8. REACTIONS OF INORGANIC SUBSTANCES
353
The first experimental approach along these lines was carried out
by Winogradsky (12) who measured the N/C ratio (nitrogen oxidized
and carbon assimilated) for cultures of nitrifying bacteria and found
an average value of 35 for Nitrosomonas and of 135 for Nitrobacter.
Under the most favorable culture conditions, Meyerhof (13) later obtained a value of 101 for the N/C ratio of Nitrobacter. These figures
show that the assimilation of a carbon atom corresponds to the oxidation by Nitrobacter of 87 molecules of nitrite to nitrate. From the heat
of combustion of nitrite, Meyerhof thus calculated the energy efficiency
of Nitrobacter and arrived at the remarkably low figure of 4.5%.
Baas-Becking and Parks (10) reopened this question by considering
not the heats of combustion but the free energy change during the
oxidation of nitrogen-containing substrates under the conditions of the
bacterial cultures:
Nitrosomonas spp.
NH 4
+ (5 X 10~
3 M) + %0 2 = N0 2 - + H 2 0 + 2H+(1()8
)
(-AV = 66.5 kcal.)
Nitrobacter spp.
N0 2 -(7 X 103 M) + }40ι = NO3(-Δ^ 298 ο = 17.5 kcal.)
Starting with the metabolic balance sheets of Winogradsky and of
Meyerhof, then, the free energy efficiency of Nitrosomonas is 5.9% and
that of Nitrobacter is 7.8%. Baas-Becking and Parks also calculated the
free energy efficiencies of several other chemo-lithotrophic bacteria by
this method. The results (Table IV) show values between 5 and 9%, except for Hydrogenomonas with a 30% efficiency; these values are much
lower than those of heterotrophic bacteria which reach or even surpass
efficiencies of 60%. This difference in total efficiency does not necessarily
indicate that the energy-transport systems which connect the sequence
of catabolic reactions with that of biosynthetic reactions are different
or less efficient in autotrophs than in heterotrophs. Recent studies on
the mechanism of carbon dioxide assimilation have shown, as will be
discussed in Section I,C, that energy is transferred by the same type of
phosphorylated compounds in the two types of microorganisms.
The manner in which the heterotrophic bacteria fix and assimilate
carbon is not yet known precisely and may actually vary from one substrate or organism to the next. As a first approximation, it is likely that
the starting point of the biosyntheses is a compound in which the carbon is in an oxidation state corresponding to (CHO), that is, intermediate between that of C0 2 and of the sugars and analogous to that
of pyruvate. It is obvious that the biosynthesis of cell constituents from
a carbon compound of this type requires fewer transfers and hence
fewer energy losses, even if each stage in the reduction of carbon has
353
The first experimental approach along these lines was carried out
by Winogradsky (12) who measured the N/C ratio (nitrogen oxidized
and carbon assimilated) for cultures of nitrifying bacteria and found
an average value of 35 for Nitrosomonas and of 135 for Nitrobacter.
Under the most favorable culture conditions, Meyerhof (13) later obtained a value of 101 for the N/C ratio of Nitrobacter. These figures
show that the assimilation of a carbon atom corresponds to the oxidation by Nitrobacter of 87 molecules of nitrite to nitrate. From the heat
of combustion of nitrite, Meyerhof thus calculated the energy efficiency
of Nitrobacter and arrived at the remarkably low figure of 4.5%.
Baas-Becking and Parks (10) reopened this question by considering
not the heats of combustion but the free energy change during the
oxidation of nitrogen-containing substrates under the conditions of the
bacterial cultures:
Nitrosomonas spp.
NH 4
+ (5 X 10~
3 M) + %0 2 = N0 2 - + H 2 0 + 2H+(1()8
)
(-AV = 66.5 kcal.)
Nitrobacter spp.
N0 2 -(7 X 103 M) + }40ι = NO3(-Δ^ 298 ο = 17.5 kcal.)
Starting with the metabolic balance sheets of Winogradsky and of
Meyerhof, then, the free energy efficiency of Nitrosomonas is 5.9% and
that of Nitrobacter is 7.8%. Baas-Becking and Parks also calculated the
free energy efficiencies of several other chemo-lithotrophic bacteria by
this method. The results (Table IV) show values between 5 and 9%, except for Hydrogenomonas with a 30% efficiency; these values are much
lower than those of heterotrophic bacteria which reach or even surpass
efficiencies of 60%. This difference in total efficiency does not necessarily
indicate that the energy-transport systems which connect the sequence
of catabolic reactions with that of biosynthetic reactions are different
or less efficient in autotrophs than in heterotrophs. Recent studies on
the mechanism of carbon dioxide assimilation have shown, as will be
discussed in Section I,C, that energy is transferred by the same type of
phosphorylated compounds in the two types of microorganisms.
The manner in which the heterotrophic bacteria fix and assimilate
carbon is not yet known precisely and may actually vary from one substrate or organism to the next. As a first approximation, it is likely that
the starting point of the biosyntheses is a compound in which the carbon is in an oxidation state corresponding to (CHO), that is, intermediate between that of C0 2 and of the sugars and analogous to that
of pyruvate. It is obvious that the biosynthesis of cell constituents from
a carbon compound of this type requires fewer transfers and hence
fewer energy losses, even if each stage in the reduction of carbon has