For example: the bottom of the Mariana Trench. This is a seam on the sea floor in the northwestern Pacific, not far from the island of Guam; it’s where the Pacific plate is sliding under the Philippine plate. The ocean is deeper here than anywhere else in the world: the seabed is 11 kilometers (almost 7 miles) below the surface of the sea. Yet even here, where the pressure of the water would crush you or me, there are bacteria. Some of them won’t grow at all unless the atmospheric pressure is at least 50 megapascals (around 7,000 pounds per square inch), and they grow better if the pressure is greater — 70 megapascals (more than 10,000 pounds per square inch). For comparison, the pressure at sea level — the pressure we have evolved to bear — is 700 times less.

Then there are the “intraterrestrials” — the organisms that live in rocks deep in the earth, the creatures of the “deep subsurface biosphere.” Bacteria have been found in rock samples taken several hundred meters below the sea floor, even when the sea floor itself is 4 kilometers (2.5 miles) below sea level.

We don’t know how many organisms are living in this (to us) alien environment. But based on what’s been found in rock samples so far, the numbers are likely to be gigantic. One recent study found between 1 million and 1 billion bacteria per gram of rock (a gram is 1/28 of an ounce). It may be that a large proportion — perhaps as many as a third — of all bacteria on Earth live in rocks below the floor of the sea. That would be a lot of bacteria.

Until recently, it was assumed that the chemical alteration and decomposition of rocks in the ocean crust was due purely to elemental forces — the circulation of seawater, the grinding of rocks against one another. But increasingly, intraterrestrial bacteria are suspected of making a contribution, too. Shards of volcanic glass from basaltic rocks hundreds of meters beneath the seabed show grooves and etchings that appear to have been made by bacteria.

Volcanic glass? When molten basaltic rock wells up from deep in the earth and meets cold water at the bottom of the ocean, the heat is quenched and the rock hardens. As it does so, it forms a characteristic shape known as a pillow basalt — for it looks as though someone has stacked up a pile of rocky pillows. (Every year, around 20 cubic kilometers — around 5 cubic miles — of new basalt forms at the mid-ocean ridges — the places where the sea floor is spreading apart as tectonic plates move away from each other.) Glass forms when the rock cools so fast that crystals don’t have time to take shape. Thus the edges and rims of pillow basalts are often highly glassy; indeed basaltic glass is an important component of the top layers of the oceanic crust.

Bacteria of the deep biosphere can’t get energy directly from sunlight, but they can get energy from a surprising array of other sources, including iron, manganese, sulfur or nitrogen compounds, methane and hydrogen. Indeed, bacteria collected from the deep biosphere show an enormous diversity of metabolic activities. And in chiseling away at rocks, in leaching out minerals to consume, and by excreting waste products, they alter the chemical composition of rocks — and also the composition of the seawater that circulates through the pores and fractures in them. Perhaps — though no one yet knows for sure — they do so on a grand scale, contributing substantially to the composition of rocks and oceans. Experiments suggest that deep biosphere bacteria may accelerate the weathering of basalt rocks by a factor of six compared to physical or chemical forces, especially at the low temperatures that prevail in much of the seabed.

And here’s something nifty. By altering the chemical composition of rocks, the bacteria may alter the magnetic properties of rocks as well.

Basaltic rocks newly arrived at the sea floor give off a strong magnetic signal. This is due to titanomagnetites — iron oxides that contain some titanium — that are present in the rocks. As a result, the mid-ocean ridges often feature a zone of high magnetization known as the central anomaly magnetization high. The strength of the magnetic signal fades as you move away from the newest rocks — as you move from the mid-ocean ridges towards the coasts. (The new rocks are gradually pulled away from the ridge by the spreading apart of the ocean plates; a rock’s distance from the ridge is thus a reflection of its age.) Soon after basalts arrive at the sea floor, then, they lose some of their magnetism. Why?

 

A recent series of experiments that looked at the loss of magnetism of young basaltic rocks shows that bacteria may greatly accelerate the loss. Rock slices were kept for 355 days in vials of seawater with or without bacteria. The bacteria in question were strains of Desulfovibrio — bacteria that give off hydrogen sulfide much as we give off carbon dioxide. Hydrogen sulfide interacts with the iron in the rocks to form iron sulfide; in doing so, it destroys the magnetic signal. And after 355 days, the rocks kept with bacteria gave off a lower signal than the rocks kept in seawater alone. Bacteria, it seems, can efficiently reduce the magnetic signal of young rocks.
No one knows, yet, how significant such effects are. Moreover, we’re only beginning to make an inventory of who’s down there — the identity of most intraterrestrials remains mysterious. But given their vast numbers, and the range of their activities, these small organisms from the weird world of the rocks below the sea will probably turn out to have a big-time influence on the composition of the oceans and the rocks of the seabed.

**********
NOTES:
For bacteria of the Mariana Trench, see Kato, C., Li, L, Nogi, Y., Nakamura, Y., Tamaoka, J. and Horikoshi, K. 1998. “Extremely barophilic bacteria isolated from the Mariana Trench, Challenger Deep, at a depth of 11,000 meters.” Applied and Environmental Microbiology 64: 1510-1513. For the term intraterrestrials, see Pedersen, K. 2000. “Exploration of deep intraterrestrial microbial life: current perspectives.” FEMS Microbiology Letters 185: 9-16.