Researchers associated with the Astrobiology Laboratory of the University of São Paulo (AstroLab-USP) have developed a new model that allows estimating the survival of microbial life subjected to ultraviolet radiation in liquid water with different concentrations of iron ions. The theoretical tool is relevant for simulating the habitability of lakes that must have existed on the primitive surface of Mars, more than 3 billion years ago.
The study was published last month in the journal Astrobiology and developed as part of two projects funded by Fapesp, one regular and one thematic. The researchers studied how iron ions (Fe+3) can protect microorganisms in liquid water by absorbing type C ultraviolet radiation, which the thin Martian atmosphere does not block – and which is particularly harmful to life.
Developed from experiments with yeast Saccharomyces boulardiia probiotic commonly used to restore intestinal flora, the model shows that microorganisms could have survived the incidence of ultraviolet rays in ancient lakes on Mars.
Scientific literature has already detailed the protective power of iron present in the regolith which covers the current dry surface of Mars. “And the research also looked at the ultraviolet protection provided by iron in aqueous solutions, but using very complex modeling, which is difficult to apply,” explains chemical engineer Gabriel Gonçalves Silva, a researcher at AstroLab.
First author of the article, Silva points out that these previous models did not allow detailed estimates of the viability of microorganisms with fine adjustments of radiation levels and ion concentrations. “We wanted a simple model that met these objectives,” he says.
To develop the model, the researchers placed samples of S. boulardii in aqueous solutions with different concentrations of Fe+3. They then subjected the samples to increasing levels of ultraviolet radiation to measure the survival rate of the microorganisms. Yeast was chosen as a model because it is very sensitive to ultraviolet light and tolerates high acidity – so much so that it passes into gastric juice when used in digestive health treatments.
Experiments have shown that even a relatively low concentration of iron ions is able to protect yeast from near-surface ultraviolet rays. The microorganisms were able to survive the impact of ultraviolet radiation long enough for their reproduction rate to compensate for the deaths caused by the radiation. “In this regard, it is not only about the amount of iron that protects, but also the time it takes for the microorganisms to reproduce and keep the population stable,” emphasizes Ana Paula Schiavo, postdoctoral researcher at the USP Institute of Chemistry and co-author of the study.
To test the validity of the model, the researchers compared their predictions about the viability of the microorganisms with experimental observations of the survival rate of the yeast. Applied to simulations of Martian lakes, the model highlights that their minimum habitable depth could be barely 1 centimeter for the yeast tested – very close to the surface – and around 1 meter for another microorganism widely studied in the Martian context, the Acidithiobacillus ferrooxydans. “The model gives us an approximation of the habitable conditions of Martian lakes,” explains Schiavo.
On the current Martian surface, the presence of jarosite, a mineral formed in liquid water with high acidity, is one of the signs that Mars once had lakes. Jezero Crater, explored by NASA’s Perseverance rover, was likely once home to a lake up to 30 meters deep. Jarosite also indicates that Martian lakes had considerable Fe+3 content, which acidifies the water.
Therefore, the study results strengthen the hypothesis that early Martian lakes were habitable, as they likely provided protection against ultraviolet rays.
For Dimas Zaia, professor at the State University of Londrina and researcher in prebiotic chemistry, the study demonstrates that microorganisms could have survived in the Martian environment despite the extreme radiation conditions that exist. “It is a relevant scientific support for the search for forms of life on Mars,” he assesses.
“Our research group has focused heavily on organisms resistant to ultraviolet radiation, because this source of stress is important in regions of Earth’s upper atmosphere and in extraterrestrial environments, such as Mars,” explains Fabio Rodrigues, director of AstroLab and co-author of the article.
In addition to USP researchers, scientists from the Federal University of São Carlos (UFScar) and the Universidade Paulista (Unip) also signed the work.
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