Primitive bacteria exist in huge numbers deep in the Earth, living on
hydrogen gas produced in rocks, a NASA scientist reports in the
spring issue of the journal Astrobiology.

Recent studies suggest that the mass of bacteria existing below
ground may be larger than the mass of all living things at the
Earth’s surface, according to recent studies cited by the paper’s
lead author, Friedemann Freund, who works at NASA Ames Research
Center in California’s Silicon Valley. Similar hydrogen-consuming
microbes may some day be discovered on Mars, raising new prospects
for the possible existence of life beyond Earth, Freund added.

“The hydrogen that could feed bacteria in the depth of the Earth
comes from a subtle chemical reaction that occurs within rocks that
were once hot or even molten. In the top 20 kilometers (12.4 miles)
of Earth’s crust,” Freund said, “the conditions are right to produce
a nearly inexhaustible supply of hydrogen. In the top 5 to10
kilometers (about 3 to 6 miles) all fissures and cracks in the rocks
are probably filled with water. Hydrogen molecules will seep out of
the mineral grains, enter the intergranular space and saturate the
water. Microorganisms that live in these water films can be expected
to use this hydrogen as their vital energy source.”

Many of the microorganisms in the ‘deep biosphere’ do not live off
the sunlight-derived energy that green plants trap during
photosynthesis, but live on chemically derived energy sources such as
hydrogen, according to Freund. “If deep microbial communities are to
thrive over long periods of time, they need a steady supply of
hydrogen,” he said.

It has long been known that hydrogen gas is produced when water
reaches freshly formed cracks in many common rocks, but Freund’s
paper describes a different hydrogen-producing reaction that occurs
inside the minerals that make up such rocks. This reaction does not
require rocks to crack – a necessarily episodic event. Instead, it
occurs in the entire rock volume during its gradual cooling as
continents slowly age over millions of years. Because the Earth’s
crust contains a huge quantity of rock, even a small amount of
hydrogen produced in each small section of rock results in a large
volume of gas.

To understand the details of this hydrogen-producing reaction, Freund
said, requires some insight into the structure of minerals where
silicon, oxygen and metals have combined to form a dense pack of
atoms and ions. When these minerals crystallize at high temperatures,
water is always present, and some water molecules are trapped in the
atomic structure of the minerals, said Freund. These water molecules
are ripped apart and change into hydroxyl anions, each of which is
negatively charged and has one oxygen ion with a proton attached.

“During cooling, at temperatures below 400 to 500 degrees C (752 to
932 degrees F), a strange reaction takes place. Pairs of these
hydroxyl anions rearrange their electrons in such a way that hydrogen
gas molecules are formed,” Freund said.

What is unusual and still not fully understood, said Freund, is that
the electrons needed to make the hydrogen molecules are taken away
from negatively charged oxygen anions. “Suddenly, some oxygen anions,
which everybody thought only existed in a doubly charged negative
state, convert to singly charged negative ions,” he said. “These
single negative oxygen anions join in pairs. In this form, they are
innocuous and can stay inactive over geological times.”

The hydrogen molecules, however, wander around inside the mineral
structure and can squeeze into the narrow spaces between the mineral
grains. If the intergranular space is filled with water, the hydrogen
molecules will dissolve in the water. If microbes live in the
intergranular water films, one can imagine, said Freund, that these
bacteria extract the dissolved hydrogen from the water and use this
hydrogen as an energy source, not unlike fish that extract oxygen
dissolved in the water of rivers, lakes and the sea to respire.

“What is potentially important,” Freund said, “is that, if and when
microorganisms in the deep underground use this hydrogen dissolved in
the intergranular water films, the rocks around them will replenish
the hydrogen supply – indefinitely, over eons of time.”

The paper by Freund and his coworkers also may help answer
non-biological questions related to the commercial viability of
tapping hydrogen reserves deep in the rocks and to questions of mine
safety. For example, sometimes, during mining and drilling
operations, enough hydrogen seeps out of wall rocks that explosive
gas mixtures can be produced, according to some reports.

“Since old, old times, the mining industry has had its share of mine
explosions in which hydrogen played a role,” Freund said, “but
hydrogen gas could also be used as an energy source and fuel in
today’s or tomorrow’s society. For years, pipelines have been
distributing hydrogen gas between different industrial partners in
the Ruhr Valley in Germany, and the experts say it can be handled
about as safely as natural gas.”