8. REACTIONS OF INORGANIC SUBSTANCES
385
not yet been clarified completely; in particular, it remains to be seen
whether the liberated hydrogen comes from the photochemical reaction,
from the oxidation of organic substrates, or from both of these sources
at the same time (94).
Among the Athiorhodaceae, hydrogenase is involved in yet another
metabolic process. In the dark, resting cells of R. rubrum (138) consume
H 2 and simultaneously reduce a variety of acceptors, e.g., oxygen, nitrate,
thiosulfate, ferricyanide, fumarate, and dyes; in this organism, then, the
function of the hydrogenase is not necessarily connected with photosynthesis. However, these cells cannot use this mode of oxidation of
hydrogen as an energy source to reduce C0 2 and develop chemolithotrophically.
In addition to chlorophyll-containing bacteria, a hydrogenase has
been demonstrated in other photosynthetic organisms, particularly in
algae. Gaffron (140) showed that the green alga Scenedesmus is capable
of photoevolution of H 2 and acquires strong hydrogenase activity when
incubated in the dark under hydrogen. The adapted cells consume H 2
either in the light, where they carry out photosynthesis similar to that
of bacteria, or in the dark, where they can use the energy derived from
the anaerobic oxidation of H 2 to reduce C0 2 chemo-lithotrophically.
Photoevolution of H 2 has also been reported in another green alga,
£hlamydomonas moewusii, by Frenkel (141), and in various blue-green
brown, and red algae by Frenkel and Rieger (142). In these algae, however, consumption of molecular hydrogen has been observed neither in
the dark nor in the light.
The physiological relationships between hydrogenase and photosynthesis are complicated by the fact that in certain phototrophic organisms the consumption or evolution of H 2 , i.e., the reactions in which
hydrogenase participates, are dissociated or take place only in the light,
whereas in others all the reactions can occur either in light or darkness.
According to Gest (143), who discusses this problem in detail, dependence on light may be secondary and connected with the photochemical
formation of an organic precursor by which the electrons are transferred to the hydrogenase system.
Krasna and Rittenberg (144) have stated that in biochemical evolution hydrogenase may have preceded the appearance of the photosynthetic system of higher plants, i.e., the system leading to the formation
of free oxygen. Moreover, the same authors pointed out that hydrogenase
may be a ferroprotein and that its molecule may be the precursor of the
hematinic respiratory enzymes of animals such as the cytochromes, catalase, and peroxidase. In the framework of this phylogenetic theory, the
chlorophyll-containing bacteria and the hydrogenase-containing algae
385
not yet been clarified completely; in particular, it remains to be seen
whether the liberated hydrogen comes from the photochemical reaction,
from the oxidation of organic substrates, or from both of these sources
at the same time (94).
Among the Athiorhodaceae, hydrogenase is involved in yet another
metabolic process. In the dark, resting cells of R. rubrum (138) consume
H 2 and simultaneously reduce a variety of acceptors, e.g., oxygen, nitrate,
thiosulfate, ferricyanide, fumarate, and dyes; in this organism, then, the
function of the hydrogenase is not necessarily connected with photosynthesis. However, these cells cannot use this mode of oxidation of
hydrogen as an energy source to reduce C0 2 and develop chemolithotrophically.
In addition to chlorophyll-containing bacteria, a hydrogenase has
been demonstrated in other photosynthetic organisms, particularly in
algae. Gaffron (140) showed that the green alga Scenedesmus is capable
of photoevolution of H 2 and acquires strong hydrogenase activity when
incubated in the dark under hydrogen. The adapted cells consume H 2
either in the light, where they carry out photosynthesis similar to that
of bacteria, or in the dark, where they can use the energy derived from
the anaerobic oxidation of H 2 to reduce C0 2 chemo-lithotrophically.
Photoevolution of H 2 has also been reported in another green alga,
£hlamydomonas moewusii, by Frenkel (141), and in various blue-green
brown, and red algae by Frenkel and Rieger (142). In these algae, however, consumption of molecular hydrogen has been observed neither in
the dark nor in the light.
The physiological relationships between hydrogenase and photosynthesis are complicated by the fact that in certain phototrophic organisms the consumption or evolution of H 2 , i.e., the reactions in which
hydrogenase participates, are dissociated or take place only in the light,
whereas in others all the reactions can occur either in light or darkness.
According to Gest (143), who discusses this problem in detail, dependence on light may be secondary and connected with the photochemical
formation of an organic precursor by which the electrons are transferred to the hydrogenase system.
Krasna and Rittenberg (144) have stated that in biochemical evolution hydrogenase may have preceded the appearance of the photosynthetic system of higher plants, i.e., the system leading to the formation
of free oxygen. Moreover, the same authors pointed out that hydrogenase
may be a ferroprotein and that its molecule may be the precursor of the
hematinic respiratory enzymes of animals such as the cytochromes, catalase, and peroxidase. In the framework of this phylogenetic theory, the
chlorophyll-containing bacteria and the hydrogenase-containing algae