[2] Together, they represent a broad variety of metabolic strategies and occupy diverse environments, ranging from obligate pathogens living within host organisms to oligotrophic groundwater ecosystems.

The Betaproteobacteria are most closely related to the Gammaproteobacteria, Acidithiobacillia and Hydrogenophilalia, which together make up a taxon which has previously been called "Chromatibacteria".

[3] The name "Procabacteriales" was also proposed for an order of endosymbionts of Acanthamoeba, but since they cannot be grown in culture and studies have been limited, the name has never been validly or effectively published, and thus is no more than a nickname without any standing in nomenclature.

[7] Various human activities, such as fertilizer production and chemical plant usage, release significant amounts of ammonium ions into rivers and oceans.

A variety of other organisms then reduces nitrate into molecular nitrogen gas (denitrification), which leaves the ecosystem and is carried into the atmosphere.