4.2 Methane Metabolism in Methane-Oxidizing Bacteria
77
pathway, NAD
+ , the oxidized form of NADH, accepts two electrons, and NADH, the
reduced form of NAD, is produced. Via the three oxidation steps, i.e., the oxidation
of methanol, formaldehyde, and formate, a total of six electrons (two electrons
per step) is produced. Two of the electrons are provided to the methane oxidation
process in the form of a reducing equivalent for simultaneous methane uptake.
The remaining four electrons are provided to the electron transport chain, which
produces adenosine triphosphate (ATP) through the proton motive force, as shown
in Fig. 4.1. Finally, these four electrons reduce oxygen molecules as the terminal
electron acceptor, forming H 2 O. The overall reaction is shown as reaction 4.1.
CH 4 + O 2 + NADH + H
+
→ CH 3 OH + H 2 O + NAD
+
(4.1)
(2) Assimilation pathway
In the assimilation pathway, formaldehyde, formate, and carbon dioxide can be
used for the synthesis of cellular components through the ribulose monophosphate
(RuMP) pathway and the serine pathway [30]. Furthermore, some methane-oxidizing
bacteria also have a reductive pentose phosphate cycle (Calvin–Benson–Bassham
(CBB) cycle) [31–37].
All the carbons incorporated into the bacterial cells via the RuMP pathway are
derived from formate produced from methane [30, 38, 39]. On the other hand, in
the serine pathway, only about 50% of the carbons that are incorporated into the
bacterial cells are derived from formaldehyde produced from methane, while the rest
are supplied from the multiple carboxylation reactions in both the serine and ethylmalonyl coenzyme A (ethylmalonyl-CoA) pathways for glyoxylate regeneration or
a glyoxylate bypass [40–43].
Based on differences in their assimilation pathways, as well as in their morphology
and physiological properties, methane-oxidizing bacteria have conventionally been
classified into three classes (Table 4.2), namely Type I, Type II, and Type III [38, 44].
Type I was recently divided into three types (Ia, Ib, and Ic) [45–51], while Type II
was divided into two types (IIa and IIb) [51, 52]. Furthermore, non-proteobacterial
methane-oxidizing bacteria classified as Type III hase been found in the phylum
Verrucomicrobia [53–55]. These bacteria use methane only as an energy source,
oxidize methane to carbon dioxide, and use the CBB cycle to fix carbon dioxide
[56].
Some proteobacteria such as the Type I species Methylococcus capsulatus and
Methylocaldum szegediense O-12 and the Type II species Methyloferula stellata
AR4, Methylocella silvestris BL2, and Methylocapsa acidiphila B2 also contain a
complete set of genes for the CBB cycle. However, the role of the CBB cycle in the
metabolism of these bacteria has yet to be experimentally validated.
Methane-oxidizing bacteria have been found in many ecosystems such as soils,
peatlands, rice paddies, sediments, freshwater and marine systems, alkaline soda
lakes, acidic hot springs, mud pots, cold ecosystems, and tissues of higher organisms
[57]. Most methane-oxidizing bacteria utilize methane exclusively as their carbon and
energy sources (obligate methanotrophs), but some methanotrophs can also utilize
77
pathway, NAD
+ , the oxidized form of NADH, accepts two electrons, and NADH, the
reduced form of NAD, is produced. Via the three oxidation steps, i.e., the oxidation
of methanol, formaldehyde, and formate, a total of six electrons (two electrons
per step) is produced. Two of the electrons are provided to the methane oxidation
process in the form of a reducing equivalent for simultaneous methane uptake.
The remaining four electrons are provided to the electron transport chain, which
produces adenosine triphosphate (ATP) through the proton motive force, as shown
in Fig. 4.1. Finally, these four electrons reduce oxygen molecules as the terminal
electron acceptor, forming H 2 O. The overall reaction is shown as reaction 4.1.
CH 4 + O 2 + NADH + H
+
→ CH 3 OH + H 2 O + NAD
+
(4.1)
(2) Assimilation pathway
In the assimilation pathway, formaldehyde, formate, and carbon dioxide can be
used for the synthesis of cellular components through the ribulose monophosphate
(RuMP) pathway and the serine pathway [30]. Furthermore, some methane-oxidizing
bacteria also have a reductive pentose phosphate cycle (Calvin–Benson–Bassham
(CBB) cycle) [31–37].
All the carbons incorporated into the bacterial cells via the RuMP pathway are
derived from formate produced from methane [30, 38, 39]. On the other hand, in
the serine pathway, only about 50% of the carbons that are incorporated into the
bacterial cells are derived from formaldehyde produced from methane, while the rest
are supplied from the multiple carboxylation reactions in both the serine and ethylmalonyl coenzyme A (ethylmalonyl-CoA) pathways for glyoxylate regeneration or
a glyoxylate bypass [40–43].
Based on differences in their assimilation pathways, as well as in their morphology
and physiological properties, methane-oxidizing bacteria have conventionally been
classified into three classes (Table 4.2), namely Type I, Type II, and Type III [38, 44].
Type I was recently divided into three types (Ia, Ib, and Ic) [45–51], while Type II
was divided into two types (IIa and IIb) [51, 52]. Furthermore, non-proteobacterial
methane-oxidizing bacteria classified as Type III hase been found in the phylum
Verrucomicrobia [53–55]. These bacteria use methane only as an energy source,
oxidize methane to carbon dioxide, and use the CBB cycle to fix carbon dioxide
[56].
Some proteobacteria such as the Type I species Methylococcus capsulatus and
Methylocaldum szegediense O-12 and the Type II species Methyloferula stellata
AR4, Methylocella silvestris BL2, and Methylocapsa acidiphila B2 also contain a
complete set of genes for the CBB cycle. However, the role of the CBB cycle in the
metabolism of these bacteria has yet to be experimentally validated.
Methane-oxidizing bacteria have been found in many ecosystems such as soils,
peatlands, rice paddies, sediments, freshwater and marine systems, alkaline soda
lakes, acidic hot springs, mud pots, cold ecosystems, and tissues of higher organisms
[57]. Most methane-oxidizing bacteria utilize methane exclusively as their carbon and
energy sources (obligate methanotrophs), but some methanotrophs can also utilize