Alphaproteobacteria. This is a class of Proteobacteria, includes non-sulphur purple bacteria and their heterotrophic descendants; respirers with ubiquinones having 10 isoprenoid units, often facultative aerobes. SAR11 bacteria are a group within the Alphaproteobacteria class. SAR11 is the numerically dominant microorganism in the ocean surface.

alpha proteobacterium HIMB59. First known cultured isolate from a clade of marine bacterioplankton which appears to share an ancient evolutionary history with the SAR11 clade of bacterioplankton.