Retroelement-guided protein diversification abounds in vast lineages of Bacteria and Archaea

Major radiations of enigmatic Bacteria and Archaea with large inventories of uncharacterized proteins are a striking feature of the Tree of Life1-5. The processes that led to functional diversity in these lineages, which may contribute to a host-dependent lifestyle, are poorly understood. Here, we show that diversity-generating retroelements (DGRs), which guide site-specific protein hypervariability6-8, are prominent features of genomically reduced organisms from the bacterial candidate phyla radiation (CPR) and as yet uncultivated phyla belonging to the DPANN (Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanoarchaeota and Nanohaloarchaea) archaeal superphylum. From reconstructed genomes we have defined monophyletic bacterial and archaeal DGR lineages that expand the known DGR range by 120% and reveal a history of horizontal retroelement transfer. Retroelement-guided diversification is further shown to be active in current CPR and DPANN populations, with an assortment of protein targets potentially involved in attachment, defence and regulation. Based on observations of DGR abundance, function and evolutionary history, we find that targeted protein diversification is a pronounced trait of CPR and DPANN phyla compared to other bacterial and archaeal phyla. This diversification mechanism may provide CPR and DPANN organisms with a versatile tool that could be used for adaptation to a dynamic, host-dependent existence

Microbial metagenomes from three aquifers in the Fennoscandian shield terrestrial deep biosphere reveal metabolic partitioning among populations

Microorganisms in the terrestrial deep biosphere host up to 20% of the earth's biomass and are suggested to be sustained by the gases hydrogen and carbon dioxide. A metagenome analysis of three deep subsurface water types of contrasting age (from &lt;20 to several thousand years) and depth (171 to 448 m) revealed phylogenetically distinct microbial community subsets that either passed or were retained by a 0.22 mu m filter. Such cells of &lt;0.22 mu m would have been overlooked in previous studies relying on membrane capture. Metagenomes from the three water types were used for reconstruction of 69 distinct microbial genomes, each with &gt;86% coverage. The populations were dominated by Proteobacteria, Candidate divisions, unclassified archaea and unclassified bacteria. The estimated genome sizes of the &lt;0.22 mu m populations were generally smaller than their phylogenetically closest relatives, suggesting that small dimensions along with a reduced genome size may be adaptations to oligotrophy. Shallow 'modern marine' water showed community members with a predominantly heterotrophic lifestyle. In contrast, the deeper, 'old saline' water adhered more closely to the current paradigm of a hydrogen-driven deep biosphere. The data were finally used to create a combined metabolic model of the deep terrestrial biosphere microbial community.Supplementary information available for this article at http://www.nature.com/ismej/journal/v10/n5/suppinfo/ismej2015185s1.html</p

Unusual biology across a group comprising more than 15% of domain Bacteria.

A prominent feature of the bacterial domain is a radiation of major lineages that are defined as candidate phyla because they lack isolated representatives. Bacteria from these phyla occur in diverse environments and are thought to mediate carbon and hydrogen cycles. Genomic analyses of a few representatives suggested that metabolic limitations have prevented their cultivation. Here we reconstructed 8 complete and 789 draft genomes from bacteria representing &gt;35 phyla and documented features that consistently distinguish these organisms from other bacteria. We infer that this group, which may comprise &gt;15% of the bacterial domain, has shared evolutionary history, and describe it as the candidate phyla radiation (CPR). All CPR genomes are small and most lack numerous biosynthetic pathways. Owing to divergent 16S ribosomal RNA (rRNA) gene sequences, 50-100% of organisms sampled from specific phyla would evade detection in typical cultivation-independent surveys. CPR organisms often have self-splicing introns and proteins encoded within their rRNA genes, a feature rarely reported in bacteria. Furthermore, they have unusual ribosome compositions. All are missing a ribosomal protein often absent in symbionts, and specific lineages are missing ribosomal proteins and biogenesis factors considered universal in bacteria. This implies different ribosome structures and biogenesis mechanisms, and underlines unusual biology across a large part of the bacterial domain