Gammaproteobacteria: a broad range of bacteria with diverse metabolic capabilities and ecological roles.

Deltaproteobacteria: They are a diverse group of bacteria that occupy various habitats, including marine sediments.

Epsilonproteobacteria: They are often found in environments with high temperatures or low oxygen concentrations.

Betaproteobacteria: They are a relatively newly discovered class, primarily known for their role in iron oxidation.