but do not constitute the largest group in terms of biomass. Evidently, they’re
working very hard to support all of the other trophic groups. The next tier in the
pyramid is made up of methanogens, as well, but these are methanogens that in
simple terms run their biochemistry backwards as anaerobic methane oxidizers
(ANMEs). ANME metabolism by itself is thermodynamically unfavorable, but the
ANMEs are functioning mutualistically with sulfate-reducing bacteria in a tight
consortium first discovered in marine sediments. The sulfide product of the sulfate
reducers in turn serves as energy source for sulfur-oxidizing bacteria. There’s no
oxygen, so the sulfur oxidizers use nitrate as their electron acceptor. Thus, this
chemolithoautotrophic pyramid nicely links the carbon, sulfur, and nitrogen cycles
to form an isolated but self-sustaining ecosystem.
7.2 Stone
Life underground, once one descends beyond surficial sedimentary layers, consists
of rock, and the microbes existing in the fluid-filled fractures in that rock exist in
rock-hosted communities (Onstott et al. 2018). If we consider “stone” to have a
similar meaning to “rock” and to “litho-,” then continental subsurface microbes live
in a stony world and get their energy from stone. In that realm, the chemolithotrophs
truly dominate.
The frequently detected subsurface bacterium D. audaxviator has a special
relationship to “stone.” It’s a sulfate reducer that gets its electron donor and electron
acceptor from water-rock (-stone) interactions, as do most subsurface
chemoautotrophs. The electron donor, H 2 , is created via water-rock interactions, as
described above. In deep South African rock-hosted habitats, the electron acceptor is
produced indirectly by water-rock interactions when reactive oxygen species from
radiolysis of water oxidize 2.9-billion-year-old pyrite to produce sulfate (Chivian
et al. 2008). D. audaxviator has another “stony” connection, too. Dr. Esta van
Heerden, biochemist/microbiologist and co-author of the original papers on this
intriguing little organism, proposed that it be named after a mythical sprite in Zulu
and Xhosa folklore called a “tokoloshe” (or “tikoloshe”). Tokoloshes are thought to
be mischievous little gnome-like creatures. Among their special powers is the ability
to become invisible, which comes about when they eat a stone, or in some versions,
when they drink water. Either way, tokoloshe is an appropriate species moniker for a
deep-dwelling bacterium that obtains its power from water-stone interactions. Incidentally, some say that tokoloshes are especially well endowed, such that they sling
their members over a shoulder. I’m not sure how that fits our sulfate reducer, except
that D. audaxviator is endowed with one or more flagella, long appendages that
extend away from the body of the bacterium for locomotion. The attribute of
locomotion does fit the species name that was actually selected; “audaxviator”
means “bold traveler,” which is appropriate to a motile microbe that has found its
way to the deep subsurface of multiple continents.
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T. L. Kieft
working very hard to support all of the other trophic groups. The next tier in the
pyramid is made up of methanogens, as well, but these are methanogens that in
simple terms run their biochemistry backwards as anaerobic methane oxidizers
(ANMEs). ANME metabolism by itself is thermodynamically unfavorable, but the
ANMEs are functioning mutualistically with sulfate-reducing bacteria in a tight
consortium first discovered in marine sediments. The sulfide product of the sulfate
reducers in turn serves as energy source for sulfur-oxidizing bacteria. There’s no
oxygen, so the sulfur oxidizers use nitrate as their electron acceptor. Thus, this
chemolithoautotrophic pyramid nicely links the carbon, sulfur, and nitrogen cycles
to form an isolated but self-sustaining ecosystem.
7.2 Stone
Life underground, once one descends beyond surficial sedimentary layers, consists
of rock, and the microbes existing in the fluid-filled fractures in that rock exist in
rock-hosted communities (Onstott et al. 2018). If we consider “stone” to have a
similar meaning to “rock” and to “litho-,” then continental subsurface microbes live
in a stony world and get their energy from stone. In that realm, the chemolithotrophs
truly dominate.
The frequently detected subsurface bacterium D. audaxviator has a special
relationship to “stone.” It’s a sulfate reducer that gets its electron donor and electron
acceptor from water-rock (-stone) interactions, as do most subsurface
chemoautotrophs. The electron donor, H 2 , is created via water-rock interactions, as
described above. In deep South African rock-hosted habitats, the electron acceptor is
produced indirectly by water-rock interactions when reactive oxygen species from
radiolysis of water oxidize 2.9-billion-year-old pyrite to produce sulfate (Chivian
et al. 2008). D. audaxviator has another “stony” connection, too. Dr. Esta van
Heerden, biochemist/microbiologist and co-author of the original papers on this
intriguing little organism, proposed that it be named after a mythical sprite in Zulu
and Xhosa folklore called a “tokoloshe” (or “tikoloshe”). Tokoloshes are thought to
be mischievous little gnome-like creatures. Among their special powers is the ability
to become invisible, which comes about when they eat a stone, or in some versions,
when they drink water. Either way, tokoloshe is an appropriate species moniker for a
deep-dwelling bacterium that obtains its power from water-stone interactions. Incidentally, some say that tokoloshes are especially well endowed, such that they sling
their members over a shoulder. I’m not sure how that fits our sulfate reducer, except
that D. audaxviator is endowed with one or more flagella, long appendages that
extend away from the body of the bacterium for locomotion. The attribute of
locomotion does fit the species name that was actually selected; “audaxviator”
means “bold traveler,” which is appropriate to a motile microbe that has found its
way to the deep subsurface of multiple continents.
136
T. L. Kieft