tions in the light, using sulfide as electron donor (Cohen et al, 1975a; Cohen et al, 1975b;
Garlick et al, 1977).
In the dark, cyanobacteria usually generate energy for maintenance and other purposes
by aerobic respiration (Smith, 1982). They oxidize their reserve polymer glycogen via the
oxidative pentose phosphate pathway (Smith, 1982). The NADPH formed is oxidized via
the respiratory chain with oxygen as terminal electron acceptor.
Cyanobacteria in microbial mats have to cope with anaerobic conditions in the dark. The
metabolism of cyanobacteria under such conditions was subject of this study. We have
shown that two cyanobacteria isolated from a marine microbial mat possess mechanisms
of anaerobic dark metabolism. Oscillatoria strain 23 can ferment endogenous carbon
reserves to lactate or, alternatively, reduces elemental sulfur to sulfide. M. chthonoplastes, on the other hand, only reduces sulfur.
There are several other reports on anaerobic dark metabolism in phototrophic organisms.
Lactic acid fermentation is known in some green algae (Gibbs, 1962). Dark sulfide
production is known in purple sulfur bacteria (Trüper and Pfennig, 1966). Van Gemerden
(1968) reported the oxidation of storage polyglucose to poly-β-hydroxybutyric acid using
endogenous sulfur as electron acceptor. The cyanobacterium Synechococcus lividus
reduces sulfate to sulfide or thiosulfate to sulfide and sulfite in the dark anaerobically
(Sheridan and Castenholz, 1968; Sheridan, 1973). Oren and Shilo (1979) reported sulfur
respiration and lactate fermentation in the cyanobacterium Oscillatoria limnetica and
sulfur respiration in Aphanocapsa halophytica.
Oscillaroria sp. strain 23 is an interesting organism to study because of its capabality to
synthesize nitrogenase. Nitrogenase activity is also observed under dark anaerobic conditions. The observation, that, in the presence of a nitrogenase reducable substrate (e.g.
acetylene), neither lactate nor sulfide is produced, but exclusively ethylene, raises several
questions. The production of lactate serves probably not only as a sink for electrons, but
the excretion of lactate into the medium eventually is an energy generating process (Otto,
1981). The absence of lactate production when acetylene is present, however, makes it
likely that lactate serves predominantly as a sink for electrons. The same argument applies
for sulfur reduction. On the other hand, in the absence of a nitrogenase-reducable
substrate, but in cells containing nitrogenase enzyme, lactate is the product even in the
presence of elemental sulfur. This is different in cells grown on nitrate and thus not
containing nitrogenase. In this case we find sulfide when elemental sulfur is added and
lactate in the absence of sulfur. We do not have a satisfactory explanation for this
observation. One possibility is that nitrogenase is coupled to an electron transport system
which also transfers electrons to sulfide. Nitrogenase may block electron transfer to sulfur
in that case.
Many organisms are able to reduce elemental sulfur to sulfide (Pfennig and Biebl, 1981).
However, in many organisms it is not clear whether this is a true dissimilatory sulfur
reduction. In 1976 the first true dissimilatory sulfur reducing bacteria were isolated.
Pfennig and Biebl (1976) isolated Desulfuromonas acetoxidans. All strains oxidize
acetate to CO2, using elemental sulfur as electron acceptor. In our organism we cannot
decide whether the sulfur reduction is dissimilatory or just a sink for electrons, without
the additional generation of energy. Also in Oscillatoria limnetica there is no evidence for
a true dissimilatory sulfur reduction.
Oscillatoria limnetica produced twice as much lactate per glucose metabolized than
Oscillatoria sp. strain 23. The amount of lactate produced per glucose metabolized in our
strain was about 0.8 moles. The same applies for the production of sulfide. For Oscillatoria as well as for M. chthonoplastes we found approximately 0.8 moles of sulfide formed
307
Garlick et al, 1977).
In the dark, cyanobacteria usually generate energy for maintenance and other purposes
by aerobic respiration (Smith, 1982). They oxidize their reserve polymer glycogen via the
oxidative pentose phosphate pathway (Smith, 1982). The NADPH formed is oxidized via
the respiratory chain with oxygen as terminal electron acceptor.
Cyanobacteria in microbial mats have to cope with anaerobic conditions in the dark. The
metabolism of cyanobacteria under such conditions was subject of this study. We have
shown that two cyanobacteria isolated from a marine microbial mat possess mechanisms
of anaerobic dark metabolism. Oscillatoria strain 23 can ferment endogenous carbon
reserves to lactate or, alternatively, reduces elemental sulfur to sulfide. M. chthonoplastes, on the other hand, only reduces sulfur.
There are several other reports on anaerobic dark metabolism in phototrophic organisms.
Lactic acid fermentation is known in some green algae (Gibbs, 1962). Dark sulfide
production is known in purple sulfur bacteria (Trüper and Pfennig, 1966). Van Gemerden
(1968) reported the oxidation of storage polyglucose to poly-β-hydroxybutyric acid using
endogenous sulfur as electron acceptor. The cyanobacterium Synechococcus lividus
reduces sulfate to sulfide or thiosulfate to sulfide and sulfite in the dark anaerobically
(Sheridan and Castenholz, 1968; Sheridan, 1973). Oren and Shilo (1979) reported sulfur
respiration and lactate fermentation in the cyanobacterium Oscillatoria limnetica and
sulfur respiration in Aphanocapsa halophytica.
Oscillaroria sp. strain 23 is an interesting organism to study because of its capabality to
synthesize nitrogenase. Nitrogenase activity is also observed under dark anaerobic conditions. The observation, that, in the presence of a nitrogenase reducable substrate (e.g.
acetylene), neither lactate nor sulfide is produced, but exclusively ethylene, raises several
questions. The production of lactate serves probably not only as a sink for electrons, but
the excretion of lactate into the medium eventually is an energy generating process (Otto,
1981). The absence of lactate production when acetylene is present, however, makes it
likely that lactate serves predominantly as a sink for electrons. The same argument applies
for sulfur reduction. On the other hand, in the absence of a nitrogenase-reducable
substrate, but in cells containing nitrogenase enzyme, lactate is the product even in the
presence of elemental sulfur. This is different in cells grown on nitrate and thus not
containing nitrogenase. In this case we find sulfide when elemental sulfur is added and
lactate in the absence of sulfur. We do not have a satisfactory explanation for this
observation. One possibility is that nitrogenase is coupled to an electron transport system
which also transfers electrons to sulfide. Nitrogenase may block electron transfer to sulfur
in that case.
Many organisms are able to reduce elemental sulfur to sulfide (Pfennig and Biebl, 1981).
However, in many organisms it is not clear whether this is a true dissimilatory sulfur
reduction. In 1976 the first true dissimilatory sulfur reducing bacteria were isolated.
Pfennig and Biebl (1976) isolated Desulfuromonas acetoxidans. All strains oxidize
acetate to CO2, using elemental sulfur as electron acceptor. In our organism we cannot
decide whether the sulfur reduction is dissimilatory or just a sink for electrons, without
the additional generation of energy. Also in Oscillatoria limnetica there is no evidence for
a true dissimilatory sulfur reduction.
Oscillatoria limnetica produced twice as much lactate per glucose metabolized than
Oscillatoria sp. strain 23. The amount of lactate produced per glucose metabolized in our
strain was about 0.8 moles. The same applies for the production of sulfide. For Oscillatoria as well as for M. chthonoplastes we found approximately 0.8 moles of sulfide formed
307