Abstract
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 environments1 and are thought to mediate carbon and hydrogen cycles2. Genomic analyses of a few representatives suggested that metabolic limitations have prevented their cultivation2,3,4,5,6. Here we reconstructed 8 complete and 789 draft genomes from bacteria representing >35 phyla and documented features that consistently distinguish these organisms from other bacteria. We infer that this group, which may comprise >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.
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Accession codes
Primary accessions
BioProject
Sequence Read Archive
Data deposits
DNA and RNA sequences have been deposited in the NCBI Sequence Read Archive under accession number SRP050083, and genome sequences have been deposited in NCBI BioProject under accession number PRJNA273161 (first versions described here). Genomes are also available through ggKbase: http://ggkbase.berkeley.edu/CPR-complete-draft/organisms. ggKbase is a ‘live data’ site, thus annotations and genomes may be improved after publication.
Change history
29 January 2016
Extended Data Table 1 was corrected on 25 January 2016
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Acknowledgements
We thank J. Cate and S. Moore for input into the ribosomal protein analysis, J. Doudna and E. Nawrocki for suggestions on the rRNA insertion analysis, and M. Markillie and R. Taylor for assistance with RNA sequencing. Research was supported by the US Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research under award number DE-AC02-05CH11231 (Sustainable Systems Scientific Focus Area and DOE-JGI) and award number DE-SC0004918 (Systems Biology Knowledge Base Focus Area). L.A.H. was partially supported by a Natural Sciences and Engineering Research Council postdoctoral fellowship. DNA sequencing was conducted at the DOE Joint Genome Institute, a DOE Office of Science User Facility, via the Community Science Program. RNA sequencing was performed at the DOE-supported Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory.
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Samples and geochemical measurements were taken by M.J.W., K.C.W. and K.H.W. B.C.T. assembled the metagenome data. I.S. implemented the ABAWACA algorithm. C.T.B. and J.F.B. binned the data and carried out the ESOM binning validation. J.F.B. closed and curated the complete genomes. C.T.B., L.A.H. and B.C.T. conducted the rRNA gene insertion analysis. C.T.B. and L.A.H. performed phylogenetic analyses. M.J.W. and K.C.W. conducted the RNA sequencing. C.T.B. carried out the 16S rRNA gene copy number, primer binding and transcript analyses. C.T.B. and J.F.B. carried out the ribosomal protein analyses. C.T.B., L.A.H., C.J.C. and J.F.B. conducted the metabolic analysis. A.S. and B.C.T. provided bioinformatics support. C.T.B. and J.F.B. drafted the manuscript. All authors reviewed the results and approved the manuscript.
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Extended data figures and tables
Extended Data Figure 1 Sampling and geochemical measurements from acetate amendment field experiment conducted in aquifer well CD-01 at the Rifle IFRC site.
a, b, Samples were collected for metagenomics and metatranscriptomics at six time points (A–F) spanning several redox transitions during acetate stimulation of groundwater microbial communities. a, Groundwater was pumped from the alluvial aquifer and filtered through serial 1.2, 0.2 and 0.1 μm filters. DNA was extracted and sequenced from both the 0.2 and 0.1 μm filters, and RNA extracted and sequenced from the 0.2 μm filters (aerial image provided by S. M. Stoller for the US DOE under contract DE-AM01-07LM00060). b, Geochemical measurements were taken throughout the time series, showing a transition from dominant iron reduction to sulfate reduction through to methane production in the sampling environment.
Extended Data Figure 2 Validation of 20 draft-quality genomes by ESOM clustering of genome fragments based on tetranucleotide sequence composition.
For validation, 20 draft genomes from a sample with a high proportion of CPR genomes (GWA2) were chosen at random. Each data point represents a 5–10 kb genome fragment. The ESOM was trained for 100 epochs with normalized tetranucleotide frequencies. Dark lines between data points indicate strong separation between regions. Data points are coloured based on the genome the fragment originated from. The ESOM shows well-delineated clusters for most of the 20 draft genomes, with few sequence fragments falling outside of these clusters. Two genomes from the same Microgenomates (OP11) phylum were not well delineated in the tetranucleotide-based ESOM (genomes 18 and 19). This shows how the method we used for binning, which takes into account abundance patterns in addition to sequence signatures, provides more accurate genome reconstructions. The white box distinguishes a single period on the repeating map. Genomes split into multiple clusters are labelled in red.
Extended Data Figure 3 Relative abundance of bacterial community members during acetate amendment.
a, b, Relative abundance was calculated based on stringent mapping of paired-read sequences from each sample to 16S rRNA gene sequences assembled from all samples. Relative abundance of cells from 0.2 μm filters (a) and from 0.1 μm (b) filters. Enrichment of CPR organisms in the 0.2 μm filtrate indicates that these organisms have ultra-small cell sizes.
Extended Data Figure 4 Features of insertion sequences encoded within 16S rRNA genes from the Silva database.
The non-redundant Silva 16S rRNA gene database (v. 115) was analysed to assess the prevalence of insertions. Only 761 of the 418,498 16S rRNA gene sequences from bacteria encode insertions. While many small insertions were identified, unlike the 16S rRNA gene sequences assembled from groundwater, these sequences (1) rarely encode large insertions, (2) do not contain both ORFs and introns, (3) do not encode ORFs that could be assigned to Pfam families, and (4) may be found in one of multiple copies of the 16S rRNA gene.
Extended Data Figure 5 16S rRNA gene copy number estimations for genomes reconstructed from groundwater metagenomics.
a, b, 16S rRNA gene copy number was estimated for all draft CPR genomes and genome bins for organisms outside the CPR. This was achieved by comparing the coverage of 16S rRNA gene regions to the coverage of the rest of the genome. Importantly, coverage was calculated only with stringently mapped reads (no mismatches were allowed) to improve the accuracy of coverage calculations. a, Histogram of the number of 16S rRNA gene sequence copies estimated for each genome by calculating (16S rRNA gene coverage)/(genome coverage). Several WWE3 genomes were estimated to have high 16S rRNA gene copy number (Supplementary Table 7), but it was later determined that these estimates were skewed by the presence of a highly abundant closely related strain. The complete WWE3 genome assembled previously3 has an identical 16S rRNA gene and confirms that it is found in only one copy for this genotype. Thus, we removed these estimates from subsequent copy number analysis. b, Density plot comparing estimated copy number of genomes for organisms found within and outside the CPR, where the longer tail for non-CPR genomes depicts the propensity for multiple 16S rRNA copies, a trait absent from the CPR.
Extended Data Figure 6 Features of insertion sequences encoded within 23S rRNA genes recovered from groundwater-associated bacteria.
Bacteria associated with the CPR encode insertions within their 23S rRNA genes (Supplementary Table 5). These insertions share many features with those identified in 16S rRNA gene sequences from CPR bacteria. Taxonomy was determined by inclusion in a genome with an established phylogeny.
Extended Data Figure 7 Analysis of the ability of PCR primers 515F and 806R to bind to recovered groundwater-associated 16S rRNA gene sequences.
a, b, PrimerProspector was used to assess the ability of primers 515F and 806R to bind a non-redundant set of assembled near-complete 16S rRNA gene sequences (clustered at 97% sequence identity). The percentage of sequences that would be amplified by these primers is shown on the left axis, the total number of sequences analysed is on the top of each bar, and the number of sequences these primers would not bind to is indicated by the shading. Many assembled groundwater-associated 16S rRNA gene sequences would evade amplification by PCR primers 515F and 806R. Results of the analysis are shown at the domain (a) and superphylum or phylum (b) levels.
Extended Data Figure 8 Metabolic potential and ribosomal protein analysis of genomes from CPR and TM6 organisms.
Assembled genomes were analysed using ggKbase (Supplementary Data 4). Shown here is a non-redundant set of complete and near-complete genomes (≥75% of single copy genes, ≤1.125 copies) organized based on a subset of a maximum-likelihood 16S rRNA gene phylogeny (Supplementary Fig. 1). CPR organisms have partial tricarboxylic acid (TCA) cycles and lack electron transport chain (ETC) complexes. In addition, they have incomplete biosynthetic pathways for nucleotides and amino acids. The Peregrinibacteria are a notable exception to some of these limitations. Several Parcubacteria exhibit a complete ubiquinol (cytochrome bo) oxidase operon, as previously seen in Saccharibacteria3. However, lack of NADH dehydrogenase and other ETC components suggests that this enzyme is involved in oxygen scavenging/detoxification rather than energy production. AA Syn., amino acid synthesis; PP, pentose phosphate pathway.
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Brown, C., Hug, L., Thomas, B. et al. Unusual biology across a group comprising more than 15% of domain Bacteria. Nature 523, 208–211 (2015). https://doi.org/10.1038/nature14486
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DOI: https://doi.org/10.1038/nature14486
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