8. REACTIONS OF INORGANIC SUBSTANCES
371
able to expect that the nitrification enzymes are entirely different in
nature from those involved in the reduction of nitrate to ammonia.
The oxidation of nitrite to nitrate by Nitrobacter most probably
takes place in a single enzymatic step, but the nature of this enzyme is
unknown. It has long been known that this reaction is inhibited by
chlorate; recently, Lees and Simpson (83) demonstrated that this inhibitor has an indirect effect. It is first transformed by the bacteria to
chlorite which is the true specific inhibitor. These authors showed that
the rate of oxidation of nitrite is proportional to the content in Nitrobacter cells of a cytochrome purified and studied by Butt and Lees
(84). This is a cytochrome c with an absorption maximum at 551 τημ
which takes on a functional role in the electron transport in the oxidation of nitrite. Finally, Zavarzin (85) indicated that molybdenum and
riboflavin take part in this reaction.
III. Oxidation of Sulfur and Sulfur Compounds
A. SULFO-OXIDIZING BACTERIA
1. Thiobacilli
The genus Thiobacillus comprises chemo-lithotrophic sulfo-oxidizing
bacteria lacking chlorophyll and intracellular sulfur granules. The principal species are shown in Table VIII.
Morphologically, these bacteria are thin and short, measuring about
0.5 by 1 to 3 μ, nonsporeforming, Gram-negative, generally mobile and,
in these instances, equipped with a single polar flagellum. Very widely
distributed in nature, the thiobacilli, like the morphologically quite
similar Pseudomonas, are constant inhabitants of soil and fresh or salt
water.
All the species of thiobacilli use for growth the energy provided by
the oxidation of sulfide and of thiosulfate:
HS- + 20 2 = S0 4
2 "
(-AFV = +171 kcal.)
S 2 0 3
2 - + 2.5 0 2 = 2SCV(-AFV = +237.6 kcal.)
Several species can also oxidize elementary sulfur, more or less
actively:
S° + 1.5 0 2 + H 2 0 = S0 4
2 - + 2H+
(-AF° 298 o = +119.9 kcal.)
These reactions, which result in the complete oxidation of sulfur and
the formation of free sulfuric acid, cause a considerable drop in the
pH of the cultures. The tolerance towards this acidity varies in bacteria
from species to species. Thus T. thioparus develops best at near-neutral
371
able to expect that the nitrification enzymes are entirely different in
nature from those involved in the reduction of nitrate to ammonia.
The oxidation of nitrite to nitrate by Nitrobacter most probably
takes place in a single enzymatic step, but the nature of this enzyme is
unknown. It has long been known that this reaction is inhibited by
chlorate; recently, Lees and Simpson (83) demonstrated that this inhibitor has an indirect effect. It is first transformed by the bacteria to
chlorite which is the true specific inhibitor. These authors showed that
the rate of oxidation of nitrite is proportional to the content in Nitrobacter cells of a cytochrome purified and studied by Butt and Lees
(84). This is a cytochrome c with an absorption maximum at 551 τημ
which takes on a functional role in the electron transport in the oxidation of nitrite. Finally, Zavarzin (85) indicated that molybdenum and
riboflavin take part in this reaction.
III. Oxidation of Sulfur and Sulfur Compounds
A. SULFO-OXIDIZING BACTERIA
1. Thiobacilli
The genus Thiobacillus comprises chemo-lithotrophic sulfo-oxidizing
bacteria lacking chlorophyll and intracellular sulfur granules. The principal species are shown in Table VIII.
Morphologically, these bacteria are thin and short, measuring about
0.5 by 1 to 3 μ, nonsporeforming, Gram-negative, generally mobile and,
in these instances, equipped with a single polar flagellum. Very widely
distributed in nature, the thiobacilli, like the morphologically quite
similar Pseudomonas, are constant inhabitants of soil and fresh or salt
water.
All the species of thiobacilli use for growth the energy provided by
the oxidation of sulfide and of thiosulfate:
HS- + 20 2 = S0 4
2 "
(-AFV = +171 kcal.)
S 2 0 3
2 - + 2.5 0 2 = 2SCV(-AFV = +237.6 kcal.)
Several species can also oxidize elementary sulfur, more or less
actively:
S° + 1.5 0 2 + H 2 0 = S0 4
2 - + 2H+
(-AF° 298 o = +119.9 kcal.)
These reactions, which result in the complete oxidation of sulfur and
the formation of free sulfuric acid, cause a considerable drop in the
pH of the cultures. The tolerance towards this acidity varies in bacteria
from species to species. Thus T. thioparus develops best at near-neutral