One of the main goals of astrobiology is to find out life or signs of life on planets beyond Earth. Therefore, we must study the limits of life on our planet and we are finding organisms which are living in a variety of extreme environmental conditions. The study of these organisms can inform us about how life began on primitive Earth and possiblity of life on other planets.
What is Extremophile?
Extremophiles are organisms that live in extremes of temperature, salinity, pressure, ionizing-radiation and etc. These organisms have the ability to adapt to these hard environmental conditions because of specialized cellular mechanisms.
We can find extremophiles in volcanoes,deep oceanic sediments, polar regions, acidic bogs and hypersaline lakes. In general, extremophilic organisms are primarily prokaryotic; these are archaea and bacterias.
Also they have eukaryotic members such as algae forming lichens, fungi, protozoa and microscopic invertebrates- tardigrades.
Why Extremophiles Are Important?
Extremophiles are important source of biocatalysis or enzymes in biotechnology. They produce specific enzymes called extremozymes which are special for their ability to remain active under extreme conditions where they live.
The extremozymes have an economic potential uses in agriculture, food beverages, detergen, leather, pharmaceutical, biomining industries, biomedical and biotechnology prosses. Also they are important in astrobiology, because of their ability to adapt extreme conditions.
Kinds of Extremophiles
Extremophiles are described according to respond stress of their environment;
- Acidophiles– they live in asidic environment (pH1-pH5),
- Alkaliphiles– live in environment with ≥pH9,
- Halophiles– they can live in high salt concentration,
- Thermophiles– they can be moderate (45-70°C), extreme(≥70°C) and hyper thermophilic (≥80°C or close to boiling point of water),
- Psychrophiles– live in cold or environment with temperature close to freezing point of water,
- Barophiles– live in hydrostatic pressure,
- Oligotrophic– grow in nutritionally limited environment,
- Endolithic– can grow within rocks or pores of mineral grains,
- Xerophiles– live in dry conditions.
Extremophilic Organisms in Environmental Extremes
Also some extremophiles are adapted to live in two or more stresses and called polyextremophiles. For example: thermoacidophiles -can live in asidic environment with high temperature or haloalkaliphiles- can live in alkaline environment with high salt concentration.
Tardigrade is a polyextremophile, which meansthey can withstand temperatures from -328°F (200 °C) up to 304 °F (151 °C), lack of water and oxygen, and a thousand times the radiation.
Polyextremophilic Organism Tardigrade
Cyanobacterias in Astrobiology
Cyanobacterias are unicellular bacterias that thought to be first organisms to do photosynthesis. They are survived for billions of years and have wide genetic diversity.
You can find cyanobacteria almost everywhere: on land or in water. Many cyanobacterias are known for their ability to live in extreme conditions. They can live in extreme terrestrial environment or deep saline ocean.
Cyanobacterias even survived outside the International Space Station (ISS) for 16 months. Outside the ISS ,they were exposed to extreme radiation levels and temperature variation, also adapted well to the coldness of vaccum.
The scytonemin is an important biomolecule produced by extremophilic cyanobacterias. The main function of this biomolecule is protection from ultra-violet (UV) radiation in extreme terrestrial environments.
The studies show that scytonemin has capability to absorb all 3 regions of UV radiation: UV-A, UV-B and UV-C. Unlike the dimer molecule, the monomer of scytonemin can absorb only UV-B and UV-C regions of radiation.
The dimerization by steric hindrance can remove lower energy UV and protect chlorophyll for photosynthesis. The result of this study provide scientists to search for similar organisms to cyanobacterias in the astrobiological researches of Mars.
Can Extremophiles Live In Space?
Yes. As mentioned above, some extremophiles by the help of special molecules can live unprotected in space for several days , months or years. Scientists have done a number of experiments on different kinds of extremophiles to know their abilities to adapt life out of Earth.
One of these experiments was on bacterium Deinococcus Radiodurans . The experiment shows that ,they can tolerate the influence of outer space; cosmic and solar UV radiation, extreme vacuum, desiccation, freezing and microgravity for 3 years.
Bacterium Deinococcus Radiodurans Under Microscope
Akihiko Yamagishi, a microbiologist at Tokyo University of Pharmacy and Life Sciences, with his team did astrobiological experiment on D.Radiodurans in April 2015. The result of this experiment shows the influence of outer space on this bacteria on a molecular level.
Firstly, astronouts set Deinococcus radiodurans in two small aluminium plates. Then one plate were pointed toward the International Space Station (ISS) and the second one were pointed to the cosmos. After 3 years in space, scientists found that bacteria in 100-micrometer thick pellets escaped morphological damages, but radiation fried their genetic material.
The ability of D. radiodurans to survive ionizing-radiation-induced DNA damage involves recombination repair, the regulation of DNA replication and the export of damaged nucleotides .
The Response of Deinococcus Radiodurans to Ionizing-Radiation
The bacterias on outer layer of pellet were dead. The ultraviolet radiation and desiccation discolored them. But those dead cells protected inner cells from UV radiation and inner cells still alive.
Yamagishi estimates that about four in every 100 of D.Radiodurans survived and this pellets with bacteria could survive 8 years floating through space. The result of this experiment show that larger organisms with such capabilities also can live through space journeys.