Hydrogen sulfide is formed only in the absence of oxygen but may occur in intensively managed tilapia ponds where heavy feeding leads to an accumulation of organic detritus, severe prolonged anoxia in bottom sediments and subsequent formation of H2S. Hydrogen sulfide occurs as a byproduct of anaerobic metabolism of heterotrophic bacteria which can use sulfate and other oxidized sulfur compounds as terminal electron receptors in oxidative phosphorylation.
Hydrogen sulfide dissociates in water into H2S, HS- and S-. Only the un-ionized H2S is toxic because it readily passes through biological membranes.
The dissociation of hydrogen sulfide in water:
H2S <==> HS- + H+ <==> S2- + H+
The dissociation of hydrogen sulfide is strongly influenced by pH and to a lesser extent temperature and salinity. The effect of temperature and pH on the percent of hydrogen sulfide as H2S is given below:
PERCENT HYDROGEN SULFIDE
IN THE TOXIC H2S FORM:
Temperature (øC) |
||||
| pH | 24 | 26 | 28 | 30 |
| 7.0 | 51 | 50 | 48 | 47 |
| 8.0 | 10 | 9.0 | 8.5 | 8.0 |
| 9.0 | 1 | 1 | 0.9 | 0.9 |
It is obvious from this table that low pH conditions dramatically increase the formation of the toxic form, H2S. Hydrogen sulfide is a potent toxicant that is an indiscriminant poison. Hydrogen sulfide kills by blocking oxidative phosphorylation. In other words, at the subcellular level H2S poisons the tilapia by depriving the animal of ATP. Clinically, animals display symptoms of tissue hypoxia (e.g. as in oxygen depletion).
In intensive tilapia ponds H2S may be present in anaerobic sediments and if these sediments are disturbed H2S can be released into the water column. Hydrogen sulfide is rapidly transformed in the presence of oxygen, but toxic levels may occur within a few centimeters of highly anoxic sediments. Hydrogen sulfide is an acute toxicant and exposure leads rapidly to death. Tilapia are apparently susceptible to toxic levels of H2S in the 10 to 200 ppb range. Several authors indicate that sublethal exposure can negatively effect growth.
The rotten egg odor of hydrogen sulfide can be detected by the human nose when it is at a concentration of 2 to 5 ppb. Hydrogen sulfide can be toxic to people if sufficient exposure occurs.