Researchers associated with the Astrobiology Laboratory at 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 Martian surface, more than 3 billion years ago.
The study was published last month in the journal Astrobiology and developed within the scope of two projects financed by Fapesp, one regular and one thematic. The researchers explored 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 is especially 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 radiation in the ancient lakes of Mars.
Scientific literature has already detailed the protective capacity of the iron present in the regolith that covers the current dry surface of Mars. “And research has also examined the ultraviolet protection provided by iron in aqueous solutions, but using very complex modeling, difficult to apply”, explains chemical engineer Gabriel Gonçalves Silva, researcher at AstroLab.
First author of the article, Silva clarifies that these previous models did not allow detailed estimates of the viability of microorganisms with fine adjustments for radiation levels and ion concentrations. “We wanted a simple model that served these purposes,” 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 and supports high acidity – so much so that it passes through gastric juice when applied in digestive health treatments.
Experiments have shown that even a relatively low concentration of iron ions is able to protect yeast from ultraviolet radiation near the surface. Microorganisms were able to survive ultraviolet radiation long enough for their reproduction rate to compensate for the deaths caused by radiation. “In this regard, it is not only how much iron protects, but also how long it takes for microorganisms to reproduce and keep the population stable,” highlights Ana Paula Schiavo, postdoctoral fellow at the USP Chemistry Institute and co-author of the study.
To test the validity of the model, the researchers compared their predictions of microorganism viability with observations of the yeast’s survival rate in experiments. Applied to simulations of Martian lakes, the model points out that their minimum habitable depth may have been just 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 ferrooxidans. “The model gives us an approximation of the habitability conditions of Martian lakes,” says 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 in the remote past. Jezero Crater, explored by NASA’s Perseverance rover, likely once housed a lake up to 30 meters deep. Jarosite also indicates that Martian lakes had a considerable content of Fe+3, which acidifies the water.
Therefore, the results of the study reinforce the hypothesis that primitive Martian lakes were habitable, as they probably provided protection against ultraviolet radiation.
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 relevant scientific support for the search for life forms on Mars”, he assesses.
“Our research group has focused heavily on organisms resistant to ultraviolet radiation, as this source of stress is important in regions of Earth’s high atmosphere and in extraterrestrial environments, such as Mars”, explains Fabio Rodrigues, director of AstroLab and co-author of the article.
In addition to researchers from USP, scientists from the Federal University of São Carlos (UFScar) and Universidade Paulista (Unip) also signed the work.
