Life thrives in the deepest, darkest recesses of Earth’s crust—from methane seeps in the ocean floor to the highest reaches of Arctic permafrost—and it is unlike anything seen on the surface. Intraterrestrials shares what scientists are learning about these strange types of microbial life—and how research expeditions to some of the most extreme locales on the planet are broadening our understanding of what life is and how its earliest forms may have evolved.
What inspired your interest in studying subsurface life?
Karen G. Lloyd: I have always been drawn to things that are mysterious yet real. In the past 25 years or so, we’ve discovered that there are more living cells inside Earth’s crust than there are stars in the universe. And many of them are on deep branches of the tree of life that were completely unknown to science until a few years ago. The idea that there are tiny living beings that are all around us, but hard to discover without the use of specialized techniques, is an idea that grabbed me and won’t let go. The best part about this unseen life is that it’s findable, for people who spend the time and energy looking for it. While I was a PhD student in Marine Sciences, I decided to be one of those people. We’re finding that microbial life is present all throughout Earth’s crust, and it’s fundamentally different than the life we’re familiar with at the surface.
Does studying subsurface life require you to go all over the world to remote places to find it?
KGL: Many of these organisms are on deep lineages on the tree of life, and they have lifestyles that are hard for us to imagine, because we can’t just extrapolate from what we know from our daily lives. This means we need to go to where these organisms are, deep inside Earth’s crust, to study at them “in the wild”. We do this by drilling down to them with giant drills (often through the ocean depths), diving down in deep-sea submersibles, or by going across the Earth in search of natural deeply-sourced springs that bring them up to us. This has taken me to the bottom of the ocean, high altitude volcanoes, and frozen soils at the northernmost Arctic land in the world. It is often very challenging to go to these places, but there are very many of us around the world who work together to do this exciting work. It’s a great way to make a living!
What makes life in the subsurface so different from the types of life we’re familiar with up at the surface?
KGL: Unlike us, their lives are not dictated by sunlight and plants. Instead, the primary producers of many of these subsurface ecosystems get energy from chemical reactions and use this energy to make biomass. This means that much of this vast ecosystem functions with little to no inputs from the surface world. They show us that our normal constraints for life (a dependency on oxygen and light, for example) are not absolute limitations for all of life. These organisms break down life into its most basic thermodynamic components—it’s all about helping chemicals to react and keeping the energy from it. There are very many chemical reactions that are there for the taking—involving respiring metals that would be toxic to most the life we’re familiar with, or even working with pure elections, with life acting more like an electrical cord than something we would recognize as life. The intraterrestrials get energy by respiring almost every element on the periodic table, not just oxygen. And they use energy in miniscule amounts that would be far too small to keep any of our human cells alive. This low energy living means that they can potentially have very long lifespans. Many of them appear to sit in a long-term stasis state for thousands of years or longer, where they repair broken cellular parts without dividing into daughter cells. This presents a fundamental challenge—how does life evolve to basically do nothing for thousands of years, or even hundreds of thousands of years? Part of my job as a scientist is to come up with scenarios for how this could be an evolutionary advantage.
Thermodynamics seems like something that belongs in the field of physics. Why is it such a focus of yours?
KGL: The intraterrestrials show us that when you break life down into its most essential needs, the laws of thermodynamics become more important than anything we might assume are important, like oxygen or sunlight. In particular, I’m interested in how life interacts with the second law of thermodynamics—the one that says entropy must always increase. Even though entropy is often associated with disorder, it’s not always disorder. Sometimes more entropy is created when you build an orderly machine, like a living cell, that creates more heat. I think the intraterrestrials could represent an extreme end-member for life, where they fulfill the second law of thermodynamics by stretching out entropy production over longer and longer time periods.
Why does subsurface life matter for us?
KGL: Currently, companies are working on inventing the industry of deep-sea mining. This would destroy many of these sub-ocean habitats before we even learn about them. We need to make sure they’re protected with proper regulation if those industries move forward. One of the reasons we need these ecosystems to stick around is that they can potentially help us with deep underground carbon storage. By and large, climate change is driven by the fact that we’ve pulled large quantities of carbon out of the subsurface and released it into our atmosphere as carbon dioxide and methane. By capturing some of this and placing it back underground, we can possibly lessen some of the effects of climate change. Understanding what subsurface life will do to this sequestered carbon when we put it under ground or in the deep sea is critical to the success of these methods.
About the Author
Karen G. Lloyd is the Wrigley Chair in Environmental Studies and Professor of Earth Sciences at the University of Southern California. Her work has appeared in leading publications such as Nature and Science.