In December 2016, Sarah Stewart Johnson flew from McMurdo Station, the US scientific outpost on the edge of Antarctica, to the shore of Lake Vanda in the middle of the Dry Valleys – the driest desert in the world. Johnson, assistant professor of planetary science at Georgetown University in Washington, DC, was looking for ancient microbes: simple creatures that had once lived and thrived in this remote region. But this wasn’t just a collecting expedition. Johnson and her team were searching for the secret of life itself.
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“Antarctica is the perfect place to investigate some of the fundamental questions about the persistence of life,” says Johnson. “For me, that was really the big draw – to go to one of the most marginal places on Earth and find out, when microbes are really pushed to their limit, how they survive.” By answering that question, she hoped to unlock a new understanding of life, both on Earth and perhaps elsewhere.
To call Lake Vanda remote is an understatement. From the nearest city, Christchurch in New Zealand, it is a seven-hour flight, one-hour drive and 45-minute helicopter trip. It sits surrounded by mountains, which in turn block the encroaching glaciers from moving through the valley. Winds, reaching a sprightly 320kph, are heated by friction from the ground as they pass over, melting, evaporating and blowing away most moisture. This leaves just gravel and a lake so shrunken that it has ten times the salinity of regular seawater. Even so, it’s covered by ice that’s four metres thick for much of the year.
It wasn’t always like this. Three thousand years ago, the lake was much deeper, reaching higher up the valley and teeming with microbes. As it receded, the lakebed sediment containing those microbes was exposed to the air. The mat of microbes dried up and over time became covered in a centimetre of windblown gravel. Not a good place to be, you’d think, for a microbe used to being in a lake. Yet, when Johnson’s team looked, the mat was still very much intact, as if it had been healing itself over those thousands of dry years. The cells had survived over an enormous amount of time. But the question was, how?
One theory suggested that the cells entered a state of dormancy. As Johnson puts it, “They hunker down and shut down all metabolic activity, and basically just wait until conditions return to a more clement state.” But if that was the case, over time the environment would have taken its toll and the cells would have been damaged, eventually beyond repair. That hadn’t happened. So Johnson began to look at another hypothesis, that the cells had stayed metabolically alive at a very low level and continued to repair themselves.
“One of the questions we wanted to ask was: over what timescale should we see dormancy as favoured, versus something maintaining a very low level of metabolic activity, at least enough to repair your genome so it doesn’t get damaged beyond repair?” Johnson says. But to do that required an understanding of the genetics behind this ability to stay “just alive”. What in the DNA of these microbes made them so tough? Previously that would have been impossible.
For the first time, the researchers on the ice were equipped with pocket-sized DNA sequencers, the MinION MK 1B made by UK company Oxford Nanopore, to sample and sequence the microbes right there on the ice, sending back not samples, but data. For something to survive in such a harsh environment means that the microbes have evolved to produce biochemicals within that are able to provide protection. These chemicals are called secondary metabolites and are very useful for humanity. One group forms the basis of modern antibiotics, for example.
The DNA sequencing of organisms that can be found thriving in harsh places allows scientists to not only add to the great catalogue of living things, but also to the catalogue of genes known to produce useful secondary metabolites.
As well as this, sequencing species in situ is important because, in many ways, it enables the re-emergence of old-school adventure biology. Where Victorian botanists would travel to, say, the eastern Himalayas to bring back rare orchids, today’s geneticists are now travelling to extreme locations to bring back genes.
But Johnson, as Georgetown’s assistant professor of planetary science, has another reason to study microbes that can exist in harsh environments – something even more adventurous than expeditions in search of extreme genetics: life on Mars. “Microbes are the big story in Antarctica,” she explains. “This is the most Mars-like place on Earth.”
This is important because there’s a chance that microbes that we find on Earth might have once been resident on Mars. At a time when the Red Planet had running water, rocks from both planets hit each other as a result of asteroid impacts. As we can be sure that the rock traffic went in both directions, it’s possible that microbial life went with it.
On Mars, life will need to have survived even after the planet’s water evaporated or froze. In Antarctica (and for a much shorter time period), that might be possible. Perhaps buried underneath some Martian gravel, there’s something similar, waiting to be found.