I just learned that microbes can hitch a ride on dust particles, getting lifted and transported long distances by strong winds. This discovery, published by Armando Azua-Bustos et al. in Scientific Reports last week, ticks all the boxes on my list of interests. Naturally, I asked Armando if he would be willing to chat with me about it.
Armando Azua-Bustos, PhD, is an astrobiologist working for Spain’s Center of Astrobiology in Madrid. On the bio side, he studies the evolution of life on Earth, especially how microbial life adapts to harsh environments. On the astro side, he uses what he learns to develop evidence-based theories about the evolution of life on other planets and to decide upon best approaches to determine whether or not other planets have or had life. In particular, he is trying to determine how life might have evolved on Mars and how to test for it.
One Earth analog for Mars is the otherworldly Atacama Desert in Chile. Some parts of the Atacama have 0% humidity, making it the driest place on Earth and drier than the surface of Mars. Even so, life will find a way (thanks for that lesson Michael Crichton). Buried in the soil of the driest region, María Elena South, bacteria thrives.
Azua-Bustos’s story is fascinating. He is Chilean, born in a small nitrate mining town in the hyperarid valley of the Atacama desert. His family was drawn there by incentives offered to miners: a salary plus free housing, health benefits, schools, and all utilities paid. When that business model became financially unsustainable and the incentives were removed, many miners decided to leave for greener pastures, so to speak. When he was just a year old, Azua-Bustos’s family moved closer to the Andes Mountains so his father could work at a copper mine.
Though he moved away from the hyperarid core, Azua-Bustos spent nearly every weekend of his teenage years adventuring in the desert with his telescope, searching for fossils, Inca roads, insects, and plants. He described this formative time as giving him an advantage in his later studies in the desert, because he knew exactly where to go and why. For example, in this TED talk he explains how he knew better than NASA where the driest part of the Atacama desert was.
When asked whether this level of curiosity and exploration was common among his high school peers, he said no. “I was the only one with a passion for science as a teenager. I don’t remember anyone else. There was someone I knew whose dad was interested in science and I had many wonderful interactions with him, many science talks, but that would be it. I was the only one.”
So much for my theory that an environment as awe inspiring as the Atacama Desert is a breeding ground for scientific inquiry. I was projecting myself onto the people who live there, imagining what it must be like to grow up with the milky way as your night light. Of course to me that is a novelty. I guess the clear, expansive view of the stars afforded by dry cloudless skies and high altitude is like anything else. We get desensitized quickly. Most people are not driven to investigate what is familiar. Azua-Bustos seems to be in a rare minority who are able to feel a sense of awe in his or her own backyard.
“I can remember walking in the Atacama on a moonless night and seeing my shadow produced by the light of the stars, a star shadow. That is amazing, and I don’t think there are many places in the world you can see that; a star shadow.” Armando Azua-Bustos, interview with Nerina Finetto for Traces.Dreams.
**Edit to original, September 6th, 2019: Now that I have been to San Pedro de Atacama, I think my theory is closer to the truth than not. If I were to use one word to define the people I have met here it is curious. They may not be doing experiments, but they are driven to explore and understand.
In 2015, Azua-Bustos established María Elena South as the driest place in the Atacama Desert, and he discovered multiple species of bacteria living in the soil there (published in Environmental Microbiology Reports). On average, the entire desert receives less than an inch of rainfall over the course of a year. In some places, like María Elena South, it typically receives none. How can life survive here, without water? And how did it get here in the first place? The former is still under investigation. Wind may be the answer to the latter.
The shaping of land by wind is called an Aeolian process. Aeolian processes are a major mover in deserts, where strong wind can lift dry topsoil (see a longer discussion of Aeolian processes and one of my favorite geology quotes about same in my previous post, Mountains and Minds).
The Atacama Desert lies between two mountain ranges, the Chilean Coastal Range and the Andes. These mountains capture almost all of the incoming moisture, leaving the Atacama in what is known as a rain shadow (a double rain shadow, in this case). In Azua-Bustos et al. 2019, they used earth.nullschool.net/ and their own measurements and models to understand how wind moves through the desert (you should definitely play around with these wind flow maps, so fun!).
earth :: a global map of wind, weather, and ocean conditions
See current wind, weather, ocean, and pollution conditions, as forecast by supercomputers, on an interactive animated…
Almost every afternoon, a strong eastward wind blows from the Pacific Ocean into the desert. Could microbe-coated dust travel inland on the wind? To find out, Azua-Bustos chose six sampling sites along two transects to place collection plates. The northern transect goes inland for 63 km from Iquique, the southern transect goes inland for 50 km from Tocopilla. The collection plates were simple enough. 50 petri dishes were placed at each of the six locations: 40 with agar plus growth medium, and 10 empty plates. For just one hour, the lids were removed from the plates. I asked if they tilted the open plates toward the wind and Azua-Bustos said no. They laid them flat on the ground so they would only collect dust that settles to the surface. Once the hour was up, the plates were sealed shut and analyzed for microbial growth 2 weeks later.
Azua-Bustos said he wasn’t expecting to see anything growing on the cultured collection plates. It was just a simple test of a wild idea. He was surprised to isolate a number of bacterial and fungal species from each location. In the Iquique transect, he found 5 bacterial and 4 fungal species. At the Tocopilla transect, he found 18 bacterial and 4 fungal species. The bacteria tended to come from relatively nearby, with collection plates from coastal sampling sites picking up ocean-based bacteria, and plates from inland sampling sites picking up bacteria likely to be found on mountain ridges. Some of the bacteria they found use spores to disperse themselves, so free flying spores may be blown inland by the wind. Other bacteria must have hitched a ride on dust.
To test this, dust collected on empty, sterile plates was subjected to DNA extraction and 16S ribosomal RNA sequencing to identify bacterial species present. Present they were. They found an even higher number of bacterial species using this approach. Azua-Bustos is now using scanning electron microscopy to image microbes attached to dust. I can’t wait to see that.
“The analysis of dust particles collected across the hyperarid core of the Atacama shows that microbial life is able to efficiently move across the driest and most UV irradiated desert on Earth unharmed. … The main driver for the transport of microbial life in this desert is the presence and speed of winds.” From Aeolian transport of viable microbial life across the Atacama Desert, Chile: Implications for Mars (Scientific Reports, 2019).
The big question now is, what happens to these microbes after they arrive in the hyperarid desert? Do some of them eventually adapt to the arid environment and thrive there? The bacteria found by Azua-Bustos in 2015 had to come from somewhere. Aeolian transport may be the answer.
I would assume that most microbes arriving in the desert die. Occasionally, they may happen to fall upon a patch of earth where they are shielded just enough from environmental stressors for the most resilient among them to cling to life. The survivors could then multiply and further adapt to their new home. But that is just a guess. This is an area of active research by Azua-Bustos and colleagues.
The finding that wind can transport microbes has big implications for life on Mars, where dust storms can cover the entire surface of the planet. Perhaps wind has played a similar role as the main driver for the transport of nascent microbial life on Mars. Hitching a ride on dust may have enabled microbes to disperse widely, then settle into and evolve within a variety of environmental niches.
“As it has been argued that the lack of continuously hospitable conditions would have deterred a continuous biological evolution on Mars, our results offer a way to sort out this problem, as the lack of inter-connectivity between dispersed habitable environments by ancient water flow or tectonics may have been balanced by wind dispersal of microorganisms during extended periods of time. As a consequence, the aeolian distribution of life may have allowed some degree of evolution of microbial Martian life forms.” From Aeolian transport of viable microbial life across the Atacama Desert, Chile: Implications for Mars (Scientific Reports, 2019).
From there our conversation veered into speculation. Could our space probes and human explorations be seeding other planets with earth microbes? How do human bodies adapt to life in the desert? Might bacteria and people form symbiotic relationships to survive in arid conditions? I think I will keep the answers in mind for future science fiction writing, but these would be interesting questions to study scientifically.
In closing, I told Armando that I am traveling to the Atacama Desert next week and asked if he had any advice. I was hoping for some sort of inside scoop on where to go or who to talk to, but he said simply, drink water continuously and wear a lot of sunscreen. Sensible advice, as I am like a coastal bacterium floating in on the wind.