fluidized bed reactor treating effluent from a methanogenic reactor. Nitrate and
ammonium fed into the column were consumed with concomitant gas production
(Mulder et al. 1995). It was soon discovered that nitrite rather than nitrate was the
electron acceptor for the ammonium oxidizing bacteria that are affiliated with the
Planctomycetes phylum (Strous and Jetten 2004). Now it is clear that the anammox
process is a major link in the global nitrogen cycle, especially in the ocean where it
causes major losses in available nitrogen in oxygen-depleted waters (Francis et al.
2007).
Among the varied company of new “actors” in the “grand microbial variety
show,” the anammox bacteria are maybe the most unusual. Hydrazine, a highly
reactive compound used as rocket fuel, is an intermediate in the oxidation of
ammonium to dinitrogen (Strous et al. 2006). The anammox bacteria possess an
intracellular compartment named the anammoxosome, composed of a specialized
membrane on which the redox processes occur with energy conservation in the form
of a transmembrane proton gradient. Possibly to protect this membrane from reactive
intermediates, this very dense membrane is built of linearly concatenated
cyclobutane lipids. These lipids contain up to five linearly fused cyclobutane
moieties with cis ring junctions. Such “ladderane” molecules were never yet encountered elsewhere in nature (Sinninghe Damsté et al. 2002).
10.4 Diverse Ways to Oxidize Methane in Anaerobic
Environments
Just like the old dogma that ammonium cannot be oxidized in the absence of
molecular oxygen, it long was believed that oxidation of methane in anaerobic
environments is not possible. The rationale was very similar: the only known
reaction in which methane could be oxidized was by methane monooxygenase,
which catalyzes the oxidation of methane to methanol in a reaction in which the
oxygen atom of methanol is derived from molecular oxygen, analogous to the
formation of hydroxylamine from ammonia in the first step of nitrification.
Occurrence of anaerobic oxidation of methane was first postulated following the
analysis of vertical profiles of methane abundance in anoxic marine systems. Methane consumption was correlated with a decrease in sulfate concentrations,
suggesting that sulfate may act as the terminal electron acceptor. Based on methane
profiles, radiotracer experiments, and stable carbon isotope data, it appears that a
large fraction of the globally produced methane is oxidized to CO 2 in anaerobic
marine sediments (Boetius et al. 2000). The process is often performed by dense
aggregates of archaea (termed ANME—ANaerobic MEthane oxidizers) and sulfatereducing bacteria, growing in clusters of which the anaerobic methane-oxidizing
archaea are surrounded by sulfate reducers. Massive anaerobic oxidation of methane
in the bottom sediments of the Black Sea results in the formation of up to 4-meterhigh carbonate buildups produced by the activity of ANME archaea and sulfate
166
A. Oren
ammonium fed into the column were consumed with concomitant gas production
(Mulder et al. 1995). It was soon discovered that nitrite rather than nitrate was the
electron acceptor for the ammonium oxidizing bacteria that are affiliated with the
Planctomycetes phylum (Strous and Jetten 2004). Now it is clear that the anammox
process is a major link in the global nitrogen cycle, especially in the ocean where it
causes major losses in available nitrogen in oxygen-depleted waters (Francis et al.
2007).
Among the varied company of new “actors” in the “grand microbial variety
show,” the anammox bacteria are maybe the most unusual. Hydrazine, a highly
reactive compound used as rocket fuel, is an intermediate in the oxidation of
ammonium to dinitrogen (Strous et al. 2006). The anammox bacteria possess an
intracellular compartment named the anammoxosome, composed of a specialized
membrane on which the redox processes occur with energy conservation in the form
of a transmembrane proton gradient. Possibly to protect this membrane from reactive
intermediates, this very dense membrane is built of linearly concatenated
cyclobutane lipids. These lipids contain up to five linearly fused cyclobutane
moieties with cis ring junctions. Such “ladderane” molecules were never yet encountered elsewhere in nature (Sinninghe Damsté et al. 2002).
10.4 Diverse Ways to Oxidize Methane in Anaerobic
Environments
Just like the old dogma that ammonium cannot be oxidized in the absence of
molecular oxygen, it long was believed that oxidation of methane in anaerobic
environments is not possible. The rationale was very similar: the only known
reaction in which methane could be oxidized was by methane monooxygenase,
which catalyzes the oxidation of methane to methanol in a reaction in which the
oxygen atom of methanol is derived from molecular oxygen, analogous to the
formation of hydroxylamine from ammonia in the first step of nitrification.
Occurrence of anaerobic oxidation of methane was first postulated following the
analysis of vertical profiles of methane abundance in anoxic marine systems. Methane consumption was correlated with a decrease in sulfate concentrations,
suggesting that sulfate may act as the terminal electron acceptor. Based on methane
profiles, radiotracer experiments, and stable carbon isotope data, it appears that a
large fraction of the globally produced methane is oxidized to CO 2 in anaerobic
marine sediments (Boetius et al. 2000). The process is often performed by dense
aggregates of archaea (termed ANME—ANaerobic MEthane oxidizers) and sulfatereducing bacteria, growing in clusters of which the anaerobic methane-oxidizing
archaea are surrounded by sulfate reducers. Massive anaerobic oxidation of methane
in the bottom sediments of the Black Sea results in the formation of up to 4-meterhigh carbonate buildups produced by the activity of ANME archaea and sulfate
166
A. Oren