400
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
Ferrous sulfate in sulfuric acid medium is oxidized rapidly by the
bacteria,
4FeS0 4 + 2H2SO4 + 0 2 -> 2Fe 2 (S0 4 )3 + 2H 2 0
and the resultant ferric sulfate finally is hydrolyzed completely to ferric
hydroxide. The oxidation of ferric sulfate accelerates the chemical oxidation of the pyrite by a mass action law effect and, moreover, Fe 2 (S0 4 ) 3
catalyzes the oxidation of sulfuritic compounds; this explains why bacterial activity speeds the over-all process considerably.
B. OXIDATION OF THIOCYANATE
Beijerinck (213) mentioned that enrichment cultures of thiobacilli
degrade thiocyanate when this compound replaces thiosulfate in the
medium. In 1937, Happold and Key (214) isolated a bacterium, later
called Thiobacillus thiocyanooxidans, from the residual waters of gasworks and from sewage sludge. This organism, which was studied in
detail by Happold et al. (215) and Youatt (216), is a strict aerobic
autotroph. It can grow, like the other members of the genus (Thiobacillus, by oxidizing elementary sulfur or thiosulfate, but it can also utilize
thiocyanate as the sole source of energy, carbon, and nitrogen. In the
latter case, its over-all metabolism corresponds to this equation:
CNS- + 20 2 + 2H 2 0 = S0 4
2 - + NH 4 + C0 2 + 220 kcal.
According to Youatt, this process comprises several successive steps.
The first is the hydrolysis of thiocyanate to cyanate and sulfide:
CNS" + H 2 0 -+ HCNO + SHSulfide is then oxidized to sulfate by the same process as in other
thiobacilli and cyanate is decomposed to carbon dioxide and ammonia
by a second hydrolysis:
HCNO + H 2 0 -> C0 2 + NH3
This last reaction yields the carbon and nitrogen sources the organism uses for growth. If the bacterium is grown on thiosulfate, carbonate
and an ammonium salt must of course be added to provide a complete
culture medium.
Sulfide is relatively toxic to T. thiocyanooxidans and its concentration in the medium may not rise above 1.6 X 10~
3
M. Youatt elegantly
demonstrated the oxidation of sulfide by growing the bacterium in a
medium containing thioacetamide or thioacetate as substrate; these
compounds are progressively hydrolyzed and yield sulfide without pushing its concentration beyond the inhibitory threshold. The observations
of this author also seem to indicate that the enzymes responsible for
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
Ferrous sulfate in sulfuric acid medium is oxidized rapidly by the
bacteria,
4FeS0 4 + 2H2SO4 + 0 2 -> 2Fe 2 (S0 4 )3 + 2H 2 0
and the resultant ferric sulfate finally is hydrolyzed completely to ferric
hydroxide. The oxidation of ferric sulfate accelerates the chemical oxidation of the pyrite by a mass action law effect and, moreover, Fe 2 (S0 4 ) 3
catalyzes the oxidation of sulfuritic compounds; this explains why bacterial activity speeds the over-all process considerably.
B. OXIDATION OF THIOCYANATE
Beijerinck (213) mentioned that enrichment cultures of thiobacilli
degrade thiocyanate when this compound replaces thiosulfate in the
medium. In 1937, Happold and Key (214) isolated a bacterium, later
called Thiobacillus thiocyanooxidans, from the residual waters of gasworks and from sewage sludge. This organism, which was studied in
detail by Happold et al. (215) and Youatt (216), is a strict aerobic
autotroph. It can grow, like the other members of the genus (Thiobacillus, by oxidizing elementary sulfur or thiosulfate, but it can also utilize
thiocyanate as the sole source of energy, carbon, and nitrogen. In the
latter case, its over-all metabolism corresponds to this equation:
CNS- + 20 2 + 2H 2 0 = S0 4
2 - + NH 4 + C0 2 + 220 kcal.
According to Youatt, this process comprises several successive steps.
The first is the hydrolysis of thiocyanate to cyanate and sulfide:
CNS" + H 2 0 -+ HCNO + SHSulfide is then oxidized to sulfate by the same process as in other
thiobacilli and cyanate is decomposed to carbon dioxide and ammonia
by a second hydrolysis:
HCNO + H 2 0 -> C0 2 + NH3
This last reaction yields the carbon and nitrogen sources the organism uses for growth. If the bacterium is grown on thiosulfate, carbonate
and an ammonium salt must of course be added to provide a complete
culture medium.
Sulfide is relatively toxic to T. thiocyanooxidans and its concentration in the medium may not rise above 1.6 X 10~
3
M. Youatt elegantly
demonstrated the oxidation of sulfide by growing the bacterium in a
medium containing thioacetamide or thioacetate as substrate; these
compounds are progressively hydrolyzed and yield sulfide without pushing its concentration beyond the inhibitory threshold. The observations
of this author also seem to indicate that the enzymes responsible for