It is more a clade containing both genera than a genus. The Bacteriological Code does not define intermediate taxa ranks like the subfamilies in zoology.

The clade is ecologically important due to its main role in the primary production in oligotrophic ocean waters.

As I read, it is often difficult to impossible to separate both genera with molecular markers, so often they are grouped under GpIIa.

http://www.nature.com/nrmicro/journal/v13/n1/box/nrmicro3378_BX1.html