branch of reductive N-oxide transformations must have increased the pool of nitrous
oxide in the anoxic dinitrogen-rich atmosphere and contributed to global warming
and ozone depletion thereby counteracting metabolic diversification and abundance
of biomass. This is one reason why the evolved capability of cyanobacteria to extract
electrons from water and generating increasing levels of free molecular oxygen as a
byproduct was such a massive liberating event, today described as the exploding
diversity of life following the GOE.
Before water was exploited as an external reductant from which electrons could
be extracted by oxidation to generate free molecular oxygen (a.k.a., oxygenic
phototrophy), water served as the dedicated intracellular source of oxygen that
permitted the anoxic oxidation of nitrite to nitrate and the oxidation of hydroxylamine to nitrite, potentially via NO as an intermediate. Both reactions are known
since decades to occur in oxic environments and molecular dioxygen was implicated
as the oxidant. This all changed in the late 1990s and early 2000s with the discovery
of peculiar bacteria affiliated with the planctomycetes that catabolized ammonium
and produced dinitrogen in the complete absence of oxygen: the anaerobic ammonium-oxidizing (anammox) bacteria. In contrast to the then well-known aerobic
ammonia-oxidizing bacteria (AOB), the anammox bacteria not only oxidized ammonium without oxygen as a co-substrate; they also did not terminate electron flow with
oxygen as happens in aerobic AOB and ammonia-oxidizing Thaumarchaeota.
Instead, the anammox process couples the extraction of electrons from ammonium
with the reduction of N-oxides to oxygen-void nitrogen compounds by
comproportionating NO and ammonium to form hydrazine (rocket fuel) as an
energy-rich, albeit dangerous, intermediate. NO is obtained by reducing nitrite
with electrons extracted from ammonium. In addition, in order to generate enough
reducing power to drive carbon assimilation, anammox bacteria anaerobically oxidize nitrite to nitrate. The discovery of this anammox catabolism was proof for how
central catabolic modules of the extant nitrogen cycle evolved and were functional in
the absence of free dioxygen and that the intermediate products of their activities
must have been available during evolutionary times before the GOE.
Before free molecular oxygen was available at sufficient concentrations, the NO
radical was the only operative oxidant released during spontaneous interconversions
of nitrogen species at different oxidation states. Nitrogen in its most reduced state,
ammonium/ammonia [NH 4
+
/NH 3 ], can oxidize successively, albeit very slowly,
without catalysis into its N-alcohol, hydroxylamine [“aminol”, H 2 N-OH
(NH 2 OH)]; its N-aldehyde, hydrogen nitrosyl [nitroxyl; N(¼O)-H, HNO]; its two
N-acids, nitrous acid [hydrogen nitrite; N(¼O)-OH, HNO 2 ] and hyponitrous acid (N
(¼N-OH)-OH, H 2 N 2 O 2 ); as well as its imine peroxide [hydroperoxy-nitrene;
N ¼ (O-O)-H, HONO]. Hyponitrous acid, which essentially is dimerized nitroxyl,
can disintegrate into two molecules of HNO; however, spontaneous dehydration
would yield water and nitrous oxide [H 2 N 2 O 2 ! H 2 O + N 2 O]. Of all these N-oxides,
NO is by far the most reactive, capable of both accepting and donating electrons, and
is particularly reactive with sulfur alcohols [Thiols; -SH]. Since the pre-cell stage of
evolving metabolism, NO and Methyl-thiol [CH 3 SH] have served as the primordial
universal oxidant and reductant, respectively, and their interactions might have thus
9 Mutagens, Radicals, Rocket Fuel, and Laughing Gas: Stringing Metabolic Modules. . .
155
oxide in the anoxic dinitrogen-rich atmosphere and contributed to global warming
and ozone depletion thereby counteracting metabolic diversification and abundance
of biomass. This is one reason why the evolved capability of cyanobacteria to extract
electrons from water and generating increasing levels of free molecular oxygen as a
byproduct was such a massive liberating event, today described as the exploding
diversity of life following the GOE.
Before water was exploited as an external reductant from which electrons could
be extracted by oxidation to generate free molecular oxygen (a.k.a., oxygenic
phototrophy), water served as the dedicated intracellular source of oxygen that
permitted the anoxic oxidation of nitrite to nitrate and the oxidation of hydroxylamine to nitrite, potentially via NO as an intermediate. Both reactions are known
since decades to occur in oxic environments and molecular dioxygen was implicated
as the oxidant. This all changed in the late 1990s and early 2000s with the discovery
of peculiar bacteria affiliated with the planctomycetes that catabolized ammonium
and produced dinitrogen in the complete absence of oxygen: the anaerobic ammonium-oxidizing (anammox) bacteria. In contrast to the then well-known aerobic
ammonia-oxidizing bacteria (AOB), the anammox bacteria not only oxidized ammonium without oxygen as a co-substrate; they also did not terminate electron flow with
oxygen as happens in aerobic AOB and ammonia-oxidizing Thaumarchaeota.
Instead, the anammox process couples the extraction of electrons from ammonium
with the reduction of N-oxides to oxygen-void nitrogen compounds by
comproportionating NO and ammonium to form hydrazine (rocket fuel) as an
energy-rich, albeit dangerous, intermediate. NO is obtained by reducing nitrite
with electrons extracted from ammonium. In addition, in order to generate enough
reducing power to drive carbon assimilation, anammox bacteria anaerobically oxidize nitrite to nitrate. The discovery of this anammox catabolism was proof for how
central catabolic modules of the extant nitrogen cycle evolved and were functional in
the absence of free dioxygen and that the intermediate products of their activities
must have been available during evolutionary times before the GOE.
Before free molecular oxygen was available at sufficient concentrations, the NO
radical was the only operative oxidant released during spontaneous interconversions
of nitrogen species at different oxidation states. Nitrogen in its most reduced state,
ammonium/ammonia [NH 4
+
/NH 3 ], can oxidize successively, albeit very slowly,
without catalysis into its N-alcohol, hydroxylamine [“aminol”, H 2 N-OH
(NH 2 OH)]; its N-aldehyde, hydrogen nitrosyl [nitroxyl; N(¼O)-H, HNO]; its two
N-acids, nitrous acid [hydrogen nitrite; N(¼O)-OH, HNO 2 ] and hyponitrous acid (N
(¼N-OH)-OH, H 2 N 2 O 2 ); as well as its imine peroxide [hydroperoxy-nitrene;
N ¼ (O-O)-H, HONO]. Hyponitrous acid, which essentially is dimerized nitroxyl,
can disintegrate into two molecules of HNO; however, spontaneous dehydration
would yield water and nitrous oxide [H 2 N 2 O 2 ! H 2 O + N 2 O]. Of all these N-oxides,
NO is by far the most reactive, capable of both accepting and donating electrons, and
is particularly reactive with sulfur alcohols [Thiols; -SH]. Since the pre-cell stage of
evolving metabolism, NO and Methyl-thiol [CH 3 SH] have served as the primordial
universal oxidant and reductant, respectively, and their interactions might have thus
9 Mutagens, Radicals, Rocket Fuel, and Laughing Gas: Stringing Metabolic Modules. . .
155