High pressure on the ocean floors of worlds like Enceladus could put a damper on DNA replication, suggests a new study — but that may not be bad news for alien life.
Chemicals that play a key role in copying DNA don’t perform well under pressure, according to a recent study. Here on Earth, the deepest of deep-sea life (creatures like the adorable dumbo octopus) lives under about 8,800 pounds per square inch (PSI) of water pressure. But at the bottom of the ocean on Europa, an icy moon orbiting Jupiter, alien sea life, if it exists, would live under the relentless weight of a 60-to-120-mile-deep ocean, to the tune of about 19,000 to 38,000 PSI.
“Deep-sea organisms have to cope with the effects of high hydrostatic pressure at the molecular level,” writes University of Grenoble Alpes biologist Lorenzo Carre and his colleagues in their recent paper, published in the journal Astrobiology. Under that kind of pressure, some proteins — the molecules that DNA actually “codes” for — get squished out of shape.
Carre and his colleagues wanted to find out if conditions in a world like Europa or Enceladus might be a problem for DNA polymerase, the enzyme that copies DNA. The answer, it turns out, is complicated.
What’s new — Carre and his colleagues took five different DNA polymerase enzymes from deep-sea hydrothermal vent microbes (archaeobacteria with evocative names like Pyrococcus abyssi, Pyrococcus furiosa, and Thermus aquaticus). The scientists put the enzymes in a pressure chamber with some DNA and measured how quickly they were able to make copies of the genetic material.
Here’s a quick biology refresher: if a living thing wants to grow or reproduce, it needs to make more cells. That process usually involves doubling up the contents of an existing cell, then splitting it into two identical cells. Repeat until you have a bustling colony of archaeobacteria. It’s up to DNA polymerase to copy the DNA so each cell has a complete genome.
At pressures above 1,450 PSI, DNA replication started to slow down a bit. Around 7,250 PSI, Carre and his colleagues noticed a sharp decrease in productivity: the enzymes were copying much less DNA. When they cranked the dial on the pressure chamber all the way up to 14,500 PSI, DNA polymerase almost completely stopped working. (Almost will turn out to be a keyword here; the enzymes were slowly but determinedly chugging along at about one or two percent of their usual rate.)
Those results are a strong hint that — assuming molecular biology works similarly on the seafloor of Enceladus as here on Earth — life might have a tough time copying genetic material to pass on to new cells, which is a pretty basic bit of biology.
But the news isn’t actually that bleak, according to Louisiana State University biologist Vincent LiCata, who commented on the recent study. LiCata studies how enzymes cope with extremely salty environments, as well as extreme temperatures.
“I think it actually tells us that we're more likely to find Earth-like DNA polymerases in those oceans,” LiCata tells Inverse.
Why it matters — The icy moons of Jupiter and Saturn are probably our best shot at finding life elsewhere in our Solar System. Thinking about the kinds of environments we might find there, and how life might adapt, can help scientists figure out where and how to search for life in those alien seas.
Astrobiologists often use our familiar Earth oceans as a starting point. The most popular idea at the moment is that life might evolve around deep-sea hydrothermal vents on a world like Europa or Enceladus, much as it did here on Earth. Hydrothermal vents provide a source of energy and nutrients for complex, thriving ecosystems in the sunless depths, so they seem like a logical place to look for life in an ocean world whose ocean is mostly buried beneath several miles of ice.
But when we’re thinking about what Europan or Encedalusian life might look like, it’s important to remember that both of those distant oceans are colder and much, much deeper than Earth’s oceans. They may also have a different mix of salts and other chemicals. In other words, these are alien oceans we’re talking about.
Digging into the details — The thing that gives LiCata hope is this: even under pressure twice what they evolved to handle, the DNA polymerases still worked, albeit with just a tiny fraction of their usual oomph.
“It’s maybe one or two percent, but it's still there,” says LiCata. And it’s easier than it sounds for microbes to adapt their enzymes to work at double, or even triple, the pressure.
“That’s not very far to evolve,” says LiCata. “They just need to be more flexible.”
He means that literally; proteins like the ones that make up DNA polymerases and other enzymes are made up of a bunch of squiggly shapes. Under pressure, they either get squashed of shape so they don’t work correctly, or they just become very stiff, so they also don’t work correctly.
“Certain amino acids and certain combinations of amino acids in sequence create more rigid backbones, and other ones create more flexible backbones. And so if you start to bias toward the ones that have more flexibility, you could start to evolve to adapt to pressure,” says LiCata.
But here on Earth, there has been — as evolutionary biologists would say — no selective pressure for those kinds of adaptations. The DNA polymerases Carre and his colleagues tested have never, in the entire history of life on Earth, needed to operate at 14,500 PSI, let alone 38,000 PSI. With the right evolutionary incentive, though, life (uh) finds a way.
“If we started growing these bacteria under increased pressures, it’s possible that we could force evolve even these polymerases to operate under higher pressures,” says LiCata.
The temperature may also be important. The team tested the enzymes at a balmy 130 to 150 degrees Fahrenheit, but they suggest that higher temperatures may make the DNA polymerase molecules, and the DNA they’re working on, more flexible. That means that in the nearly boiling waters around a deep-sea hydrothermal vent, the process of DNA copying may go pretty smoothly, even under extreme pressure.
Only two things will tell us that for sure: more experiments, or eventual data from the ocean of an icy moon.
