Regressive Evolution of Photosynthesis in the Roseobacter Clade

The Roseobacter clade constitutes a significant fraction of marine microbial communities. The clade is functionally heterogeneous as it consists of both photoheterotrophic as well as chemoheterotrophic species. Such functional diversity can be explained by two different evolutionary scenarios: (1) the progenitors of the Roseobacter species were originally photoautotrophic, but lost part or all of their photosynthesis genes over time; or (2) they were originally heterotrophs adopting the photosynthesis genes via horizontal gene transfer (HGT). To address these hypotheses, we analysed genomic information from several Roseobacter species and compared it with other phototrophic organisms. The analyses suggest that the photosynthesis genes were not acquired via HGT, but rather that Roseobacter species present in today's oceans descend from ancient phototrophic bacteria, and radiated after the oxygenation of the oceans during the Neoproterozoic era. Later, several Roseobacter lineages lost their photosynthesis genes forming strictly heterotrophic species. © 2013 Elsevier Ltd.