceous amber in North America, but are essentially modern forms, and therefore likely have a significantly earlier origin, as they diverged from their closest relatives in the Cambrian, over 500 million years ago.
Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power microscope, making them accessible to students and amateur scientists.
Unicellular organisms:
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Thermophilic species G. thermantarcticus is a good example of a microorganism that could survive space travel. It is a bacterium of genus Bacillus. It forms spores, which allows for it to survive extreme environments while still being able to restart cellular growth. It is capable of effectively protecting its DNA, membrane and proteins integrity in different extreme conditions (desiccation, temperatures up to -196 °C, UVC and C-ray radiation). It is also able to repair the damage produced in the harsh space environment.
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By understanding how extremophilic organisms can survive the Earth's extreme environments, we can also understand how microorganisms could have survived space travel and how the panspermia hypothesis could be possible.
New research on the bacterium Tepidibacillus decaturensis shows that it could be a model organism for what might live on Mars. This microorganism has been shown to be moderately tolerant of heat and salt and able to persist in an anoxic environment.
New ongoing research:
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1. Extremophiles Survival to Simulated Space Conditions: An Astrobiology Model Study
Abstract:
In this work we investigated the ability of four extremophilic bacteria from Archaea and Bacteria domains to resist the space environment by exposing them to extreme conditions of temperature, UV radiation, desiccation coupled to low pressure generated in a Mars’ conditions simulator. All the investigated extremophilic strains (namely Sulfolobus solfataricus, Haloterrigena hispanica, Thermotoga neapolitana and Geobacillus thermantarcticus) showed a good resistance to the simulation of the temperature variation in the space; on the other hand irradiation with UV at 254 nm affected only slightly the growth of H. hispanica, G. thermantarcticus and S. solfataricus; finally exposition to Mars simulated condition showed that H. hispanica and G. thermantarcticus were resistant to desiccation and low pressure.
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(Citation: Mastascusa, V., Romano, I., Di Donato, P. et al. Extremophiles Survival to Simulated Space Conditions: An Astrobiology Model Study. Orig Life Evol Biosph 44, 231–237 (2014). https://doi.org/10.1007/s11084-014-9397-y)
2. Tepidibacillus decaturensis sp. nov., a microaerophilic, moderately thermophilic iron-reducing bacterium isolated from 1.7 km depth groundwater.
Abstract:
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A Gram-stain-negative, microaerophilic rod-shaped organism designated as strain Z9T was isolated from groundwater of 1.7 km depth from the Mt. Simon Sandstone of the Illinois Basin, Illinois, USA. Cells of strain Z9T were rod shaped with dimensions of 0.3×(1-10) µm and stained Gram-negative. Strain Z9T grew within the temperature range 20-60 °C (optimum at 30-40 °C), between pH 5 and 8 (optimum 5.2-5.8) and under salt concentrations of 1-5 % (w/v) NaCl (optimum 2.5 % NaCl). In addition to growth by fermentation and nitrate reduction, this strain was able to reduce Fe(III), Mn(IV), Co(III) and Cr(VI) when H2 or organic carbon was available as the electron donor, but did not actively reduce oxidized sulfur compounds (e.g. sulfate, thiosulfate or S0). The G+C content of the DNA from strain Z9T was 36.1 mol%. Phylogenetic analysis of the 16S rRNA gene from strain Z9T showed that it belongs to the class Bacilli and shares 97 % sequence similarity with the only currently characterized member of the genus Tepidibacillus, T.fermentans. Based on the physiological distinctness and phylogenetic information, strain Z9T represents a novel species within the genus Tepidibacillus, for which the name Tepidibacillus decaturensis sp. nov. is proposed. The type strain is Z9T (=ATCC BAA-2644T=DSM 103037T).
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(Citation: Dong Y, Sanford RA, Boyanov MI, Kemner KM, Flynn TM, O'Loughlin EJ, Locke RA, Weber JR, Egan SM, Fouke BW. Tepidibacillus decaturensis sp. nov., a microaerophilic, moderately thermophilic iron-reducing bacterium isolated from 1.7 km depth groundwater. Int J Syst Evol Microbiol. 2016 Oct;66(10):3964-3971. doi: 10.1099/ijsem.0.001295. Epub 2016 Jul 11. PMID: 27406851.)