356
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
(pH 2.0) and a large proportion of the energy furnished by the substrate may possibly be used to regulate the endocellular pH.
Lees (14) postulates that in other chemo-lithotrophic bacteria the
energy efficiency of growth may also be reduced by the necessity of
regulating the composition of the endocellular milieu. For example,
nitrite produced by Nitrosomonas from ammonium ions is toxic for this
organism and is possibly excreted by an active transport system at a
considerable expenditure of energy.
b. Photo-lithotrophic Bacteria. This group includes the purple sulfur
bacteria or Thiorhodaceae and the green sulfur bacteria or Chlorobacteriaceae. These organisms possess a chlorophyll which is chemically
very similar to plant chlorophylls and they develop anaerobically in a
purely inorganic medium, where C0 2 is the sole carbon source, if light
and an exogenous electron donor are provided. The typical donors are
sulfide and, for certain species, thiosulfate or tetrathionate, which are
oxidized to elementary sulfur and sulfate. The sulfur compounds may
be replaced by molecular hydrogen which is oxidized to water, and the
purple sulfur bacteria can also utilize various organic compounds such
as mono- or dicarboxylic acids as electron donors.
The nature of the source of energy used by these bacteria to assimilate carbon dioxide and carry out their biosyntheses has been the subject of a lengthy and interesting controversy. Winogradsky (15), who
discovered them in 1888, considered these organisms as chemo-lithotrophs using for growth the chemical energy produced by the aerobic
oxidation of sulfide to sulfate. Later, it was recognized that they are
photosynthetic organisms related to the purple nonsulfur bacteria or
Athiorhodaceae which Engelmann (16) had discovered in 1882; however, their physiology and energy metabolism remained obscure until
1931, when van Niel (17) began his masterful series of investigations on
this subject. On the basis of a large body of experimental evidence, van
Niel was led to postulate that the fundamental reaction of photosynthesis,
in green plants as in bacteria, consists of the photolysis of water catalyzed
by chlorophyll, with the formation of a reducing constituent (H) which
the organism uses to hydrogenate C0 2 , and of an oxidizing constituent
(OH). In green plants the (OH) constituent is finally eliminated as free
oxygen. The important finding that the oxygen formed during chlorophyll-mediated assimilation in green plants is derived from water and
not, as believed earlier, from C0 2 was confirmed experimentally some
years later by Calvin and his school with water labeled with heavy
oxygen.
Moreover, van Niel showed that the green and purple sulfur bacteria are strict anaerobes and that, contrary to Winogradsky's conclu-
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
(pH 2.0) and a large proportion of the energy furnished by the substrate may possibly be used to regulate the endocellular pH.
Lees (14) postulates that in other chemo-lithotrophic bacteria the
energy efficiency of growth may also be reduced by the necessity of
regulating the composition of the endocellular milieu. For example,
nitrite produced by Nitrosomonas from ammonium ions is toxic for this
organism and is possibly excreted by an active transport system at a
considerable expenditure of energy.
b. Photo-lithotrophic Bacteria. This group includes the purple sulfur
bacteria or Thiorhodaceae and the green sulfur bacteria or Chlorobacteriaceae. These organisms possess a chlorophyll which is chemically
very similar to plant chlorophylls and they develop anaerobically in a
purely inorganic medium, where C0 2 is the sole carbon source, if light
and an exogenous electron donor are provided. The typical donors are
sulfide and, for certain species, thiosulfate or tetrathionate, which are
oxidized to elementary sulfur and sulfate. The sulfur compounds may
be replaced by molecular hydrogen which is oxidized to water, and the
purple sulfur bacteria can also utilize various organic compounds such
as mono- or dicarboxylic acids as electron donors.
The nature of the source of energy used by these bacteria to assimilate carbon dioxide and carry out their biosyntheses has been the subject of a lengthy and interesting controversy. Winogradsky (15), who
discovered them in 1888, considered these organisms as chemo-lithotrophs using for growth the chemical energy produced by the aerobic
oxidation of sulfide to sulfate. Later, it was recognized that they are
photosynthetic organisms related to the purple nonsulfur bacteria or
Athiorhodaceae which Engelmann (16) had discovered in 1882; however, their physiology and energy metabolism remained obscure until
1931, when van Niel (17) began his masterful series of investigations on
this subject. On the basis of a large body of experimental evidence, van
Niel was led to postulate that the fundamental reaction of photosynthesis,
in green plants as in bacteria, consists of the photolysis of water catalyzed
by chlorophyll, with the formation of a reducing constituent (H) which
the organism uses to hydrogenate C0 2 , and of an oxidizing constituent
(OH). In green plants the (OH) constituent is finally eliminated as free
oxygen. The important finding that the oxygen formed during chlorophyll-mediated assimilation in green plants is derived from water and
not, as believed earlier, from C0 2 was confirmed experimentally some
years later by Calvin and his school with water labeled with heavy
oxygen.
Moreover, van Niel showed that the green and purple sulfur bacteria are strict anaerobes and that, contrary to Winogradsky's conclu-