<P> Several groups of bacteria can use hydrogen sulfide as fuel, oxidizing it to elemental sulfur or to sulfate by using dissolved oxygen, metal oxides (e.g., Fe oxyhydroxides and Mn oxides), or nitrate as electron acceptors . </P> <P> The purple sulfur bacteria and the green sulfur bacteria use hydrogen sulfide as electron donor in photosynthesis, thereby producing elemental sulfur . (In fact, this mode of photosynthesis is older than the mode of cyanobacteria, algae, and plants, which uses water as electron donor and liberates oxygen .) </P> <P> The biochemistry of hydrogen sulfide is a key part of the chemistry of the iron - sulfur world . In this model of the origin of life on Earth, geologically produced hydrogen sulfide is postulated as an electron donor driving the reduction of carbon dioxide . </P> <P> Hydrogen sulfide is lethal to most animals, but a few highly specialized species (extremophiles) do thrive in habitats that are rich in this compound . </P>

Hydrogen sulfide (h2s) is a covalent compound