8. REACTIONS OF INORGANIC SUBSTANCES
403
Anaerobic bacteria can also metabolize carbon monoxide. Kluyver
and Schnellen (228) showed that the strict anaerobic methane-forming
Methanobacterium barkerii can produce methane from CO and H 2 :
CO + 3H 2 -> CH 4 + H 2 0
This bacterium also produces methane in the absence of H 2 , in an
atmosphere containing a mixture of CO and nitrogen or 100% CO.
Under these conditions, where the energy source is an organic substrate, significant amounts of C0 2 are produced during the development
of the cultures and if this is absorbed by alkali the accumulation of
appreciable quantities of H 2 in the gaseous phase is noted. These observations suggest a mechanism involving two successive reactions. The first
is the hydrolytic oxidation of CO with the formation of C0 2 and H 2 , and
the second is the hydrogenation of C0 2 by molecular hydrogen in the
hydrogenase reaction which is also carried out by the other methane
bacteria. The entire reaction sequence is as follows:
4CO + 4H 2 0 -> 4C0 2 + 4H 2
C0 2 + 4H 2 -» CH 4 + 2H 2 0
4CO + 2H 2 0 -> 3C0 2 + CH 4
M. formicicium behaves in an analogous manner but tolerates only
lower partial pressures of CO than M. barkerii. That carbon monoxide is
initially oxidized to C0 2 and is not, as might be thought, an intermediate in the reduction of C0 2 to methane is shown by the behavior
of M. omelicmskii, which produces CH 4 from H 2 and C0 2 but not
from CO.
Recently, Yagi (229) showed that cell-free extracts of Desulfovibrio
desulfuricans oxidize carbon monoxide to C0 2 anaerobically by a
process coupled with the reduction of sulfite to sulfide:
3CO + H 2 S0 3 -> 3C0 2 + H 2 S
In this instance, as in that of methane formation by M. barkerii, it
seems that the first reaction step involves the hydrolytic oxidation of CO
to C0 2 and H 2 . At the expense of the molecular hydrogen thus produced,
sulfite is reduced secondarily with the participation of hydrogenase and
the sulfite reductase enzyme system.
References
1. M. Stephenson, "Bacterial Metabolism," 3rd ed., p. 241. Longmans, Green,
London, 1949.
2. W. W. Umbreit, Bacterial. Revs. 11, 157 (1947).
S. C. B. van Niel, Ann. Rev. Microbiol. 8, 105 (1954).
4. H. Lees, "Biochemistry of Autotrophic Bacteria." Butterworths, London, 1955.
403
Anaerobic bacteria can also metabolize carbon monoxide. Kluyver
and Schnellen (228) showed that the strict anaerobic methane-forming
Methanobacterium barkerii can produce methane from CO and H 2 :
CO + 3H 2 -> CH 4 + H 2 0
This bacterium also produces methane in the absence of H 2 , in an
atmosphere containing a mixture of CO and nitrogen or 100% CO.
Under these conditions, where the energy source is an organic substrate, significant amounts of C0 2 are produced during the development
of the cultures and if this is absorbed by alkali the accumulation of
appreciable quantities of H 2 in the gaseous phase is noted. These observations suggest a mechanism involving two successive reactions. The first
is the hydrolytic oxidation of CO with the formation of C0 2 and H 2 , and
the second is the hydrogenation of C0 2 by molecular hydrogen in the
hydrogenase reaction which is also carried out by the other methane
bacteria. The entire reaction sequence is as follows:
4CO + 4H 2 0 -> 4C0 2 + 4H 2
C0 2 + 4H 2 -» CH 4 + 2H 2 0
4CO + 2H 2 0 -> 3C0 2 + CH 4
M. formicicium behaves in an analogous manner but tolerates only
lower partial pressures of CO than M. barkerii. That carbon monoxide is
initially oxidized to C0 2 and is not, as might be thought, an intermediate in the reduction of C0 2 to methane is shown by the behavior
of M. omelicmskii, which produces CH 4 from H 2 and C0 2 but not
from CO.
Recently, Yagi (229) showed that cell-free extracts of Desulfovibrio
desulfuricans oxidize carbon monoxide to C0 2 anaerobically by a
process coupled with the reduction of sulfite to sulfide:
3CO + H 2 S0 3 -> 3C0 2 + H 2 S
In this instance, as in that of methane formation by M. barkerii, it
seems that the first reaction step involves the hydrolytic oxidation of CO
to C0 2 and H 2 . At the expense of the molecular hydrogen thus produced,
sulfite is reduced secondarily with the participation of hydrogenase and
the sulfite reductase enzyme system.
References
1. M. Stephenson, "Bacterial Metabolism," 3rd ed., p. 241. Longmans, Green,
London, 1949.
2. W. W. Umbreit, Bacterial. Revs. 11, 157 (1947).
S. C. B. van Niel, Ann. Rev. Microbiol. 8, 105 (1954).
4. H. Lees, "Biochemistry of Autotrophic Bacteria." Butterworths, London, 1955.