The deeply branching clade of abundant, globally distributed aquatic alpha-Proteobacteria known as “SAR11”, are adapted to nutrient-poor environments such as the surface waters of the open ocean. Unknown prior to 1990, uncultured until 2002, members of the SAR11 clade can now be cultured in artificial, defined media to densities three orders of magnitude higher than in unamended natural media. Cultivation in natural and defined media has confirmed genomic and metagenomic predictions such as an inability to reduce sulfate to sulfide, a requirement for pyruvate, an ability to oxidize a wide variety of methylated and one-carbon compounds for energy, and an unusual form of conditional glycine auxotrophy. Here we describe the metabolism of the SAR11 type strain Candidatus “Pelagibacter ubique” str. HTCC1062, as revealed by genome-assisted studies of laboratory cultures. We also describe the discovery of SAR11 and field studies that have been done on natural populations.