preceded the Great Oxidation Event of 2.4 Gya, although they suggested an environment more akin to the salt-saturated, arsenic-rich extreme of Searles Lake than
Mono Lake (Oremland et al. 2005; Kulp et al. 2006, 2007).
Whenever one posits a chicken vs. egg precedence argument in biology, there are
always countering positions taken by those whose research is more invested with
chickens than with eggs. Such was the argument posited by van Lis et al. (2013)
who, although they agreed with the antiquity of a biological arsenic cycle, they
countered that aerobic arsenite oxidation arose first, achieved by oxygen-linkage of
the aioBA arsenite oxidases, which then in turn created niches for anaerobic arsenate
respiration via arrAB genes from which anaerobic arsenite oxidation via arxAB later
evolved. They used largely bioinformatic approaches which showed that while
neither arxA nor arrA occur in the domain Archaea, those of aioA do and in addition
are present in the domain Bacteria, thereby implying a deep-time origin in the Last
Universal Common Ancestor (LUCA).
A recent study by Wells et al. (2020) demolished this counter argument. Making
much greater use of the broad sequence databases now available (~1568 species) and
employing more sophisticated current bioinformatics techniques, they investigated
the deep origins of several CISM-genes (Complex Iron Sulfur Molybdoenzymes)
which include the arsenic reductases and oxidases. They showed clearly that arxA
preceded the origin of arrA, and that aioA arose later in a different CISM realm,
likely from a sub-group of assimilatory nitrate reductases. While there was a deep
time origin for the arrA and arxA genes it came after LUCA diverged into Bacteria
and Archaea, occurring in the former but not the latter. Although aioA genes are
found in the Archaea, they occur only in extreme halophiles and not in extreme
thermophiles, the latter thought to be models of the LUCA physiology. Hence, their
acquisition occurred much later in time via lateral gene transfer events and did not
arise independently in LUCA, and thence to Archaea and Bacteria, but first arose in
aerobic Bacteria and from thence to some Archaea. Curiously, some extremely
thermophilic archaea can respire arsenate; they achieve this via a respiratory
tetrathionate reductase TtrA, the gene for which aligns closely with the lateremerging bacterial arxA and arrA (Wells et al. 2020).
31.2.6 Arsenic in DNA?
I would be remiss if I did not offer in this retrospective piece a final, brief commentary upon our claim of arsenic substituting for phosphorus in the DNA of Halomonas
strain GFAJ-1 (Wolfe-Simon et al. 2011). Although almost 10 years have elapsed
since the work first appeared in the journal Science, aspects of it are still too painful
for me to recount in detail.
Wolfe-Simon proposed a highly original and very bold idea, namely that arsenate
could act as a substitute for phosphate in the 3
0 and 5
0 di-ester linkages of the
deoxyribose molecule in the outer helix that binds the nucleotides together. I liked
the idea and offered that she use Mono Lake sediment as an inoculum source, its
31 Salty, Alkali-Laced Tales (Mostly True) from the Great Basin Desert, California. . .
679
Mono Lake (Oremland et al. 2005; Kulp et al. 2006, 2007).
Whenever one posits a chicken vs. egg precedence argument in biology, there are
always countering positions taken by those whose research is more invested with
chickens than with eggs. Such was the argument posited by van Lis et al. (2013)
who, although they agreed with the antiquity of a biological arsenic cycle, they
countered that aerobic arsenite oxidation arose first, achieved by oxygen-linkage of
the aioBA arsenite oxidases, which then in turn created niches for anaerobic arsenate
respiration via arrAB genes from which anaerobic arsenite oxidation via arxAB later
evolved. They used largely bioinformatic approaches which showed that while
neither arxA nor arrA occur in the domain Archaea, those of aioA do and in addition
are present in the domain Bacteria, thereby implying a deep-time origin in the Last
Universal Common Ancestor (LUCA).
A recent study by Wells et al. (2020) demolished this counter argument. Making
much greater use of the broad sequence databases now available (~1568 species) and
employing more sophisticated current bioinformatics techniques, they investigated
the deep origins of several CISM-genes (Complex Iron Sulfur Molybdoenzymes)
which include the arsenic reductases and oxidases. They showed clearly that arxA
preceded the origin of arrA, and that aioA arose later in a different CISM realm,
likely from a sub-group of assimilatory nitrate reductases. While there was a deep
time origin for the arrA and arxA genes it came after LUCA diverged into Bacteria
and Archaea, occurring in the former but not the latter. Although aioA genes are
found in the Archaea, they occur only in extreme halophiles and not in extreme
thermophiles, the latter thought to be models of the LUCA physiology. Hence, their
acquisition occurred much later in time via lateral gene transfer events and did not
arise independently in LUCA, and thence to Archaea and Bacteria, but first arose in
aerobic Bacteria and from thence to some Archaea. Curiously, some extremely
thermophilic archaea can respire arsenate; they achieve this via a respiratory
tetrathionate reductase TtrA, the gene for which aligns closely with the lateremerging bacterial arxA and arrA (Wells et al. 2020).
31.2.6 Arsenic in DNA?
I would be remiss if I did not offer in this retrospective piece a final, brief commentary upon our claim of arsenic substituting for phosphorus in the DNA of Halomonas
strain GFAJ-1 (Wolfe-Simon et al. 2011). Although almost 10 years have elapsed
since the work first appeared in the journal Science, aspects of it are still too painful
for me to recount in detail.
Wolfe-Simon proposed a highly original and very bold idea, namely that arsenate
could act as a substitute for phosphate in the 3
0 and 5
0 di-ester linkages of the
deoxyribose molecule in the outer helix that binds the nucleotides together. I liked
the idea and offered that she use Mono Lake sediment as an inoculum source, its
31 Salty, Alkali-Laced Tales (Mostly True) from the Great Basin Desert, California. . .
679