Speciation and ecological success in dimly lit waters: horizontal gene transfer in a green sulfur bacteria bloom unveiled by metagenomic assembly

11 páginas, 6 figuras.A natural planktonic bloom of a brown-pigmented photosynthetic green sulfur bacteria (GSB) from the disphotic zone of karstic Lake Banyoles (NE Spain) was studied as a natural enrichment culture from which a nearly complete genome was obtained after metagenomic assembly. We showed in situ a case where horizontal gene transfer (HGT) explained the ecological success of a natural population unveiling ecosystem-specific adaptations. The uncultured brown-pigmented GSB was 99.7% identical in the 16S rRNA gene sequence to its green-pigmented cultured counterpart Chlorobium luteolum DSM 273T. Several differences were detected for ferrous iron acquisition potential, ATP synthesis and gas vesicle formation, although the most striking trait was related to pigment biosynthesis strategy. Chl. luteolum DSM 273T synthesizes bacteriochlorophyll (BChl) c, whereas Chl. luteolum CIII incorporated by HGT a 18-kbp cluster with the genes needed for BChl e and specific carotenoids biosynthesis that provided ecophysiological advantages to successfully colonize the dimly lit waters. We also genomically characterized what we believe to be the first described GSB phage, which based on the metagenomic coverage was likely in an active state of lytic infection. Overall, we observed spread HGT and we unveiled clear evidence for virus-mediated HGT in a natural population of photosynthetic GSB.This research was funded by grant DARKNESS CGL2012- 32747 from the Spanish Office of Science (MINECO) to EOC and by the Global Ocean Sampling Project supported by the Beyster Family Foundation Fund of the San Diego Foundation and the Life Technology Foundation (to JCVI). Work on BChl e biosynthesis and the genomics of GSB in the laboratory of DAB was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG02-94ER20137.Peer reviewe