The "Betaproteobacteria" are a class comprising over 75 genera and 400 species of bacteria. Together, the "Betaproteobacteria" represent a broad variety of metabolic strategies and occupy diverse environments from obligate pathogens living within host organisms to oligotrophic groundwater ecosystems. Whilst most members of the "Betaproteobacteria" are heterotrophic, deriving both their carbon and electrons from organocarbon sources, some are photoheterotrophic, deriving energy from light and carbon from organocarbon sources. Other genera are autotrophic, deriving their carbon from bicarbonate or carbon dioxide and their electrons from reduced inorganic ions such as nitrite, ammonium, thiosulfate or sulfide — many of these chemolithoautotrophic. "Betaproteobacteria" are economically important, with roles in maintaining soil pH and in elementary cycling. Other economically important members of the "Betaproteobacteria" are able to use nitrate as their terminal electron acceptor and can be used industrially to remove nitrate from wastewater by denitrification. A number of "Betaproteobacteria" are diazotrophs, meaning that they can fix molecular nitrogen from the air as their nitrogen source for growth - this is important to the farming industry as it is a primary means of ammonium levels in soils rising without the presence of leguminous plants.