Chemosynthesis uses chemical nutrients as an energy source, unlike photosynthesis, which uses sunlight. Although most life on Earth uses photosynthesis to obtain energy, there are groups of bacteria known as chemosynthetic autotrophs that use chemosynthesis instead.
Chemosynthetic autotrophs survive by oxidizing simple inorganic molecules, such as sulfates, ammonia and methane. This makes them an important part of the nitrogen cycle. Chemosynthetic autotrophs can survive in extremely harsh environments; many are found around hydrothermal vents on the ocean floor. Due to their ability to thrive in less that ideal conditions, it is a widely held belief that organisms capable of chemosynthesis were some of Earth's earliest life forms.