DNA miracle in ‘Martian’ Dry Valleys

Herald science reporter Jamie Morton is profiling a series of new studies taking place in Antarctica, before his return to the frozen continent next month. Today, he talks to the University of Waikato’s Dr Adele Williamson.

Antarctica’s McMurdo Dry Valleys are so hostile to life they’ve often been likened to conditions we might find on Mars. It’s even possible this brutal environment might eventually guide our search for life beyond Earth.

What scant moisture there is in this landscape — which, contrary to how we picture the frozen continent, is actually ice-free — gets blasted from the ground by some of the fiercest winds on the planet.

Temperatures plunge as low as minus 60C in the winter.

Only when they edge above 0C in summer, for as few as 25 days, is liquid water available through the melting of subsurface ice.

Seasonal swings cause their own problems. In only a day, the mercury can shift by up to 20C, between multiple, dramatic cycles of freezing and thawing. On top of that, the summer light brings damaging levels of UVA and UVB.

It defies all logic that any organisms survive here. But they do.

More remarkably, scientists have discovered a fascinating amount of diversity among the microbes that cling to life within the soils of this extreme polar desert. But, even for these hardy bacteria, living in the Dry Valleys comes at a cost.

The unforgiving conditions are very damaging to DNA, which stores the hereditary information needed to build and maintain any organism.

This means that, to exist, these microbes have to boast incredibly efficient repair systems.

Even so, conditions in the valley systems are so toxic to genetic material they can break strands of DNA completely — and the scant nutrients they can draw from the soil give them few options to repair the damage.

“I want to know if Dry Valley microbes have different DNA repair pathways compared to model organisms — and a sneak peek at the sequencing data we analysed have so far indicates that yes, in some cases, they do,” says the University of Waikato’s Dr Adele Williamson.

“Do these repair pathways use completely different enzymes? Are the enzymes faster and do they have unique ways of recognising or repairing damages? Do they work better at low temperatures?” she says.

“DNA repair generally involves cooperation of several enzymes in different steps: how do the individual components interact? How do they co-ordinate their activities?”

This question is especially important, because most of what we understand about bacterial DNA repair — and in fact about most cellular processes — comes from a few well-studied organisms that are easy to grow in a lab. But it is estimated fewer than 1 per cent of bacteria can be cultured this way.

“What we know so far about the microbiota inhabiting in the Dry Valleys is they have very little similarity at the DNA level to these model organisms and very few can be grown in the laboratory,” Williamson says.

“So, this is an opportunity to take a direct look at a part of life on our planet that we have never seen before.” And perhaps that life can tell us about other planets.

“As well as increasing our understanding of the organisms inhabiting the harsh yet fragile Antarctic ecosystem, the Dry Valleys are considered a good model of the Martian climate,” she says.

“Studying the molecular mechanisms that enable its inhabitants to survive can give clues to how extraterrestrial life could look.”

Since many of the Dry Valley microbes won’t grow under lab conditions Williamson describes as comparatively “cushy”, she’s drawing on a metagenomics approach to discover their DNA repair genes.

“Briefly, for a particular sample site, the aggregate DNA of all organisms within the soil sample will be sequenced, and I will use computer algorithms to predict which DNA sequences encode proteins involved in repair processes.”

Once she has a reasonable idea of how these repair proteins function, she will transplant a group of genes from one of the repair pathways into E. coli to test whether this imparts DNA-damage resistance — or whether she can effectively change this mesophilic laboratory “pet” into an extremophile.

Her ultimate hope is to glean more insights into how these hardy bugs survive where most life can’t.

“In particular, I want to understand how specific differences between the protein complements of these bacteria and model organisms endow them with such resilience.

“We would also like to understand novel DNA repair processes from the enzyme’s point of view: Are there different routes to repairing the same damages from what we already know? Are there different enzyme structures that can be used to do this?”

● Williamson’s study is supported by a $300,000 grant from the Marsden Fund.

[The study] can give clues to how extraterrestrial life could look. Adele Williamson (above)