that’s warm or even hot; it has readily available and utilizable geochemically
generated energy sources, notably hydrogen (H 2 ) and methane, and it has abundant
mineral surfaces that may have catalyzed and formed a template for the formation of
more complex organic molecules.
Hydrogen is a potent energy source that could one day fuel our cars, but it may
have fueled early life, as well. It’s produced abiotically and thus was also produced
prebiotically in the subsurface. A variety of water-rock interactions generate H 2 ,
including serpentinization of peridotite minerals (Schrenk et al. 2013), oxidation of
ferrous silicate in basalts and other minerals (Stevens and McKinley 2000), and
radiolysis of water (Lin et al. 2006). In the last of these, the energy of radioactive
decay splits water into H 2 , O 2 , and reactive oxygen species (H 2 O 2 ). Carbon monoxide (toxic to us but an energy source for microbes) is also present in deep fractures
and is thought to have a geochemical origin. Further sources of energetic reduced
gases include outgassing of mantle rocks (Nealson et al. 2005) and shearing of
silicate minerals during seismic activity (Sugisaki et al. 1983). These are generally
sluggish reactions, but may be accelerated when the rocks and water are jostled.
Lippmann-Pipke et al. (2011) detected spikes of H 2 production coinciding with
seismic events; thus, earthquakes may stimulate microbial activity. H 2 can accumulate in the subsurface to concentrations that support life, e.g., 2–3 mM (Kieft et al.
2005; Lin et al. 2006). When dissolved groundwater gases are exsolved, they can
reach 30% by volume and more (Sherwood Lollar et al. 2014). H 2 is also relatively
easily taken up and metabolized, requiring minimal biochemical machinery, mostly
hydrogenase enzymes. Due to its presence on early Earth and its ease of metabolic
use, H 2 is one of the prime candidates to have been the original electron donor for
microbial life. Hydrogen can also react with CO 2 and CO to form methane and shortchain hydrocarbons (ethane, propane, butane) via Fischer-Tropsch-type syntheses.
These abiotic hydrocarbons can also provide energy for microbial metabolism.
Pedersen (2000) coined the term “geogas” for these geochemically generated
energy-rich gaseous subsurface compounds; Stevens and McKinley (1995)
described metabolism of H 2 by the methanogens in basaltic aquifers as part of
what they termed “subsurface lithoautotrophic microbial ecosystems,” or SLiMEs.
Besides being common members of SLiMEs, methanogens have also been proposed
to be among the earliest if not the first form of life on Earth.
A further advantage that may have been conferred by the subsurface onto early
cells was protection from radiation and from obliteration during the Late Heavy
Bombardment in the Hadean period, when massive asteroids slammed into early
Earth, some with energy that may have been sufficient to vaporize the oceans. The
timing and intensity of these impacts are not fully constrained, but any nascent life
exposed at Earth’s surface would have suffered setbacks if not outright extinction.
Most scenarios place the most intense impact period in the 3.5–3.7 to 4.1 billionyears-ago time frame, which is close to (in the grand scheme of geologic time) the
window of time associated with the origin and early evolution of life, ~3.5–4.0
billion years. Incidentally, the emergence of earthly life occurred at a time when
Mars appears to have had conditions that were habitable to microbial life as we know
it. If life appeared there and if it arose in or invaded Mars’ nether regions, then it
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T. L. Kieft
generated energy sources, notably hydrogen (H 2 ) and methane, and it has abundant
mineral surfaces that may have catalyzed and formed a template for the formation of
more complex organic molecules.
Hydrogen is a potent energy source that could one day fuel our cars, but it may
have fueled early life, as well. It’s produced abiotically and thus was also produced
prebiotically in the subsurface. A variety of water-rock interactions generate H 2 ,
including serpentinization of peridotite minerals (Schrenk et al. 2013), oxidation of
ferrous silicate in basalts and other minerals (Stevens and McKinley 2000), and
radiolysis of water (Lin et al. 2006). In the last of these, the energy of radioactive
decay splits water into H 2 , O 2 , and reactive oxygen species (H 2 O 2 ). Carbon monoxide (toxic to us but an energy source for microbes) is also present in deep fractures
and is thought to have a geochemical origin. Further sources of energetic reduced
gases include outgassing of mantle rocks (Nealson et al. 2005) and shearing of
silicate minerals during seismic activity (Sugisaki et al. 1983). These are generally
sluggish reactions, but may be accelerated when the rocks and water are jostled.
Lippmann-Pipke et al. (2011) detected spikes of H 2 production coinciding with
seismic events; thus, earthquakes may stimulate microbial activity. H 2 can accumulate in the subsurface to concentrations that support life, e.g., 2–3 mM (Kieft et al.
2005; Lin et al. 2006). When dissolved groundwater gases are exsolved, they can
reach 30% by volume and more (Sherwood Lollar et al. 2014). H 2 is also relatively
easily taken up and metabolized, requiring minimal biochemical machinery, mostly
hydrogenase enzymes. Due to its presence on early Earth and its ease of metabolic
use, H 2 is one of the prime candidates to have been the original electron donor for
microbial life. Hydrogen can also react with CO 2 and CO to form methane and shortchain hydrocarbons (ethane, propane, butane) via Fischer-Tropsch-type syntheses.
These abiotic hydrocarbons can also provide energy for microbial metabolism.
Pedersen (2000) coined the term “geogas” for these geochemically generated
energy-rich gaseous subsurface compounds; Stevens and McKinley (1995)
described metabolism of H 2 by the methanogens in basaltic aquifers as part of
what they termed “subsurface lithoautotrophic microbial ecosystems,” or SLiMEs.
Besides being common members of SLiMEs, methanogens have also been proposed
to be among the earliest if not the first form of life on Earth.
A further advantage that may have been conferred by the subsurface onto early
cells was protection from radiation and from obliteration during the Late Heavy
Bombardment in the Hadean period, when massive asteroids slammed into early
Earth, some with energy that may have been sufficient to vaporize the oceans. The
timing and intensity of these impacts are not fully constrained, but any nascent life
exposed at Earth’s surface would have suffered setbacks if not outright extinction.
Most scenarios place the most intense impact period in the 3.5–3.7 to 4.1 billionyears-ago time frame, which is close to (in the grand scheme of geologic time) the
window of time associated with the origin and early evolution of life, ~3.5–4.0
billion years. Incidentally, the emergence of earthly life occurred at a time when
Mars appears to have had conditions that were habitable to microbial life as we know
it. If life appeared there and if it arose in or invaded Mars’ nether regions, then it
132
T. L. Kieft