8. REACTIONS OF INORGANIC SUBSTANCES
367
the bacterial agents responsible for this phenomenon; one, Nitrosomonas, oxidizes ammonia to nitrite
NH 4
+ + K0 2 = NO2- + H 2 0 + 2H+
(-Δ^° 29 8° = 66.5 kcal.)
while the other, Nitrobacter, oxidizes nitrite to nitrate
NO2- + K0 2 = NO3(-Δ^°298° = 17.5 kcal.)
Winogradsky established that these organisms depend entirely on
the oxidation of their respective inorganic substrates for energy, and
this discovery led him to formulate for the first time the concept of the
chemo-autotrophic way of life. He also showed that the two nitrification
reactions proceed successively in the soil or in enrichment cultures; the
oxidation of nitrite to nitrate by Nitrobacter is inhibited by NH 4
+ and
cannot begin until all the ammonia initially present has been oxidized
to nitrate by Nitrosomonas.
"Bergey's Manual" (61) describes two species in the genus Nitrosomonas and four other genera of chemo-lithotrophic bacteria which
oxidize nitrite to nitrate. Of the latter four, the existence of only one,
Nitrosococcus, described by Winogradsky himself (60), has been unquestioned. The three others (N itrosospira, Nitrosocystis, and Nitrosogloea) are of doubtful validity, according to several authors (62).
Among the organisms which carry out the nitrite-nitrate step, the genus
Nitrobacter includes a mobile species described and isolated by Nelson
(63) in addition to the type species which is the immobile form discovered by Winogradsky. A second genus, Nitroaystis, brings together
forms endowed with the same physiological activity but which are
morphologically distinct because of the production of zoogloea.
The literature on the taxonomy and physiology of nitrifying bacteria
was analyzed recently by Bisset and Grace (62) and Meiklejohn (64).
2. Heterotrophic Nitrifying Microorganisms
There are numerous heterotrophic bacteria which oxidize ammonium
salts anaerobically to nitrite or nitrate but cannot utilize the resulting
energy for growth. According to Cutler and Crump (65), 104 different
species produce traces of nitrite from ammonium ions in their cultures.
More recently, Quastel et al. (66) showed by their soil perfusion
method that several heterotrophic bacteria which cannot oxidize NH 4
+
nevertheless form nitrite and nitrate from pyruvic oxime. In contrast
to Nitrobacter and Nitrosomonas, these organisms are not inhibited by
methionine, thiourea, or ethyl urethan and can even oxidize the latter
compound with the formation of nitrate. Jensen (67) made similar observations with bacteria of the genera Alcaligenes and Aerobacterium
and with the mold Nocardia.
367
the bacterial agents responsible for this phenomenon; one, Nitrosomonas, oxidizes ammonia to nitrite
NH 4
+ + K0 2 = NO2- + H 2 0 + 2H+
(-Δ^° 29 8° = 66.5 kcal.)
while the other, Nitrobacter, oxidizes nitrite to nitrate
NO2- + K0 2 = NO3(-Δ^°298° = 17.5 kcal.)
Winogradsky established that these organisms depend entirely on
the oxidation of their respective inorganic substrates for energy, and
this discovery led him to formulate for the first time the concept of the
chemo-autotrophic way of life. He also showed that the two nitrification
reactions proceed successively in the soil or in enrichment cultures; the
oxidation of nitrite to nitrate by Nitrobacter is inhibited by NH 4
+ and
cannot begin until all the ammonia initially present has been oxidized
to nitrate by Nitrosomonas.
"Bergey's Manual" (61) describes two species in the genus Nitrosomonas and four other genera of chemo-lithotrophic bacteria which
oxidize nitrite to nitrate. Of the latter four, the existence of only one,
Nitrosococcus, described by Winogradsky himself (60), has been unquestioned. The three others (N itrosospira, Nitrosocystis, and Nitrosogloea) are of doubtful validity, according to several authors (62).
Among the organisms which carry out the nitrite-nitrate step, the genus
Nitrobacter includes a mobile species described and isolated by Nelson
(63) in addition to the type species which is the immobile form discovered by Winogradsky. A second genus, Nitroaystis, brings together
forms endowed with the same physiological activity but which are
morphologically distinct because of the production of zoogloea.
The literature on the taxonomy and physiology of nitrifying bacteria
was analyzed recently by Bisset and Grace (62) and Meiklejohn (64).
2. Heterotrophic Nitrifying Microorganisms
There are numerous heterotrophic bacteria which oxidize ammonium
salts anaerobically to nitrite or nitrate but cannot utilize the resulting
energy for growth. According to Cutler and Crump (65), 104 different
species produce traces of nitrite from ammonium ions in their cultures.
More recently, Quastel et al. (66) showed by their soil perfusion
method that several heterotrophic bacteria which cannot oxidize NH 4
+
nevertheless form nitrite and nitrate from pyruvic oxime. In contrast
to Nitrobacter and Nitrosomonas, these organisms are not inhibited by
methionine, thiourea, or ethyl urethan and can even oxidize the latter
compound with the formation of nitrate. Jensen (67) made similar observations with bacteria of the genera Alcaligenes and Aerobacterium
and with the mold Nocardia.