•The second largest class of Proteobacteria.
•Most species are obligate aerobes or facultative aerobes and many are oligotrophic (grow in the environment that has low nutrient concentration).
•Examples: Rhizobiales, Rickettsiales, Rhodobacteriales, Sphingomonadales, etc.
•Metagenomicdata from oceanic Alphaproteobacteriainhabiting in oxygen-rich environment suggested that this group of bacteria is likely to be the origin of mitochondria.