A chemoautotroph is an organism that produces its cellular energy, usually in the form of ATP, directly from the environment using a chemical source.
There are a number of groups of bacteria that produce energy this way. One is the methanogens, a group which are commonly found in environments that are low in oxygen but rich in carbon compounds. Methanogens typically reduce carbon dioxide, capturing energy and releasing methane or a related compound such as methanol as a waste product.
Nitrifying bacteria such as Nitrobacter species obtain energy by converting nitrate compounds into nitrate compounds. These bacteria are very important in the nitrogen cycle.
Some members of the bacterial genus Planctomycetes oxidize environmental ammonia in order to harvest energy. This is a particularly interesting group; in the process of making cellular energy, they can produce hydrazine, which is an important rocket fuel.