Chemosynthesis is a metabolic pathway used by some bacteria to synthesize new organic compounds such as carbohydrates by using energy derived from the oxidation of inorganic molecules—hydrogen sulfide (H₂S) or ammonia (NH₃). Chemosynthesis can occur in environments such as the deep ocean around hydrothermal vents, where sunlight does not penetrate, but where chemicals like hydrogen sulfide are available. Chemosynthesis is also a critical part of the nitrogen cycle, where bacteria that live in the soil, or in special plant structures called heterocysts, utilize ammonia for energy and produce nitrates and nitrites which can subsequently be used as nutrients for plants. Some bacteria can also utilize hydrogen gas (H₂) and carbon dioxide (CO₂) in a chemosynthetic pathway that results in the production of new organic compounds and methane (CH₄).