Polyhydroxybutyrate production in freshwater SAR11 (LD12)

SAR11 bacteria (order Pelagibacterales) are oligotrophs and often the most abundant bacterioplankton in aquatic environments. A subset of sequenced SAR11 genomes, predominantly in the brackish and freshwater SAR11 subclades, contain homologs of pha genes, which in other organisms confer the ability to store carbon and energy via polyhydroxyalkanoate (PHA) polymers. Here, we investigated the relevance of PHA production to SAR11 biology. Phylogenetics showed that Pha proteins occurred on a long branch and provided evidence for origin at the common ancestor of the brackish IIIa and freshwater LD12 subclades, followed by horizontal transfer within SAR11. Using the LD12 representative "Candidatus Fonsibacter ubiquis" strain LSUCC0530, we found that a large minority of LSUCC0530 cells contained a single Nile red-staining granule, confirmed that the cells produced polyhydroxybutyrate (PHB), and estimated the total PHB content in the cells. We heterologously expressed the LSUCC0530 phaCAB locus in Escherichia coli, finding it to be functional and the likely origin of the PHB. We also determined that, irrespective of changes to carbon, nitrogen, and phosphorus concentrations, a similar fraction of LSUCC0530 cells generated PHB granules and expression of the phaCAB locus remained constant. We suggest that PHB synthesis in LSUCC0530 may be constitutively active due to the slow growth dynamics and minimal regulation that characterize SAR11 bacteria. This is the first characterization of polymer storage in SAR11, providing new insights into the likely fitness advantage for cells harboring this metabolism.