Descend 30 miles below ground or 100 miles beneath the ocean, and living conditions get extreme. In fact, even at room temperature, the pressures at such depths would compact a single bacterial cell and squeeze any exterior water to solid ice. Because of these hostile conditions, most researchers have concluded that only some exotic forms of life might survive at such depths and high pressures--about 16 thousand times sea level pressures. But a recent study published in Science magazine highlights what might reveal a large and subterranean biomass, even for common surface bacteria. These findings suggest looking for life deep underneath planetary bodies like the surface of Mars or Europa, where a habitable zone of high-pressure survivors might pool.
|Single cells of Shewanella oneidensis, shown adhering to a force microscope cantilever and their adherence to the metallic nutrients measured with a laser beam deflection change.
Credit: S. Lower, M. Hochella, R. Weaver, & M. Fortney / Virginia Tech
At the Geophysical Laboratory of the Carnegie Institution of Washington, the scientific team headed by Dr. Anurag Sharma, a geophysicist, and James Scott, a microbiologist, tested such biological limits for high-pressure life. The tools they chose to probe some common bacteria combined their expertise in both physics and biology. They put these bacterial cultures in a vice-like grip, exerting pressure equivalent to plumbing the depths of the Earth.
Using a vice-like, diamond anvil traditionally found in high-pressure physics and geology laboratories, the team clamped down on two particular bacterial species -- E. coli, a bacterium commonly found in the human gut, and the metal-reducing bacteria called Shewanella oneidensis. Shewanella is an important agent in the weathering of rocks, since the organisms can essentially respire 'on dust' by reducing metal components to derive their energy. As an alternative control, E. coli was studied as one of the most common and well-described microbes. By most measures, more scientific knowledge has been accumulated on the E. coli organism, (its biochemistry, genetics and metabolism) than any other single species on Earth.
Even on Earth, the question of a subterranean world turns out to be far from academic. Nearly half of the Earth's surface is considered to be deep sea (or 75 percent of the total ocean water, which covers more than 70 percent of the Earth's surface). Below 10 kilometers (6 miles), such deep water is often considered a biological wasteland because typical enzyme reactions begin to fail under extreme pressure. But while the exact estimates for subsurface biomass are difficult to gauge, by some accounts life on the surface is indeed the rarer, and not the more common, terrestrial biology at work. Sharma indicated that subsurface biomass is vast, with "estimates varying from 30 to more than 90 percent of the Earth surface diversity. This is only a speculative guess, since one has to consider much more restricted energy and environmental controls."
Living outside ordinary pressure limits?
According to Dr. Sharma, one feature of their approach was its holistic look at biological prerequisites for survival under extreme conditions. In previous work, "pressure limits were considered based on the limits to enzyme activity and cell membrane viability defined by biophysics studies. However, such studies were never done on whole organisms, until our study, which shows that pressure is not a limiting factor in the viability of life."
|E. coli, a common human gut bacteria.
But in the high-pressure laboratory, Sharma and Scott first tried to peg the room temperature survival of their bacterial cultures. Their first tests included staining both kinds of bacteria (which will highlight only those cells having an active metabolism and thus able to uptake a colored dye). A second test traced the biochemistry of how those surviving organisms might get their energy source. These studies showed survival but not necessarily reproduction or growth at high pressures. As a measure of the organisms' livelihood and resilience, the team plans further studies that will try to determine bacterial growth and replication rates.
Even in the absence of oxygen such resilient bacteria can shift their metabolism to radically different pathways. In the case of the E. coli and Shewanella particularly, the bacteria drive their metabolism towards an energy-conserving respiration. Such remarkable dormancy hinges on the presence of a chemical called formate. Formate is the simplest organic acid. For the Carnegie scientists, formate metabolism also proved valuable as a traceable indicator that indeed the bacteria remained viable at the highest pressures yet studied on whole cells.
Planetary challenges for plumbing the depths for life
One high priority to study next is whether the high-pressure organisms develop a stable population by selection or adaptation. According to Scott, "One of the fundamental questions that needs to be asked now is whether the response exhibited by the bacteria is due to adaptation or selection. Our results raise important questions about the impact of pressure on the evolution of life".
The answer to how such bacteria first got this survival trait will likely determine if a unique habitable zone can be found outside the laboratory on Earth or elsewhere in the solar system, and also whether such bacterial traits are common or appear only among their more exotic counterparts, the so-called extremophiles. In the simplest view of evolution, natural selection relies on survival of the fittest, where some percentage of the initial bacterial culture must have the capability to remain robust even at high pressure. But adaptation would likely rely on a more dynamic change within the population as a whole, where a different kind of survival pathway developed over generations to adjust to the inhospitable pressure.
|Subsurface oceanic probe of Europa, one of Jupiter's moons (Icepick, or the Europa Ocean Explorer development study) thought to have a liquid layer heated by tidal forces while orbiting eccentrically around Jupiter, much the way friction heats up a paperclip when bent repeatedly.
Credit: NASA JPL
How such pressures stack up on other planetary bodies remains unknown. Results from the Galileo spacecraft showed that Europa almost definitely has a layer of ice between 60-120 miles (100 and 200 km) thick, but at such depths, most density probes have been unable to differentiate a liquid or solid. Other than the Earth, Europa is currently considered the most likely (and only) candidate in the solar system for finding any liquid water (as much as 50 km or 30 miles deep). Of the 61 moons in the solar system only four others (Io, Ganymede, Titan and Triton) are known to have atmospheres.
Probing subsurface bacteria is a frontier that spans both biology and physics. The subsurface of a planetary body for instance, would likely still need a way to recycle or refresh its nutrient supply to the surface atmosphere and back. According to Sharma: "Eventually the metabolic components have to be recycled otherwise the whole system will eventually stall. Hence there needs to be at least some interaction with the atmosphere."
In the wild, some of the deepest terrestrial samples were collected from Antarctica, underneath Lake Vostok, where viable organisms were recovered from a depth of two and half miles (~4 km) and at below freezing (14 F) or -10 C temperatures. One school of thought first proposed by Thomas Gold of Cornell indicated that the frequent coexistence of helium and petroleum in deep drilling pointed to a viable ecosystem many miles below the Earth's surface. According to Gold, the concentration of such helium is a possible biological tracer or signature of subterranean ecosystems.
"Soon the only thing that should limit our investigation of the survivability of life on Earth and beyond is our imagination," concludes Scott.
For the Carnegie science team, the challenges and opportunities of using a diamond anvil cell, or DAC, as a kind of petri dish are only beginning to be explored actively. As Sharma notes, many experiments are planned or already in the works. For instance, "although there is no evidence for cell division at these extreme conditions, the cells have however, shown changes in shape, increased size, deformations etc.," said Sharma. "We are in the middle of completing experiments to determine whether there is any cell division ('growth') after the system has undergone decompression, which will answer quite a lot of important questions on implications of this study."
"I think another challenge will be to be able to take advantage of molecular biological tools and tags in the DAC. Labeling cells with RNA probes to test for protein synthesis and cell replication might pin down the growth question." E. coli particularly contains over 1,000 known soluble enzymes, many of which can be traced in the complex chains of its life cycle. "Also," notes Sharma, "the challenge of doing analyses with the cells after coming out of the DAC is still an important challenge, as there are many analyses that can not be done inside the diamond cell. How to quench cells without damaging them might be a challenge...At present we are working towards developing experimental techniques to answer such questions."