The Sun was once thought to provide energy for all life on Earth - meaning that life could not survive without it. In the 20th century, as astrobiologists began to explore the Earth's most remote and harsh environments, scientists began to question that assumption.

We now know that numerous microorganisms are able to obtain the energy they need for life through chemical reactions that do not involve sunlight. These incredible organisms can be found in many environments - from sediments below the dark ocean floor to microscopic pockets of water inside solid rock.

Many questions about these microbial ecosystems remain. How do microbes get into the deep subsurface in the first place? Are communities capable of growing, or do they just sort of sit there in the rock recycling nutrients and carbon from dead cells? How much of the deep biosphere is actually living, and how much of it is just dead matter trapped in the slow, grinding motion of our planet's geology?

Field studies have revealed that subsurface microorganisms can and do live active lives, even when buried kilometers under the surface. But we're still not entirely sure how large the living subsurface biosphere is, how deep it actually goes, and how it originated.

Previous Studies: A Community Harvest Organisms in the deep subsurface can be identified by simply digging up samples, sticking them under a microscope, and then seeing what's there. The problem is, even though microbes might be present, it's sometimes hard to tell if they're active - or how they behave in their native environment.

Previously, scientists have tried to define the depth limit for life based on environmental constraints like temperature. In general, the environment gets hotter and hotter as you get closer and closer to the Earth's core. Life simply cannot survive when it gets too hot. However, it's hard to tell just how close to that boundary a living community can get.

"The reality is that in order to live at high temperatures, you are forced to replace your proteins very frequently," said Tullis Onstott, a geoscience professor at Princeton University. "If you do not have enough metabolic energy to support that replacement then you, as an individual cell, cannot live."

High temperature environments can be challenging for life. Cellular components break down at an increased rate. If a cell cannot actively repair the damage, the conditions quickly take a turn toward the uninhabitable. Proteins stop working, causing metabolism to grind to a halt. Cell membranes, cell walls and DNA also begin to deteriorate. So it's not temperature alone that affects habitability, it also comes down to an organism's ability to repair the damage that high temperatures cause.

"You will die at a lower temperature even though under energy and nutrient-rich situations you can live at higher temperatures and to much greater depths," said Onstott. "The most important constraint that this places on deep life is its abundance as a function of depth."

The depth and abundance of living organisms in Earth's subsurface depends on how active they are, and how quickly they can repair and reproduce. This is a question of resources and energy. Previous studies have often focused on the resource part of the question - specifically the resource of organic carbon.

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Microbes, How Low Can You Go?