Читать книгу Fundamentals of Aquatic Veterinary Medicine - Группа авторов - Страница 34

Hydrogen Sulfide (H₂S) is a poisonous gas with a “rotten egg” smell, produced by anaerobic decomposition of organics. Sulfur is an essential element for plants, animals and bacteria. H₂S is present in natural waters and in aquaculture systems, mainly as the sulfate ion. Sulfide can occur in water because it is a metabolite of Desulfovibrio species and certain other bacteria found in anaerobic zones, usually in sediment. These bacteria use oxygen from sulfate as an alternative to molecular oxygen in respiration. There are three forms of sulfide (H₂S, HS^– and S₂^–), and they exist in a pH and temperature‐dependent equilibrium. As pH increases, the proportion of H₂S declines, and that of HS^– rises until the two forms have roughly equal proportions at pH 7. At greater pH, HS^– is the dominant form, and there is no S₂^– until the pH is above 11. Hydrogen sulfide is toxic to aquatic animals because it interferes with reoxidation of cytochrome a3 in respiration. This effect is caused almost entirely by H₂S, while HS^– is essentially non‐toxic. Even if it is toxic, S₂^– is not an issue, because it does not occur at pH values found in aquaculture systems.