Pelagibacterales

Pelagibacterales is an order of bacteria within the class Alphaproteobacteria. It encompasses the SAR11 clade and related marine lineages and is widely recognized as among the most abundant groups of microorganisms in the world’s oceans. Members are free-living heterotrophs that tend to be extremely small, often about 0.2–0.4 micrometers in diameter, and are noted for possessing some of the smallest known microbial genomes.

Genomes and metabolism are characterized by extreme streamlining. Pelagibacterales genomes are compact, with limited redundancy and a minimal set of biosynthetic capabilities. They rely heavily on dissolved organic carbon from the surrounding environment and possess numerous high-affinity transporters to scavenge scarce nutrients. Some members carry proteorhodopsin genes, enabling light-driven energy generation that can supplement respiration in sunlit waters.

Ecologically, Pelagibacterales are widespread in marine environments, particularly in nutrient-poor, oligotrophic regions of surface waters, and can comprise a substantial fraction of bacterial cells in many ecosystems. They play a meaningful role in marine carbon cycling and in the turnover of sulfur-containing organics such as dimethylsulfoniopropionate (DMSP). Their abundance and metabolic specialization have made them central to studies of ocean biogeochemistry and microbial genome minimization.

Taxonomically, the group was defined through 16S rRNA gene surveys in the 1990s, with the first cultured representative, Pelagibacter ubique, described in 2002. The order Pelagibacterales currently includes families such as Pelagibacteraceae (containing Pelagibacter) and Puniceispirillaceae, among others, reflecting diverse lineages adapted to marine life.