with the generally anoxic conditions deep underground. The anoxic nature of these
habitats and the anaerobic lifestyles of the inhabitant microbes are also part and
parcel of these rock-hosted ecosystems that function totally independently from
photosynthesis; they not only don’t require photosynthetic carbon; they have no
need for and would be damaged by oxygen from photosynthesis. The widely
distributed and recently isolated (Karnachuk et al. 2019) sulfate-reducing autotrophic bacterium Desulforudis audaxviator utilizes the Wood-Ljungdahl pathway
(Chivian et al. 2008). In one case, D. audaxviator was found to be not just the
foundation of the ecosystem, but the sole organism in the community (Lin et al.
2006). Methanogenic archaea are also common primary producers in deep SLiMEs.
They can use H 2 as electron acceptor while fixing CO 2 into methane and organic C.
One might be tempted to view these chemoautotrophically based microbial
ecosystems as rare and remote oddities (the living world is liberally peppered with
quaint and improbable forms), but subsurface SLiMEs are globally widespread and
likely form a huge fraction of the Earth’s prokaryotic cells. The minerals required for
H 2 -forming water-rock interactions are commonplace. Barbara Sherwood Lollar
et al. (2014) have estimated that Precambrian crust, which comprises 70% of the
Earth’s continental area, produces ~10
10
–10
11 moles/year of H 2 . Basalt flows, with
the potential for oxidation of Fe to release H 2 , occur in large areas of the continents
and also the oceans. Rocks bearing uranium, thorium, and other radioisotopes that
drive radiolysis of water are common; one of the mechanisms for concentration of
uranium occurs during granite formation. Put this altogether and you can envision
SLiMEs as a dominant life form.
Astronomer Tommy Gold (1992) considered the possibility of H 2 - and methanefueled deep life almost 30 years ago and posited a vast “deep hot biosphere,”
essentially SLiMEs growing on geogas on a grand scale. While parts of his controversial hypothesis have not been supported (e.g., mantle-derived petroleum hydrocarbons), the basic premise of a vast underground biosphere powered by
geochemically generated energy-rich substrates has now been borne out in many
reports (Colman et al. 2017). Gold estimated the vastness of the subsurface biosphere based in part on the upper temperature limit for life, using a somewhat
speculative value from the time, 150
C. He was certainly correct that temperature
controls the depth limit, but 122
C is more currently accepted. The geothermal
gradient varies across the Earth’s continents from ~8 to ~30
C/km, which corresponds to a depth limit for life ranging up to 12 km or even more in isolated areas.
However, while a few extremophiles may be able to function at temperatures as high
as 122
C in especially energy-rich environs such as deep-sea hydrothermal vents
where H 2 and H 2 S spew out of the seafloor, life at high temperatures comes with a
high energetic cost for repair and replacement of macromolecules, a cost that most
subsurface microbes can’t pay. Racemization of L-amino acids in proteins (Onstott
et al. 2014) and DNA damage are accelerated at high temperatures and thus pose a
special problem for thermophiles. Most subsurface microbes experience relatively
low energy fluxes, probably too low to handle expensive maintenance costs. The
actual upper temperature for most subsurface life might be in the neighborhood of
85
C. Magnabosco et al. (2018) used this information combined with cell
134
T. L. Kieft
habitats and the anaerobic lifestyles of the inhabitant microbes are also part and
parcel of these rock-hosted ecosystems that function totally independently from
photosynthesis; they not only don’t require photosynthetic carbon; they have no
need for and would be damaged by oxygen from photosynthesis. The widely
distributed and recently isolated (Karnachuk et al. 2019) sulfate-reducing autotrophic bacterium Desulforudis audaxviator utilizes the Wood-Ljungdahl pathway
(Chivian et al. 2008). In one case, D. audaxviator was found to be not just the
foundation of the ecosystem, but the sole organism in the community (Lin et al.
2006). Methanogenic archaea are also common primary producers in deep SLiMEs.
They can use H 2 as electron acceptor while fixing CO 2 into methane and organic C.
One might be tempted to view these chemoautotrophically based microbial
ecosystems as rare and remote oddities (the living world is liberally peppered with
quaint and improbable forms), but subsurface SLiMEs are globally widespread and
likely form a huge fraction of the Earth’s prokaryotic cells. The minerals required for
H 2 -forming water-rock interactions are commonplace. Barbara Sherwood Lollar
et al. (2014) have estimated that Precambrian crust, which comprises 70% of the
Earth’s continental area, produces ~10
10
–10
11 moles/year of H 2 . Basalt flows, with
the potential for oxidation of Fe to release H 2 , occur in large areas of the continents
and also the oceans. Rocks bearing uranium, thorium, and other radioisotopes that
drive radiolysis of water are common; one of the mechanisms for concentration of
uranium occurs during granite formation. Put this altogether and you can envision
SLiMEs as a dominant life form.
Astronomer Tommy Gold (1992) considered the possibility of H 2 - and methanefueled deep life almost 30 years ago and posited a vast “deep hot biosphere,”
essentially SLiMEs growing on geogas on a grand scale. While parts of his controversial hypothesis have not been supported (e.g., mantle-derived petroleum hydrocarbons), the basic premise of a vast underground biosphere powered by
geochemically generated energy-rich substrates has now been borne out in many
reports (Colman et al. 2017). Gold estimated the vastness of the subsurface biosphere based in part on the upper temperature limit for life, using a somewhat
speculative value from the time, 150
C. He was certainly correct that temperature
controls the depth limit, but 122
C is more currently accepted. The geothermal
gradient varies across the Earth’s continents from ~8 to ~30
C/km, which corresponds to a depth limit for life ranging up to 12 km or even more in isolated areas.
However, while a few extremophiles may be able to function at temperatures as high
as 122
C in especially energy-rich environs such as deep-sea hydrothermal vents
where H 2 and H 2 S spew out of the seafloor, life at high temperatures comes with a
high energetic cost for repair and replacement of macromolecules, a cost that most
subsurface microbes can’t pay. Racemization of L-amino acids in proteins (Onstott
et al. 2014) and DNA damage are accelerated at high temperatures and thus pose a
special problem for thermophiles. Most subsurface microbes experience relatively
low energy fluxes, probably too low to handle expensive maintenance costs. The
actual upper temperature for most subsurface life might be in the neighborhood of
85
C. Magnabosco et al. (2018) used this information combined with cell
134
T. L. Kieft