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CLAUDE FROMAGEOT AND JACQUES C. SENEZ
ize acetate and the lower fatty acids; however this species was not reisolated and its existence still requires proof.
D. desulfuricans was one of the first organisms in which Stephenson
and Stickland (133) demonstrated the presence of a hydrogenase which
enables the bacterium to reduce sulfate by utilizing gaseous hydrogen:
4H 2 + H 2 S0 4 -> H 2 S + 4H 2 0
The free energy produced by this reaction, taking into account the
hydrolysis of the resultant sulfide (134), is 29.7 kcal, per mole H 2 consumed. Starkey and Wight (135), Butlin and Adams (136), and Senez
and Volcani (137) reported experimental results showing that D. desulfuricans can utilize this energy source to grow on a completely inorganic medium and must therefore be classed as a facultative chemolithotroph; but the assimilation of C0 2 by this organism and its incorporation into the cell constituents has not yet been demonstrated.
D. desulfuricans is remarkable because it is the only strictly anaerobic
bacterium with a cytochrome, demonstrated by Postgate (134) and
described as cytochrome c 3 . It shares the spectroscopic properties of
mammalian cytochromes c but differs by its very low oxidation-reduction
potential (E' 0 = —0.205 volt) and its autoxidizability. Like mammalian
cytochrome c, cytochrome c 3 has a molecular weight around 12,000 but
its molecule is bifunctional and contains two hemin groups. Postgate
(134) adduced evidence for the functional participation of cytochrome
c 3 in electron transport from hydrogenase to sulfate and other inorganic
sulfur compounds.
2. Phototrophic Organisms
As seen above (cf. Sections I,B,2 and III,A,3), the green and purple
sulfur bacteria possess hydrogenase and oxidize H 2 under anaerobiosis
in the light, but do not utilize the resultant energy for their biosynthetic
activities.
The same mode of photosynthesis is carried out by the nonsulfur
purple bacteria or Athiorhodaceae. These photo-heterotrophs also contain a hydrogenase, demonstrated in several members of the group and
extracted and purified by Gest (138) from cells of Rhodospirillum rubrum. Under certain experimental conditions, the Athiorhodaceae, like
the purple sulfur bacteria, produce gaseous hydrogen during photosvnthesis. This photoevolution of H 2 , discovered by Gest and Kamen
(139), occurs when cells are incubated anaerobically in the presence
of various organic substrates such as acetate, propionate, and n-butyrate.
It is certainly connected with the activity of the hydrogenase which
mediates the liberation of gaseous hydrogen, but its mechanism has
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
ize acetate and the lower fatty acids; however this species was not reisolated and its existence still requires proof.
D. desulfuricans was one of the first organisms in which Stephenson
and Stickland (133) demonstrated the presence of a hydrogenase which
enables the bacterium to reduce sulfate by utilizing gaseous hydrogen:
4H 2 + H 2 S0 4 -> H 2 S + 4H 2 0
The free energy produced by this reaction, taking into account the
hydrolysis of the resultant sulfide (134), is 29.7 kcal, per mole H 2 consumed. Starkey and Wight (135), Butlin and Adams (136), and Senez
and Volcani (137) reported experimental results showing that D. desulfuricans can utilize this energy source to grow on a completely inorganic medium and must therefore be classed as a facultative chemolithotroph; but the assimilation of C0 2 by this organism and its incorporation into the cell constituents has not yet been demonstrated.
D. desulfuricans is remarkable because it is the only strictly anaerobic
bacterium with a cytochrome, demonstrated by Postgate (134) and
described as cytochrome c 3 . It shares the spectroscopic properties of
mammalian cytochromes c but differs by its very low oxidation-reduction
potential (E' 0 = —0.205 volt) and its autoxidizability. Like mammalian
cytochrome c, cytochrome c 3 has a molecular weight around 12,000 but
its molecule is bifunctional and contains two hemin groups. Postgate
(134) adduced evidence for the functional participation of cytochrome
c 3 in electron transport from hydrogenase to sulfate and other inorganic
sulfur compounds.
2. Phototrophic Organisms
As seen above (cf. Sections I,B,2 and III,A,3), the green and purple
sulfur bacteria possess hydrogenase and oxidize H 2 under anaerobiosis
in the light, but do not utilize the resultant energy for their biosynthetic
activities.
The same mode of photosynthesis is carried out by the nonsulfur
purple bacteria or Athiorhodaceae. These photo-heterotrophs also contain a hydrogenase, demonstrated in several members of the group and
extracted and purified by Gest (138) from cells of Rhodospirillum rubrum. Under certain experimental conditions, the Athiorhodaceae, like
the purple sulfur bacteria, produce gaseous hydrogen during photosvnthesis. This photoevolution of H 2 , discovered by Gest and Kamen
(139), occurs when cells are incubated anaerobically in the presence
of various organic substrates such as acetate, propionate, and n-butyrate.
It is certainly connected with the activity of the hydrogenase which
mediates the liberation of gaseous hydrogen, but its mechanism has