Gazing up into the night sky with the stars twinkling down, it is easy to wonder if we are alone in the universe. With all the stars and planets around us, it is possible that life similar to that on Earth exists – or existed. But how would we know?
Understanding how life could exist on other planets requires scientists from many disciplines working together. Astronomers determine the environmental conditions on potentially habitable planets. They can estimate the temperature, oxygen levels, and presence of water, all vital to life as we know it. The biologists (or astrobiologists) determine what types of life forms may live on the planets described by the astronomers. Often, they seek out extreme places on Earth similar to the predicted conditions on other planets called analog environments: the coldest, darkest, and most oxygen-depleted regions on the planet. Microbes, like bacteria, can thrive in these extreme conditions. Much of this research is focused on determining what organisms can live or could have lived on Mars, the closest planet to Earth..
Mars is 182.51 million miles (293.72 million km) away from Earth and experiences a much colder environment due to its distance from the sun. Mars also has more intense solar radiation since the atmosphere is thinner. Therefore, for life to thrive on Mars, the organisms must be adapted to cold temperatures and high radiation. However, astrobiologists hypothesize that lava tubes, caves formed by solidified lava are remnants of Mars’ volcanic history and may provide an ideal environment for microbial life. The lava tubes provide shelter and are protected from solar radiation. To test this theory, astrobiologists used an analog environment – the lava tubes of the Mauna Loa volcano in Hawaii.
C.B. Fishman and colleagues from Georgetown University described the microbes living in the Mauna Loa lava tubes using various DNA sequencing techniques. They used this information to infer what types of microbes may be able to survive in the Martian lava tubes.
The researchers collected microbes from lava tube walls from different light conditions, high light, dim light, and darkness. Scientist predict that These light gradients are also present in lava tubes on Mars. Interestingly, they found a clear spatial separation of the microbial communities meaning most microbes were only found in one place in the lava tube. Over 25% of the microbes identified were only found in one sample. This led to the conclusion that the microbial communities utilize different ecological niches – or each community is specialized for a particular environment. For example, one sample (C12_13) had 322 microbes not found in any other place, likely because the microbes within this community were highly specialized for the dark environment and the minerals present on the cave wall in that particular spot. Astrobiologists can guess what microbes might have existed on Mars by comparing the conditions in the Mauna Loa lava tube and predictions about the minerals and light in the Martian lava tube.
The microbial community analyses revealed distinct types of microbes in each environment and more microbes specialized for extreme environments in the dark samples. Furthermore, when the researchers attempted to identify the microbes, they discovered that 83.1% of the DNA had yet to be determined. This is called “microbial dark matter” and indicates the presence of rare microbial species or an understudied environment. If evidence of past or present microbial life is discovered on Mars, they may be taxonomically similar to the unidentified microbes in earthly microbial dark matter.
This research allows scientists to analyze the types of microbes growing in the lava tubes. With this baseline information from the microbial DNA, they can determine what the microbes eat, the ideal light, temperature, and oxygen conditions, and what types of lava rock they prefer to live on. Then, they can start to understand the limitations of life in lava tubes. All of this information informs astronomers designing equipment to search for extraterrestrial life so they can look for similar signals of life on other planets or moons.
Astronomers and microbiologists seem like an unlikely team. Astronomers study the biggest objects in the universe. Microbiologists study the smallest microscopic organisms. When scientists combine their knowledge about how life survives in extreme places on Earth with their extraterrestrial knowledge, they can find answers about whether earthly beings are alone in this universe.
Citation: Fishman, C. B., Bevilacqua, J. G., Hahn, A. S., Morgan-Lang, C., Wagner, N., Gadson, O., et al. (2023). Extreme Niche Partitioning and Microbial Dark Matter in a Mauna Loa Lava Tube. Journal of Geophysical Research: Planets, 128, e2022JE007283. https://doi.org/10.1029/2022JE007283