Life underground, meet the intraterrestrials

 By Mathew Goldstein

Karen Lloyd is an Associate Professor, University of Tennessee in the Department of Microbiology.  Yet her recent book, Intraterrestrials: Discovering the Strangest Life on Earth, currently ranks higher on Amazon’s bookstore under the category physics of entropy than under the categories microbiology or ecology. The internet site of her deep subsurface biosphere lab provides a hint on how her work intersects with science beyond biology, including physics. It describes the lab’s focus as follows: “We work on determining the carbon and energy sources for the vast uncharacterized majority of subsurface microorganisms in hydrothermal vents/springs, cold methane seeps, deep oceanic sediments, coastal estuaries and bays, and subduction zones.” Following is my synopsis of the contents of this book.

Life is more varied and more plentiful then we previously realized. The current tree of life on earth is far from complete because of the unknown number of single celled organisms yet to be discovered living underground. Even with the latest technology that is enabling intrepid researchers to identify these organism for the first time, identifying underground microbes is still a difficult and time consuming task. Microbes are found in the deepest mines (5 kilometers below sea level) and in a wide variety of conditions including high pressure, acidic, salty, hot, cold, radioactive, etc.

Multicellular life relies on mitochondria to extract energy by transferring electrons from sugar to oxygen. When oxygen gains an electron it is “reduced” and when sugar loses an electron it is “oxidized”.  This “redox” reaction, which in a biological context is called respiration, produces enough energy to support multicellular life. But redox reactions are not limited to using oxygen for reduction and sugar for oxidization. Aerobic oxygen breathers are an atypical, late arriving, minority in the menagerie of life organisms.

Although some bacteria are aerobic, unicellular organisms are often anaerobic. It now appears that almost any redox reaction that is feasible is utilized by some type of life to obtain energy. At least 20 of the elements in the periodic table of elements are known to be respired by some microbe, including iron, manganese, selenium, vanadium, chromium, arsenic, uranium, and gold. These other redox reactions are less energetic than oxygen respiration which is probably why they occur only in single celled organisms. 

Underground life that respires elements found in rocks survive in a relatively dormant condition, with very slow metabolism because they rely on low energy redox chemistry. The logical implication is that individual microbes live a very long time, biding their time on geological timeframes until they experience a change to their local environment that enables them to obtain the additional energy needed to reproduce. Under what circumstances these ultra-long lived microbes, referred to as “aeonophiles”, reproduce is one of the details of their lifecycle and evolutionary history that is unknown. Their DNA is unique, they belong to new phyla, some of them do not even appear to fit comfortably within the existing three domains of archaea, bacteria, or eukoryates.

Life maintains dissipative structures that provide more opportunity for entropy production. Dissipative structures dissipate energy while maintaining a low entropy, and thus ordered, state. A chemical reaction is said to be autocatalytic if one of the reaction products is also a catalyst for the same reaction. In far from equilibrium autocatalytic contexts, entropy production, somewhat paradoxically, creates locally low entropy dissipative structures. A virus can be considered a type of a dissipative structure because it exchanges energy and matter with its environment when it infects a host cell and replicates.

Non-life features similar entropy creating processes that are locally ordered. A key difference is that life organizes itself around self-maintaining the non-equilibrium autocatalytic conditions that sustains the dissipative structures. To enable this, life encases its dissipative structures inside a membrane. The book provides the following example of the role of the membrane “All living cells continually push protons outside of their membranes. This buildup of protons outside the cell leads to an imbalance in chemical concentration that drives protons back into the cell to even it out again. This chemical/electrical pressure is called the proton motive force. It’s called “motive” because when those protons rush back into the cell, they kick a flywheel of a protein called ATP synthase that makes ATP for us. ATP provides the energy we use whenever we need to make something happen inside our cells…”

Life tends to spread entropy production over longer timescales than non-life. The second law of thermodynamics says entropy increases overall. It says nothing about the rate of increase or localized entropy decrease. From this perspective the phenomena of life exists on a continuum with non-life.