Planetary Protection of Planet Mars – Potential risks of contamination by sulphate-reducing organisms from Earth?

Mars, commonly referred to as the Red Planet due to the red color of iron oxides present in Martian regolith, is considered the most Earth-like planet in our solar system and has fascinated humanity from for centuries. One of the key questions in Mars exploration is whether life exists or has existed on the planet. To answer this question, evidence for present or past microbial life on Mars is being sought. With the increasing number of research missions reaching Mars, planetary protection has gained significant importance to prevent contamination by terrestrial microorganisms. Endospore-forming sulfate-reducing bacteria of the family Peptococcaceae represent a group of obligate anaerobic organisms that can be found on Earth under extreme conditions in various subsurface environments including freshwater and marine sediments, mines, oil reservoirs, and aquifers at depths of up to 3,000 meters. In addition to their ability to form highly resistant endospores, they are capable of chemolithoautotrophic growth using sulfate, H₂, and CO₂. This characteristic, combined with their adaptability to extreme conditions, theoretically positions them as potential candidates to become stowaways to Mars and survive the harsh environment in the form of spores.

In this study, two representatives of the Peptococcaceae family were investigated regarding their ability for autotrophic growth in the presence of Martian regolith and their tolerance to desiccation and radiation as present on Mars. The experiments demonstrate that the two species, Desulforamulus putei TH-11^T and Desulfosporosinus lacus STP12^T, can easily grow autotrophically in an H₂/CO₂ atmosphere using the sulfate present in the Mars regolith simulant MGS-1S as an electron acceptor. Since CO₂ is part of the Martian atmosphere, water could be present occasionally and H₂ could be formed through serpentinization of iron oxides where there might be a possibility that endospores who reach Mars could germinate and grow. Furthermore, we demonstrate that spores formed during growth on MGS-1S showed high resistance to an exposure to elevated temperatures, oxygen, polychromatic UV radiation and desiccation and could potentially survive dormant on Mars and may reach Mars as highly resistant contaminants of space probes. The results provide a first hint to the potential risk of contaminating Mars by spore-forming sulphate-reducing bacteria regarding topics like planetary protection and search for possible life on Mars.