complete reduction of ethene to ethane, apparently as a co-metabolism. Hinrichs
et al. (2006) concluded that biogenesis was responsible for the observed formation of
ethane and propane detected in deep sea sediments primarily based on their relatively
12 C-enriched stable C isotopic compositions of recovered ethane and propane
(“б
13 C-C 2 H 6 ” and “б
13 C-C 3 H 8 ”) and theoretical reaction kinetics with availability
of acetate and H 2 . The experimental incubation approach and broad substrate testing
was taken up by Xie et al. (2013) who examined ethane and propane formation by
sediments amended with ethene, ethane thiol, or propane thiol. Again, some H 2 was
required for the reactions to proceed, and a mixed enrichment culture composed of
methanogens and acetogens was involved in what appeared to be a co-metabolism,
but nonetheless could convert what is a well-recognized and naturally occurring
gaseous alkene of biogenic origin (ethene) to ethane.
30.8 Working Backward to Move Forward?
The phenomenon of “AOM” (Anaerobic Oxidation of Methane) has now been wellestablished and extensively studied (e.g., Wegener et al. 2015; 2016). It involves a
backward-running MCR in archaea similar to methanogens that converts ambient
CH 4 outside the cell to CH 3 -S-CoM inside the cell. The back reaction continues in
reverse along the methanogenic pathway until the eventual product of CO 2 is
formed, usually in syntrophy with other anaerobes. More recently, evidence has
accumulated that in thermogenic natural gas seeps that contain appreciable higher
alkanes besides CH 4 (e.g., C 2 H 6 , C 3 H 8 , C 4 H 10 ) analogous archaea are also present
that are specifically adapted to the anaerobic oxidation of these higher hydrocarbons,
which would include ethane (Chen et al. 2019; and commentary by Ragsdale 2019)
and butanes (Laso-Perez et al. 2016). These reactions are initiated by homologues of
MCR, namely specific alkyl-CoM reductases (e.g., ECR, BCR) that function specifically for the given alkane, be it ethane, propane, or butane.
Aside from the excitement of new discoveries herein, there is also a practical
interest in “reverse engineering” these reverse-running microorganisms. The thinking is that they could be coaxed to produce ethane, propane, or butane from CO 2 ,
thereby undercutting the need to mine these gases via petroleum drilling or hydrofracking. It is relevant that ethane, for example, can be easily converted catalytically
to ethene, thereby making it a valuable industrial feedstock.
In nature, there is usually a yin to answer to a yang. Hence, perhaps there are
naturally occurring archaea that can potentially make a living by reducing CO 2 all
the way to ethane or propane, using specific ACRs as their final step, just as
methanogens use MCR. Hence, instead of being viewed as a co-metabolism carried
out by methanogens converting ESH, DES, or ethene to ethyl-S-CoM, but requiring
a simultaneous operative methanogenic pathway to yield energy, there could exist
genuine ethanotrophs, propanotrophs, and butanotrophs. Maybe the reason they
haven’t been found yet is that no one has bothered to look for them, or that their
activities and bioenergetics are so poor that they cannot compete with the more hale
648
R. S. Oremland
et al. (2006) concluded that biogenesis was responsible for the observed formation of
ethane and propane detected in deep sea sediments primarily based on their relatively
12 C-enriched stable C isotopic compositions of recovered ethane and propane
(“б
13 C-C 2 H 6 ” and “б
13 C-C 3 H 8 ”) and theoretical reaction kinetics with availability
of acetate and H 2 . The experimental incubation approach and broad substrate testing
was taken up by Xie et al. (2013) who examined ethane and propane formation by
sediments amended with ethene, ethane thiol, or propane thiol. Again, some H 2 was
required for the reactions to proceed, and a mixed enrichment culture composed of
methanogens and acetogens was involved in what appeared to be a co-metabolism,
but nonetheless could convert what is a well-recognized and naturally occurring
gaseous alkene of biogenic origin (ethene) to ethane.
30.8 Working Backward to Move Forward?
The phenomenon of “AOM” (Anaerobic Oxidation of Methane) has now been wellestablished and extensively studied (e.g., Wegener et al. 2015; 2016). It involves a
backward-running MCR in archaea similar to methanogens that converts ambient
CH 4 outside the cell to CH 3 -S-CoM inside the cell. The back reaction continues in
reverse along the methanogenic pathway until the eventual product of CO 2 is
formed, usually in syntrophy with other anaerobes. More recently, evidence has
accumulated that in thermogenic natural gas seeps that contain appreciable higher
alkanes besides CH 4 (e.g., C 2 H 6 , C 3 H 8 , C 4 H 10 ) analogous archaea are also present
that are specifically adapted to the anaerobic oxidation of these higher hydrocarbons,
which would include ethane (Chen et al. 2019; and commentary by Ragsdale 2019)
and butanes (Laso-Perez et al. 2016). These reactions are initiated by homologues of
MCR, namely specific alkyl-CoM reductases (e.g., ECR, BCR) that function specifically for the given alkane, be it ethane, propane, or butane.
Aside from the excitement of new discoveries herein, there is also a practical
interest in “reverse engineering” these reverse-running microorganisms. The thinking is that they could be coaxed to produce ethane, propane, or butane from CO 2 ,
thereby undercutting the need to mine these gases via petroleum drilling or hydrofracking. It is relevant that ethane, for example, can be easily converted catalytically
to ethene, thereby making it a valuable industrial feedstock.
In nature, there is usually a yin to answer to a yang. Hence, perhaps there are
naturally occurring archaea that can potentially make a living by reducing CO 2 all
the way to ethane or propane, using specific ACRs as their final step, just as
methanogens use MCR. Hence, instead of being viewed as a co-metabolism carried
out by methanogens converting ESH, DES, or ethene to ethyl-S-CoM, but requiring
a simultaneous operative methanogenic pathway to yield energy, there could exist
genuine ethanotrophs, propanotrophs, and butanotrophs. Maybe the reason they
haven’t been found yet is that no one has bothered to look for them, or that their
activities and bioenergetics are so poor that they cannot compete with the more hale
648
R. S. Oremland