could be there still, metabolizing the energetic products of water-rock interactions.
Tullis Onstott and colleagues (Onstott et al. 2018) make a case for the deep
subsurface of Mars remaining habitable ever since those early days (or “sols,” as
they’re called on the Red Planet), and they further propose to search for life in fluidfilled fractures of deep Martian rocks. I suggest that we explore the depths of our
own planet more thoroughly first, but then I’m all for seeking rock-hosted life
elsewhere.
Another meaning of the term “foundation” is as the physical underpinning or base
of a structure. One way of considering this with reference to subsurface microbes is
to consider a trophic dynamic pyramid, for which the primary producers, i.e., the
autotrophs, serve as the base, the compartment having the greatest biomass. This
autotrophic base is overlain by successive layers, primary consumers, secondary
consumers, etc., culminating in the lowest total biomass group, the top predators,
e.g., wolves, sharks. These various levels of consumers are all heterotrophs. In
terrestrial biomes, the autotrophic base is primarily plants. In fact, multicellular
plants have been estimated to make up 450 gigatons of biomass carbon (Gt C) out
of an estimated total biomass on the planet of 550 Gt C (Bar-On et al. 2018). The
sources of organic carbon, i.e., the primary producers, in the oceans are primarily
microbes and are dominated by a few genera of photosynthetic cyanobacteria, e.g.,
Prochlorococcus and Synechocystis. The sources of organic carbon in the continental subsurface vary with depth. In soils and at depths to a few tens or hundreds of
meters, organic carbon is nearly all photosynthate, in other words, plant-derived
organic carbon that has been buried underground or has been transported to the
subsurface. The microbes in the shallow subsurface are therefore mostly heterotrophs. Deeper subsurface localities are removed both in distance and in time from
the surface world, and therefore photosynthetically generated organic carbon is
scarce to nonexistent (Kieft et al. 2018), except in isolated petroleum deposits,
which actually make up only a small fraction of the volume of the deep subsurface.
One might expect that the biomass of microbes would decline to zero in the absence
of surface-derived organic C, but this is not the case. Even at great depth, 3 km and
more, bacteria and archaea are found, albeit in lower numbers, ~1000–10,000 cells
per ml of fracture water; and these microbes are dominantly chemoautotrophs
functioning in SLiMEs. In other words, the foundation layer of the trophic pyramid
in the deep subsurface is comprised of chemoautotrophs, a.k.a. chemolithotrophs.
They make their living by combining H 2 and other geogas components as electron
donors (fuel) with a variety of electron acceptors (oxidants) that include CO 2 ,
sulfate, and nitrate.
As primary producers, these subsurface chemoautotrophs fix CO 2 into organic
C. There are approximately six different biochemical pathways for CO 2 fixation
(Fuchs 2011). The majority of photoautotrophs use the Calvin-Benson-Basham
cycle, which relies on ribulose bisphosphate carboxylase oxidase, possibly the
most abundant enzyme on Earth. However, many subsurface autotrophs including
dissimilatory sulfate reducers and methanogens use the more ancient and also more
efficient Wood-Ljungdahl pathway, also known as the reductive acetyl CoA pathway (Cotton et al. 2018). This pathway occurs only in anaerobes, which goes along
7 Lithotrophic (“Stone-Eating”) Microbes Provide the Foundation for Deep. . .
133
Tullis Onstott and colleagues (Onstott et al. 2018) make a case for the deep
subsurface of Mars remaining habitable ever since those early days (or “sols,” as
they’re called on the Red Planet), and they further propose to search for life in fluidfilled fractures of deep Martian rocks. I suggest that we explore the depths of our
own planet more thoroughly first, but then I’m all for seeking rock-hosted life
elsewhere.
Another meaning of the term “foundation” is as the physical underpinning or base
of a structure. One way of considering this with reference to subsurface microbes is
to consider a trophic dynamic pyramid, for which the primary producers, i.e., the
autotrophs, serve as the base, the compartment having the greatest biomass. This
autotrophic base is overlain by successive layers, primary consumers, secondary
consumers, etc., culminating in the lowest total biomass group, the top predators,
e.g., wolves, sharks. These various levels of consumers are all heterotrophs. In
terrestrial biomes, the autotrophic base is primarily plants. In fact, multicellular
plants have been estimated to make up 450 gigatons of biomass carbon (Gt C) out
of an estimated total biomass on the planet of 550 Gt C (Bar-On et al. 2018). The
sources of organic carbon, i.e., the primary producers, in the oceans are primarily
microbes and are dominated by a few genera of photosynthetic cyanobacteria, e.g.,
Prochlorococcus and Synechocystis. The sources of organic carbon in the continental subsurface vary with depth. In soils and at depths to a few tens or hundreds of
meters, organic carbon is nearly all photosynthate, in other words, plant-derived
organic carbon that has been buried underground or has been transported to the
subsurface. The microbes in the shallow subsurface are therefore mostly heterotrophs. Deeper subsurface localities are removed both in distance and in time from
the surface world, and therefore photosynthetically generated organic carbon is
scarce to nonexistent (Kieft et al. 2018), except in isolated petroleum deposits,
which actually make up only a small fraction of the volume of the deep subsurface.
One might expect that the biomass of microbes would decline to zero in the absence
of surface-derived organic C, but this is not the case. Even at great depth, 3 km and
more, bacteria and archaea are found, albeit in lower numbers, ~1000–10,000 cells
per ml of fracture water; and these microbes are dominantly chemoautotrophs
functioning in SLiMEs. In other words, the foundation layer of the trophic pyramid
in the deep subsurface is comprised of chemoautotrophs, a.k.a. chemolithotrophs.
They make their living by combining H 2 and other geogas components as electron
donors (fuel) with a variety of electron acceptors (oxidants) that include CO 2 ,
sulfate, and nitrate.
As primary producers, these subsurface chemoautotrophs fix CO 2 into organic
C. There are approximately six different biochemical pathways for CO 2 fixation
(Fuchs 2011). The majority of photoautotrophs use the Calvin-Benson-Basham
cycle, which relies on ribulose bisphosphate carboxylase oxidase, possibly the
most abundant enzyme on Earth. However, many subsurface autotrophs including
dissimilatory sulfate reducers and methanogens use the more ancient and also more
efficient Wood-Ljungdahl pathway, also known as the reductive acetyl CoA pathway (Cotton et al. 2018). This pathway occurs only in anaerobes, which goes along
7 Lithotrophic (“Stone-Eating”) Microbes Provide the Foundation for Deep. . .
133