Life on Earth isn’t limited to the surface: the biosphere has deep roots. In a new scientific paper in FEMS Microbiology Ecology, my PhD supervisor and I put deep continental biomass on the weighing scales.

 

What’s beneath our feet? Beneath mine: the soles of my slippers; then a thin carpet and some insulated plywood flooring; then the foundations of my building with their various concealed wires and tubes (completely mysterious to me); then a slightly moist, compact, chemically polluted soil full of old clay pipe-stems, potsherds and fragments of glass, brick and stone, woven through with thick pipes carrying water, gas and sewage; then finally the rocky crust: perhaps a few hundred metres of sandstones perched on a 30-kilometre-thick slab of granites and metamorphic rocks, gliding over the Earth’s mantle.

Is anything alive down there? Certainly. My slippers provide the ideal habitat for my feet and their rich microscopic ecosystem. The carpet abounds with dust mites, mould and bacteria rejoicing in the gentle rain of organic matter (dead skin, crumbs and coffee) from above, rather like filter feeders on the sea floor. Water and especially sewage in the plumbing also support innumerable microorganisms. Soil, of course, is teeming with life, from bacteria—several billion of them per millilitre of groundwater—to fungi, insects, worms, mites, spiders, small mammals, and the tiny swimming ciliates. What about the rocks below that?

It used to be generally assumed that life petered out in the airless darkness just a little way below the land’s surface and the sea-floor. But in the last 20 years or so, drilling projects have revealed multitudes of bacteria (and the bacteria-like archaea), as well as fungi and worms, kilometres underground. Many important questions remain about this “deep biosphere”. Prof. John Parnell (my PhD supervisor) and I recently tried to answer one of them: how much life is under the land?

Quantities of living matter are usually compared by their “biomass”—the mass of carbon they contain. By combining over a hundred measurements of microbial population density in groundwater with a model of how that water is distributed, we found that between 14 and 140 billion tonnes of carbon are contained in life below the continents. (See the paper for details of the methodology). This result updates the rather similar range derived from a much smaller dataset by W. B. Whitman et al. in 1998. It confirms that the deep sub-continental biosphere contains a substantial proportion of the Earth’s total biomass, possibly exceeding that below the seafloor.

The cell-counts used for our study were obtained from waters between ten metres and four kilometres underground. Here in Aberdeen, temperature increases by about 20 °C with each kilometre of depth. This means that microscopic life in pores and fractures can probably be found as deep as five or even six kilometres (three or four miles) under the city; below that, it gets too hot. In other places where the crust is unusually cool, that depth could be ten kilometres or more*. That seems very deep on a human scale but it’s still incredibly shallow compared to the radius of the Earth, which is about 6,400 km. The poet Robert Bridges wrote some good lines on this theme:

“… this crust whereon we dwell
whereon our loves and shames are begotten and buried,
our first slime and ancestral dust: ‘Tis, to compare,
thinner than o’er a luscious peach the velvet skin
that we rip off to engorge the rich succulent pulp:
Were but our planet’s sphere so peel’d, flay’d of the rind
that wraps its lava and rock, the solar satellite
Would keep its motions in God’s orrery undisturb’d.”

—The Testament of Beauty (1929).

 

* (Because cell counts and groundwater availability both diminish exponentially with depth, such ultra-deep organisms make a negligible contribution to the total biomass.)

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