In certain sub-marine sediments, the divorce rate of the average microbe is only one in 1000 years. Such a rate does not seem to be at all compatible with evolution, but the existence of cells recovered long below the ocean floor continues to challenge biologists to broaden the definition of a “lifetime.”
This has been argued by microbial biogeochemist Karen G. Lloyd who says that organisms inhabiting the crust of the planet, or more properly, existing within its crust, who are known as “intraterrestrials” perceive the planet at a pace much slower than daylight or season. These microbes are buried so deep that sunlight never reaches them, and in an almost inert state which can take hundreds of thousands or millions of years, geology rewires their world in massive silence. Plate motion, formation of fractures and long delayed changes in fluid flow are the rhythms that count in that framing.
The question of whether deep life exists is a hard question, but how it remains available in the face of food scarcity and infrequent reproduction is a harder question. In low energy sediments microbes seem to survive by slowing metabolism to the maintenance levels instead of competing to grow. Research in ocean drilling has summarized work in the counting of cells, metabolic constraints to demonstrate how extreme that slow motion can be: community scale rates would imply generation times of centuries to millennia with cells perhaps much of their lifetime repairing molecules and preserving energy instead of making biomass. That is a practical mystery still: what is the source of power that keeps the lights going when organic matter is buried and no longer available?
Chemical energy as a result of natural radioactivity is one of the responses. Radiation may also be used to break the water molecules in crustal rocks and in sediments to produce hydrogen that microbes may utilize as energy in a process known as radiolysis. Experiments and modeling have demonstrated that sediments can enhance radiolytic hydrogen production many times, up to 30 times in a few laboratory systems, and make specific minerals effective radiation-to-chemical energy converters. That slow trickle may even become the primary energy source in very old marine sediments, although that may amount to a minor portion of the world energy of sediment.
The menu is also expanded with radiolysis other than hydrogen. Radiolytic reactions associated with abnormally high concentrations of small organic molecules, such as acetate and formate, in deep fractures waters in Canadian mine systems have been linked to isotopic signatures of abiotic origin. To microbes that survive in a chronic scarcity, those compounds may serve as pre-made sources of carbon and energy-starter molecules, which are able to support a community even after nutrients available at the surface have disappeared.
There has also been strengthened evidence that one does not “dead” even when he is “sleeping.” In the laboratory research on very nutrient-depleted sediment, researchers have shown that microorganisms are capable of reviving and growing when placed under appropriate conditions, including 100 million years old sample samples taken in South Pacific Gyre. The accumulation process of those sediments was so gradual that oxygen reached unusually deep levels, and formed a prolonged habitat in which aerobic cells could survive on the geologic timescale before being provoked to grow again.
When considered as one, the deep biosphere starts to resemble less of a frozen cemetery and more of a distributed sick ecosystem. The idea by Lloyd that intraterrestrials are “waiting for something that only happens thousands of years later” suits well with processes that provide delayed opportunity, so the shifting cracks or the migrating fluids, the burial and resurfacing of sediments or the chemical pulses arriving as the rock ages and reacts. Cells that have grown in deprivation can subsequently have an advantage when conditions change, a concept also validated by laboratory results on the so-called“growth advantage in stationary phase,” where long-starved bacteria outcompete fresh competitors whenever resources become unavailable again.
This is not a speed miracle, but the persistence of engineering: a biosphere that slows the clocks of the earth, to a slow drawl, making part of its energy by radiological chemistry, and able to reappear in the fast track of life when the deep environment at last provides a motivation to resurrect it.
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