First genomic insights into members of a candidate bacterial phylum responsible for wastewater bulking

Abstract

Filamentous cells belonging to the candidate bacterial phylum KSB3 were previously identified as the causative agent of fatal filament overgrowth (bulking) in a high-rate industrial anaerobic wastewater treatment bioreactor. Here, we obtained near complete genomes from two KSB3 populations in the bioreactor, including the dominant bulking filament, using differential coverage binning of metagenomic data. Fluorescence in situ hybridization with 16S rRNA-targeted probes specific for the two populations confirmed that both are filamentous organisms. Genome-based metabolic reconstruction and microscopic observation of the KSB3 filaments in the presence of sugar gradients indicate that both filament types are Gram-negative, strictly anaerobic fermenters capable of non-flagellar based gliding motility, and have a strikingly large number of sensory and response regulator genes. We propose that the KSB3 filaments are highly sensitive to their surroundings and that cellular processes, including those causing bulking, are controlled by external stimuli. The obtained genomes lay the foundation for a more detailed understanding of environmental cues used by KSB3 filaments, which may lead to more robust treatment options to prevent bulking.

Full text

Submitted 14 October 2014
Accepted 5 January 2015
Published 27 January 2015
Corresponding authors
Yuji Sekiguchi,
y.sekiguchi@aist.go.jp
Philip Hugenholtz,
p.hugenholtz@uq.edu.au
Academic editor
Kimberly Bishop-Lilly
Additional Information and
Declarations can be found on
page 19
DOI 10.7717/peerj.740
Copyright
2015 Sekiguchi et al.
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Creative Commons CC-BY 4.0
OPEN ACCESS
First genomic insights into members of a
candidate bacterial phylum responsible
for wastewater bulking
Yuji Sekiguchi1, Akiko Ohashi1, Donovan H. Parks2,
Toshihiro Yamauchi3, Gene W. Tyson2,4 and Philip Hugenholtz2,5
1Biomedical Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba, Ibaraki, Japan
2Australian Centre for Ecogenomics, School of Chemistry and Molecular Biosciences, The University of
Queensland, St. Lucia, Queensland, Australia
3Administrative Management Department, Kubota Kasui Corporation, Minato-ku, Tokyo, Japan
4Advanced Water Management Centre, The University of Queensland, St. Lucia, Queensland, Australia
5Institute for Molecular Bioscience, The University of Queensland, St. Lucia, Queensland, Australia
ABSTRACT
Filamentous cells belonging to the candidate bacterial phylum KSB3 were previously
identified as the causative agent of fatal filament overgrowth (bulking) in a high-rate
industrial anaerobic wastewater treatment bioreactor. Here, we obtained near
complete genomes from two KSB3 populations in the bioreactor, including the
dominant bulking filament, using differential coverage binning of metagenomic data.
Fluorescence in situ hybridization with 16S rRNA-targeted probes specific for the
two populations confirmed that both are filamentous organisms. Genome-based
metabolic reconstruction and microscopic observation of the KSB3 filaments in the
presence of sugar gradients indicate that both filament types are Gram-negative,
strictly anaerobic fermenters capable of non-flagellar based gliding motility, and
have a strikingly large number of sensory and response regulator genes. We propose
that the KSB3 filaments are highly sensitive to their surroundings and that cellular
processes, including those causing bulking, are controlled by external stimuli.
The obtained genomes lay the foundation for a more detailed understanding
of environmental cues used by KSB3 filaments, which may lead to more robust
treatment options to prevent bulking.
Subjects Biodiversity, Biotechnology, Genomics, Microbiology, Taxonomy
Keywords KSB3 phylum, Candidate phylum, Wastewater treatment, Anaerobic biotechnology,
Filamentous bulking, Metagenomics
INTRODUCTION
Anaerobic digestion is a major type of biological treatment extensively used around the
world (Ahring, 2003a) that is not only cost effective for treating organic waste and wastewa-
ter, but also can frequently produce energy in the form of methane (biogas) (Angelidaki
et al., 2011). Over the last thirty years, a set of high rate anaerobic digestion reactor
configurations have been developed, of which the upflow anaerobic sludge blanket (UASB)
technology is the most successful and commercialized configuration (Kleerebezem &
Macarie, 2003;van Lier, 2008). Despite the success of this technology, serious performance
How to cite this article Sekiguchi et al. (2015), First genomic insights into members of a candidate bacterial phylum responsible for
wastewater bulking. PeerJ 3:e740;DOI 10.7717/peerj.740

issues have emerged such as the sudden washout of granular sludge biomass due to over-
growth of filamentous bacteria (bulking), which can lead to complete loss of performance.
Bulking of anaerobic digestion systems can be caused by a variety of filamentous
microorganisms (HulshoffPol et al., 2004;Li et al., 2008;Yamada & Sekiguchi, 2009)
and a phylogenetically novel filament was previously reported to be the cause of bulking
in an industrial UASB reactor treating sugar manufacturing wastewater (Yamada et al.,
2007;Yamada et al., 2011). Small subunit ribosomal RNA (16S rRNA) gene-based analyses
of the bulking sludge (Yamada et al., 2007) revealed that the dominant filament type
belongs to candidate bacterial phylum KSB3, originally proposed by Tanner et al. (2000)
based on an environmental 16S rRNA gene clone sequence obtained from a sulfur-rich
marine sediment (Tanner et al., 2000). Fluorescence in situ hybridization (FISH) with
KSB3-specific 16S rRNA-directed probes revealed that the KSB3 filaments are localized at
the outer layer of healthy granules (Yamada et al., 2007) which become substantially thicker
during bulking. The study of filamentous KSB3 bacteria will undoubtedly contribute to
our understanding of and ability to prevent bulking in anaerobic wastewater treatment sys-
tems, but has been hampered by an inability to obtain a pure culture despite repeated and
long term isolation efforts (Yamada et al., 2011). However, culture-independent molecular
and imaging methods are beginning to provide clues regarding the ecophysiology of these
organisms. This includes their ability to uptake simple carbohydrates, particularly maltose
and glucose, under anaerobic conditions and from these observations it was proposed that
high carbohydrate loading in the UASB reactor may trigger proliferation of KSB filament
populations (Yamada et al., 2011).
Here, we obtained near complete genomes from in situ populations of the dominant
bulking KSB3 filament type and a second moderately related low abundance KSB3
filament via differential coverage binning (Albertsen et al., 2013) using metagenomic data
previously reported from a full-scale UASB reactor (Soo et al., 2014). Differential coverage
binning groups together anonymous metagenomic fragments (contigs) belonging to the
same population based on the similarity of their sequencing coverage across multiple
related metagenomes (Albertsen et al., 2013). These genomes represent the first genomic
information for candidate phylum KSB3 and provide insights into the metabolism of KSB3
filaments and their ability to cause bulking.
METHODS
Samples
Methanogenic sludge samples reported in a previous study (Soo et al., 2014) were used in
the present study for shotgun sequencing and fluorescence in situ hybridization (FISH).
Briefly, two sludge samples (A1 and A2) were taken from the system at different sampling
dates (A1, 25th December, 2012; A2, 16th September, 2010), and sample A1 was further
separated into two parts (flocculant sludge [F1] and granular sludges [G1]) by gravimetric
settlement (Soo et al., 2014) (Table S1). Each sample had been divided into two parts: one
used for obtaining DNA via bead-beating and phenol chloroform extraction and the other
fixed in 4% paraformaldehyde for FISH (Soo et al., 2014).
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Fluorescence in situ hybridization
KSB3-specific FISH probes were designed in ARB v5.5 (Ludwig et al., 2004) using 16S
rRNA genes identified in the KSB3 genomes and KSB3 16S rRNA gene sequences available
in the current Greengenes database (May 2013 version) (McDonald et al., 2012). In order to
maximize the specificity and fluorescence intensity of the probes, helper probes were also
designed (Table S5). FISH was performed as described previously (Sekiguchi et al., 1999)
using the probes listed in Table S5 and all probes were hybridized overnight. Probes were
labeled with either Alexa488 or Cy3 fluorophores, and probes with different fluorophores
were used together for dual-staining FISH. Images were captured using an epifluorescence
microscope (Axioplan 2; Carl Zeiss) equipped with a cooled charge-coupled device (CCD)
camera (DP72; Olympus) and subsequently processed using imaging software (DP2-BSW,
version 2.2; Olympus). Super-imposed images were generated using Adobe Photoshop
(CS5.1; Adobe).
Metagenome sequencing
Previously sequenced paired-end and mate-pair metagenomes for samples A1, A2, F1, and
G1 (Soo et al., 2014) were supplemented with additional data generated in this study using
the same DNA. Paired-end Nextera libraries were prepared for each sample according
to the manufacturer’s instructions, quantified using the QuantIT kit (Molecular Probes)
and sequenced (2 ×250 bp paired end) on an Illumina MiSeq using the Reagent Kit v2
(Illumina) at the National Institute of Advanced Industrial Science and Technology, Japan
(AIST; Table S1). The extra metagenome sequencing generated 7.3 Gb, 13.1 Gb, 1.9 Gbp,
and 7.9 Gbp for A1, A2, F1, and G1 samples, respectively (yielding a total of 56.2 Gb for
all samples combined). For scaffolding, two additional large-insert mate-pair libraries
(∼3 kbp and ∼7.5 kbp) were constructed from sample A1 using the Mate Pair Library
Preparation Kit v2 (Illumina) and sequenced on Illumina MiSeq system (MiSeq Reagent
Kit v2) yielding 2.3 Gb and 3.0 Gb, respectively (for a total of 7.3 Gb from sample A1 when
combined with mate-pair data; Soo et al., 2014).
Community profiling
16S rRNA gene amplicon sequencing of all UASB sludge samples using the Illumina MiSeq
system was previously reported (Soo et al., 2014). Community composition was also
examined by extracting all 16S rRNA reads from the metagenome datasets using the closed
reference picking script in QIIME v1.6.0 (Caporaso et al., 2010b) with the Greengenes
database (de-replicated dataset at 97%, March 2013 McDonald et al., 2012) as reference
(otu picking method, uclust ref; similarity cutoffvalue, 0.95). All reads from paired end
sequencing were quality filtered using a QIIME script (split library fastq.py) with the
following stringent parameters to ensure only high quality reads were included in the
analysis: read trimming with a Phred quality threshold of 17 (-q 17) and discarding reads
shorter than 50% of the input read length (-p 0.5), and then the quality filtered single-end
reads were used for the closed reference picking to generate OTU tables.
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Assembly and binning
Metagenome assembly and population genome binning followed the approach previously
described (Albertsen et al., 2013). A graphical illustration of the workflow is shown in
Fig. S1. Briefly, paired-end metagenome reads in fastq format were merged with SeqPrep
(https://github.com/jstjohn/SeqPrep) using default settings and Illumina sequencing
adapters were removed. Unmerged reads were quality trimmed and filtered using
Nesoni v0.112 (https://github.com/ Victorian-Bioinformatics-Consortium/nesoni) with
removing low quality bases from reads with a Phred quality threshold of 17, removing
homopolymers reads, and eliminating trimmed reads shorter than 30 bases (clip –quality
17 –homopolymers yes –length 30). The merged and trimmed reads from the four
metagenomes were co-assembled using SPAdes v2.5.0 with the following parameters suited
to a complex metagenomic assembly: –only-assembler -k 67 –sc. Reads from respective
samples were separately mapped to scaffolds using BWA v0.7.4 (Li & Durbin, 2010)
with the BWA-MEM algorithm using default parameters. Population genome binning
using differential coverage (Albertsen et al., 2013) was performed using GroopM v0.1
(Imelfort et al., 2014) with the initial core formation based on contigs/scaffolds longer than
1,500 bp. Manual refinement of population genome bins, and subsequent recruitment of
contigs/scaffolds longer than 500 bp was performed using the GroopM tools.
Identification of conserved marker genes
All contigs/scaffolds in each genome bin were translated into six reading frames, and
hmmsearch in HMMER3 (Eddy, 2011) was used to identify 111 single copy marker
genes conserved in most bacteria (Dupont et al., 2012), 83 phylogenetically-informative
marker genes (Soo et al., 2014), and the 38 marker genes proposed by PhyloSift (Darling
et al., 2014). To determine the completeness and contamination of each genome bin, the
distribution and number of the 111 conserved single copy marker set was determined
using CheckM (Parks et al., 2014) with default settings.
Refinement of population genome bins
Scaffolding of metagenome contigs using the mate-pair data was performed with SSPACE
v2.0 (Boetzer et al., 2011). SSPACE was run with the following two sets of parameters: lower
stringency for minor population genomes with relatively low coverage (e.g., UASB270),
-k 2 (minimum number of links to compute scaffold) -a 0.7 (maximum link ratio
between two best contig pairs) -x 0 (no extention of the contigs using paired reads)
-p 1 (making .dot file for visualization) and higher stringency for major population
genomes (e.g. UASB14): -k 4 -a 0.7 -x 0 -p 1. The resulting dot files were used for
visualizing contig connections using Cytoscape v.2.8.1 (Shannon et al., 2003). In addition,
Cytoscape attribute files were generated with coverage, length, and bin number (bin name)
information for each contig/scaffold. Based on the coverage information and number
of connections between contig/scaffolds, external contig/scaffolds are manually added
to each bin. In addition, contig/scaffolds with a small number of connections to other
contig/scaffolds in their respective bins were excluded. Refined sets of contig/scaffolds were
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then scaffolded with SSPACE with the following low stringency parameters: -k 2 -a 0.7 -x 0
-p 1. For further refinement, shotgun mate-pair reads were mapped to the newly generated
scaffolds using CLC genomic workbench v6.0 (CLC Bio) using default parameters with
the exception of a similarity fraction of 0.98 and exported in SAM format. The assembly
was visualized using Circos (Krzywinski et al., 2009) and used for manual inspection of the
assembly as previously described (Albertsen et al., 2013). Manual correction of misassembly
and mis-scaffolding was performed using the microbial genome finishing module in CLC
genomic workbench v6.0 (CLC Bio).
Genome tree
Finished bacterial and archaeal genomes were downloaded from IMG (release
4.1) (Markowitz et al., 2014), from which the 38 universally (Darling et al., 2014) or
83 single-copy proteins broadly conserved in bacteria were identified using HMM
searches (Soo et al., 2014). To evaluate the robustness of the protein trees (genome trees),
four different outgroup taxon configurations (two data sets for 38 marker genes, two data
sets for 83 marker genes) were made (Table S3). Homologous proteins obtained from the
KSB3 and reference genomes in each taxon configuration were aligned using hmmalign
in HMMER3, and subsequently concatenated. A mask was generated for the concatenated
alignment using Gblocks (Talavera & Castresana, 2007) with only conserved positions
found in more than half of the sequences considered. All tree topologies were tested for
robustness using the maximum likelihood methods from FastTree v2.1.7 (with default
parameters, JTT model, CAT approximation) (Price, Dehal & Arkin, 2009) and RAxML
v7.7.8 (JTT and Gamma models with rapid 100 times bootstrapping) (Stamatakis, 2006).
The PHYLIP SEQBOOT module (Felsenstein, 1989) was used to generate 100 resampled
alignments and FastTree was used to analyze the resampled alignments (-n 100). A script
(CompareToBootstrap.pl) included in the FastTree package was used to compare the
original tree to the resampled trees and generate bootstrap values. Generated trees were
imported into ARB (Ludwig et al., 2004), where they were rooted, left-hand ladderized
using the “beautify” tool and grouped into phylum-level clades. A representative tree (tree
no. 1; Table S4) was exported from ARB and visualized using iTOL (Letunic & Bork, 2011).
16S rRNA gene phylogeny
KSB3 related 16S rRNA genes were manually curated using the Greengenes database
(version May 2013; McDonald et al., 2012) in ARB (Ludwig et al., 2004). 16S rRNA genes
from binned population genomes were aligned with PyNAST (Caporaso et al., 2010a),
imported into ARB, and the alignments were manually corrected using the ARB EDIT
tool. Sets of taxa (>1,300 nt) were selected in ARB and their alignments were exported
applying Lane mask filtering. One set of taxa included representatives across all recognized
bacterial phyla to determine the relative position of KSB3 in the bacterial domain (Fig. S6).
A second set of taxa included all KSB3 sequences to determine the relative position of
the two UASB filament genomes within the KSB3 phylum (Fig. 1A). Neighbor joining
trees were calculated from the masked alignments with LogDet distance estimation
using PAUP* 4.0 (Swofford, 2003) with 100 bootstrap resamplings. Maximum likelihood
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trees were calculated based on the masked alignments using RAxML v7.7.8 (GTR and
Gamma models +I) with rapid 100 time bootstrapping. Maximum parsimony trees were
calculated using PAUP* 4.0. A heuristic search was used with a random stepwise addition
sequence of 10 replicates and nearest-neighbor-interchange swapping. Bootstrap analyses
on the maximum parsimony trees were run with 100 times resampling for each best tree.
Generated trees were re-imported into ARB for visualization.
Genome analysis
The two assembled KSB3 population genomes were initially annotated with PROKKA
v1.7 using default settings (Seemann, 2014), and manually curated by comparison to
UniRef90 (Suzek et al., 2007), IMG (Integrated Microbial Genomes, finished genomes,
release 4.0) (Markowitz et al., 2014), COGs (Clusters of Orthologous Groups) (Tatusov
et al., 2000), PFAM (Punta et al., 2012), and KEGG (Kyoto Encyclopedia of Genes and
Genomes) (Aoki-Kinoshita & Kanehisa, 2007) databases. Bi-directional best-blast matches
were performed for proteins with matches to UniRef90 and IMG using a bit score threshold
of 300, and one-way BLASTP matches with a bit score of 60 (Castelle et al., 2013).
For COGs, RPS-BLAST against COG PSSMs from the CDD database (Marchler-Bauer
et al., 2013) was performed using an e-value cutoffof 0.01, with the top hit retained
for each protein domain. The amino acid sequences were also searched for conserved
motifs with PFAM (Punta et al., 2012) using HMMR3 (Eddy, 2011) and PfamScan
with default settings (with family noise cutoff). Protein domain structure of some gene
products were additionally evaluated using InterProScan search (Quevillon et al., 2005).
For manual annotation of the KSB3 genomes, we ranked the resulting annotations as
follows: bi-directional best-blast matches with UniRef90 and IMG data; one-way matches
with UniRef90, IMG, and COGs; PFAM matches; hypothetical proteins (Castelle et al.,
2013). For comparison of gene sets with other genomes, we downloaded the full IMG
database (release 4.1) containing all genomes in IMG and their annotations (e.g. PFAMs
and COGs). In addition, a list of all finished bacterial genomes and associated metadata
(e.g. taxonomic affiliation and genome size) was obtained though IMG. Ribosomal RNA
copy number was estimated by determining the ratio of average genome coverage to 16S
rRNA gene coverage for each KSB3 genome calculated using BWA read mapping. CRISPR
loci were identified using CRT v1.2 (Bland et al., 2007). Presence/absence of some gene
sets related to cell envelope structure (Albertsen et al., 2013), complex bacterial lifestyle,
and adaptability to fluctuating environmental conditions (‘social IQ’) (Sirota-Madi et
al., 2010) were evaluated based on IMG annotation for finished genomes and annotated
KSB3 genomes, and the resulting abundance matrix was visualized using R and ggplot2.
Orthologous proteins between the two KSB3 genomes were identified using pairwise
bi-directional best hit BLASTP searches. Glycoside hydrolases were identified using
the CAZy database (Lombard et al., 2014) (dbCAN HMMs v3.0, Yin et al., 2012) with
HMMER3 (default settings). Transmembrane proteins were predicted using TMHMM
Server v. 2.0 (Moller, Croning & Apweiler, 2001).
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Gram-staining and gliding motility
Gram-staining for KSB3 filaments was performed based on the method of
Hucker, (Doetsch, 1981). Gliding motility of KSB3 filaments was evaluated using fresh
sludge samples examined under an epifluorescence microscope (Axioplan 2; Carl Zeiss)
equipped with an automatic thermo-control system (Thermo Plate, MATS-55SFG-FT;
Tokai Hit). Fresh sludge samples were placed on glass slides, and a cover glass carefully
positioned over the sample to minimize exposure to air. To maintain anaerobic conditions
on the slides, reducing agents (Na2S and/or L-cysteine) were added to the samples. The
temperature of the microscopic stage was maintained at 37 ◦C, and time-lapse images
were recorded with a cooled CCD camera (DP72; Olympus) equipped with the imaging
software (DP2-BSW, version 2.2; Olympus). Glucose, maltose, ribose, mannose, galactose,
arabinose, raffinose, sucrose, xylose, fructose, lactate, ethanol, propionate, nitrate, nitrite
(final concentration of 5–10 mM) and yeast extract (0.1%) were used as candidate
stimuli to induce motility. Each potential stimulant was mixed with the cells prior to
the observation, or placed at the edge of the glass cover creating a gradient as the stimulant
diffused into the sample.
RESULTS
KSB3 populations in the UASB system
Two UASB sludge samples taken two years apart (A1 and A2), and flocular (F1) and
granular (G1) fractions derived from sample A1, reported previously (Soo et al., 2014),
were used in the present study. The UASB system had a history of periodic bulking
caused by KSB3 filaments (Yamada et al., 2007;Yamada et al., 2011). Inspection of 16S
rRNA gene amplicon community profiles of these samples (Soo et al., 2014) revealed two
KSB3 16S rRNA phylotypes accounting for 4.9 and 3.7% of total sequencing reads from
samples A1 and A2, respectively. The dominant phylotype, representing ∼94% of the
KSB3 reads, was identical to the previously reported bulking phylotype (clone YM-1,
AB218870;Yamada et al., 2007), and the minor phylotype, representing ∼6% of the KSB3
reads, was identical to a low abundance clone detected in the UASB reactor during normal
operation (clone SmB78fl, AB266927;Narihiro et al., 2009). The internal transcribed
spacer (ITS) region of the bulking phylotype was sequenced to confirm that it was the same
strain present in the bulking and normally operating UASB sludge (Fig. S2). Previously
reported metagenomes (Soo et al., 2014) and additional shotgun sequencing of samples
A1, F1, G1 and A2 were used for recovering high quality draft population genomes of the
two KSB3 phylotypes (Table S1). Based on detection of 16S rRNA genes in the shotgun
paired-end read datasets, the KSB3 phylotypes comprised up to 10 and 11% of the A1 and
A2 metagenomes respectively with the dominant KSB3 phylotype having approximately
10 fold higher abundance than the minor phylotype (Fig. S3), broadly consistent with the
amplicon results.
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Recovery of KSB3 population genomes
The four metagenomes (59 Gb in total, Table S1) were co-assembled, generating 504,757
contigs/scaffolds (>500 bp) with a combined length of 906 Mb, an N50 of 3 kb and a
longest scaffold of 506 kb. Population genomes were recovered from the assembly by
exploiting variations in population abundance (coverage) between individual sample
metagenomes (differential coverage binning, Albertsen et al., 2013) using the automated
binning tool GroopM (Imelfort et al., 2014). The completeness and contamination of the
population genomes were estimated by detection of single copy marker gene sets widely
conserved in the domain Bacteria (Dupont et al., 2012). Thirty-nine bacterial population
genome bins were obtained with >65% completeness (>73/111 markers) and <10%
contamination (<11/111, marker genes with >1 copy in a population genome indicate
presumptive contamination with another organism). These genomes were refined by
tracking mate-pair reads in network graphs to further improve the completeness and
reduce contamination of the bins, and to recruit repeat sequences, notably ribosomal
RNA operons, which can evade differential coverage binning if present in multiple copies
(Albertsen et al., 2013).
We identified 16S rRNA gene sequences in two refined population genome bins
(UASB14 and UASB270, Fig. S4) that were identical to the amplicon sequences from
the dominant and minor KSB3 phylotypes, respectively (Table 1). Despite careful manual
curation of both genomes, their estimated completeness based on 111 conserved single
copy marker genes (Dupont et al., 2012) is only ∼93%, and both also have an inferred
∼6% contamination based on these markers. Inspection of the marker genes with no or
>1 hit, however, show a high degree of overlap between the genomes, suggesting that these
particular genes are either actually absent, duplicated, or laterally transferred based on
phylogenetic inference and gene neighborhood (Table S2). This may not be unexpected
given the phylogenetic novelty of the lineage. A revised estimate of completeness and
contamination based on a prediction that six of the 111 marker genes are absent and five
are duplicated is >98% and <2% respectively (Table 1). To estimate the number of rRNA
operons in each KSB3 genome, we compared average genomic coverage to 16S rRNA gene
coverage, which indicated that UASB14 and UASB270 have three and two rRNA operons
respectively (Fig. S5).
KSB3 phylogeny and morphology
The relative position of the two KSB3 genomes within the phylum was assessed by
comparative analysis of their 16S rRNA gene sequences with publicly available full-length
sequences. UASB14 and UASB270 represent two of several major lines of descent in the
KSB3 phylum (Fig. 1A). According to Greengenes classification (McDonald et al., 2012),
UASB14 belongs to an unnamed class-level lineage and UASB270 is a member of class
MAT-CR-H3-D11 for which we propose the names Moduliflexia and Vecturitrichia,
respectively (Table 1;Supplemental Information 1). To confirm the status of KSB3 as a
candidate phylum, as inferred by 16S rRNA comparative analyses (Yamada et al., 2007,
Fig. S6), we constructed phylogenetic trees based on a larger genomic sampling. Two sets
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Table 1 Features of the Modulibacteria KSB3 population genomes.
Genome bin identifier UASB14 UASB270
Candidatus name Moduliflexus flocculans Vecturathrix granuli
Closest environmental 16S clone YM-1 (AB218870) SmB78fl (AB266927)
No. of scaffolds 8 21
Total length (bp) 7,147,157 8,384,694
N50 1,183,318 597,372
GC (%) 50.6 47.2
Average coverage 278 38
Genome completenessa92.8% (103/111) 93.6% (104/111)
Revised genome completenessa98.1% (103/105) 99.0% (104/105)
Genome contaminationa5.4% (6/111) 6.3% (7/111)
Revised genome contaminationa0.0% (0/105) 1.9% (2/105)
Relative abundance in UASB metagenomes (%)b9.22 0.40
No. tRNA genes 54 43
rRNA genes found in genome 5S, 16S, 23S 5S, 16S, 23S
Inferred no. of rRNA operonsc3 2
No. CDS 5,989 7,048
No. CRISPR array 4 (125 repeats in total) 5 (550 repeats in total)
Coding density 84.6 % 84.3 %
Putative glycoside hydrolasesd
Cellulase 5 (0.1%) 14 (0.2%)
Amylase 19 (0.3%) 8 (0.1%)
Debranching enzyme 3 (0.1%) 2 (0.1%)
Amino-sugar-degrading enzyme 35 (0.6%) 45 (0.6%)
Oligosaccharide-degrading enzyme 43 (0.7%) 23 (0.3%)
Putative protease/peptidased
Protease 27 (0.5% of total ORFs) 28 (0.4% of total ORFs)
Peptidase 60 (1.0%) 78 (1.1%)
Putative environmental signaling system genes
Transmembrane sensore135 (2.3%) 114 (1.6%)
Response regulator containing CheY-like domainf131 (2.2%) 116 (1.6%)
Proposed class Moduliflexia Vecturatrichia
Proposed order Moduliflexales Vecturatrichales
Proposed family Moduliflexaceae Vecturatrichaceae
Notes.
aGenome completeness and contamination were estimated based on the presence/absence of a 111 single-copy gene set from Dupont et al. (2012). Revised genome
completeness and contamination were calculated based on a revised total of 105 single-copy genes estimated to be present in the KSB3 genomes (Table S2 ). Numbers in
parentheses indicats detected number of genes per total number of each gene set.
bRelative genome abundance for each KSB3 genome was determined based on 16S rRNA gene profiling using shotgun metagenome data (Table S1).
cNumber of rRNA operons in the KSB3 genomes were inferred based on relative coverage profiles of KSB3 16S rRNA genes and the genome averages (Fig. S2).
dCounts (% of total ORFs).
ePredicted number of transmembrane sensors based on the possession of a sensor domain (Galperin, 2004) and >1 transmembrane segments (Table S8).
fNumber of all two-domain response regulators containing CheY-like domains estimated from PSI-BLAST searches of domain-specific profiles against the protein set
described in Galperin (2004) (Table S8).
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Figure 1 Phylogenetic structure of the Modulibacteria (KSB3) phylum based on comparative analysis of 16S rRNA gene sequences, and imaging
of KSB3 cells. (A) Maximum-likelihood phylogenetic tree (RAxML) of public data (accession numbers shown) and the 16S rRNA sequence
determined in this study for UASB14. Sequences from the bacterial phyla Nitrospirae, Tenericutes, and Chloroflexi were used to root the tree
(not shown). Reproducible interior nodes are indicated as a black circle (>90% bootstrap support for neighbor-joining [NJ], maximum parsimony
[MP], and maximum-likelihood [ML] inferences), open circle (>80% support); or open rectangle (>70% support). Nodes without symbols were
not reproducible between trees. The scale bar represents 5% estimated sequence divergence. Class-level clades are bracketed to the right of the figure
in black. The target ranges of KSB3-specific FISH (fluorescence in situ hybridization) probes used in this study are indicated by colored brackets with
the colors corresponding to cell color in (B) and (D). (B) 16S rRNA-targeted FISH detection of UASB14 and UASB270 filaments in the UASB sludge.
The abundant UASB14 filaments are labeled green and the low abundance UASB270 filaments are labeled red. (C) Total KSB3 filament abundance
highlighted by a phylum-level FISH probe relative to (D) all cells present in the same field (phase-contrast image). Bars in (B–D) represent 10 µm.
of marker genes broadly conserved in all domains of life (38 markers) (Darling et al., 2014)
or in Bacteria (83 markers) (Soo et al., 2014) were obtained from the KSB3 genomes and
up to 354 publicly available reference genomes (Markowitz et al., 2014). Each gene family
was independently aligned and ambiguous and/or non-informative positions removed,
and then the filtered alignments were concatenated for maximum-likelihood inference.
Sekiguchi et al. (2015), PeerJ, DOI 10.7717/peerj.740 10/24

Four sets of outgroup configurations were used including representatives of all major
genomically sampled bacterial phyla (Table S3). The two KSB3 genomes form a robustly
monophyletic group in all analyses, and did not reproducibly affiliate with any other phyla
(Fig. 2,Fig. S7,Table S4), consistent with the original proposal that KSB3 is a candidate
bacterial phylum (Yamada et al., 2007). The average amino acid identity (AAI) between
UASB14 and UASB270 is 60.3% (Fig. S8), and supports their assignment to separate classes
as it falls within the range of known class-level AAI values (44–61%, Konstantinidis &
Tiedje, 2005).
To confirm the filamentous morphology and relative abundance of the KSB3
phylotypes, we designed 16S rRNA-targeted fluorescence in situ hybridization (FISH)
probes specific at the phylum and class level and combined them with previously applied
KSB3-specific probes (Yamada et al., 2007) (Table S5). We detected only filamentous KSB3
morphotypes in the UASB sludge and these comprised the majority of observed filaments
in sample F1 (Figs. 1C–1D). The relative abundance of the two KSB3 phylotypes inferred
from both amplicon and metagenome data was also consistent with FISH analyses; that
is, filaments belonging to the class Moduliflexia (presumably mostly UASB14) greatly
outnumbered those belonging to the Venturitrichia (presumably mostly UASB270)
(Fig. 1B), noting that the two filaments were indistinguishable by light microscopy alone
(Figs. 1C–1D).
General features of the KSB3 genomes
Both KSB3 genomes are large by bacterial standards, >7 Mb (Fig. S9) and have median
GC content, ∼50% (Table 1). Since UASB14 and UASB270 are not close relatives, large
genome size may be a characteristic feature of the KSB3 phylum or at least of the two
classes that they represent (Fig. 1A). A total of 5,989 and 7,048 open reading frames
(ORFs) were identified in the UASB14 and UASB270 genomes, respectively (Table 1).
For both genomes, approximately two thirds of the ORFs had a predicted function and
the remaining third were hypotheticals. Reciprocal BLASTP best matches between the
predicted gene products of the two KSB3 genomes indicate a shared set of 3,296 orthologs,
representing approximately half of the gene inventories in each genome. Included in this
common set are conserved genes for translation, nucleotide transport and metabolism,
and construction of a diderm (Gram negative) cell envelope including lipopolysaccharide
synthesis (Fig. S10). We identified a full complement of rRNA and tRNA genes in UASB14,
but not UASB270, which were likely missed in the latter genome (Table 1;Table S6).
The UASB14 rRNA genes are estimated to be present as three nearly identical operons
(Fig. S5) collapsed into a single large repeat during the assembly process. A number of
large clustered regularly interspaced short palindromic repeats (CRISPR) were identified in
both genomes (Table 1). CRISPR, together with associated cas genes, constitute a recently
described defense mechanism against invading foreign DNAs and have been found in a
majority of bacterial genera and most Archaea (Sorek, Kunin & Hugenholtz, 2008). A COG
category analysis of the KSB3 genomes indicates that both have high relative proportions
of carbohydrate metabolism and transport (G) and signal transduction (T) relative to the
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Figure 2 Maximum-likelihood phylogenetic inference of Modulibacteria (KSB3) population genomes among known bacterial phyla. The tree
was constructed using RAxML based on up to 38 marker genes (using taxon-outgroup configuration Config 3, Table S3) and sequences were
collapsed at the phylum level except for classes in the Proteobacteria. Ranks are indicated by prefix; p (phylum), c (class). KSB3 genomes obtained
in this study are highlighted in red. Superphyla (Terrabacteria, Patescibacteria, Fibrobacteres-Chlorobi-Bacteroidetes [FCB], and Planctomycetes-
Verrucomicrobia-Chlamydiae [PVC]) are highlighted with color ranges. Taxa comprising cultivated representatives are shown in black; taxa with no
cultivated representatives are indicated by outline. Reproducible associations (>80% bootstrap values from 100 resamplings) are indicated by dots
on interior nodes. Alignments of homologous proteins from archaeal genomes were used to root the tree (not shown). The scale bar represents 10%
estimated sequence divergence.
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bacterial average (Fig. 3). More detailed inferred metabolic properties of the two KSB3
genomes are described below.
Strictly fermentative metabolism
Both KSB3 representatives have an incomplete tri-carboxylic acid (TCA) cycle and
lack most electron-transport chain complexes including terminal oxidases, indicating
a strictly fermentative metabolism (Fig. 4). They encode, however, both superoxide
reductase and thioredoxin reductase, suggesting oxidative stress tolerance. Both genomes
have a large complement of transporters and enzymes for importing and degrading
complex and simple carbohydrates, which can then be fed into a complete glycolysis
(Embden-Meyerhof-Parnas) pathway (Fig. 4; COG category G in Fig. 3). Both filaments
also are likely capable of hydrolyzing polymers such as cellulose and starch via a range
of glycoside hydrolases (Table S7). They redundantly encode four different enzymes for
converting pyruvate to acetyl–coenzyme A (acetyl-CoA), namely pyruvate dehydrogenase,
pyruvate-formate lyase, pyruvate ferredoxin oxidoreductase, and pyruvate-flavodoxin
oxidoreductase. Both generate adenosine triphosphate (ATP) by converting acetyl-CoA to
acetate via two enzymes (acetate kinase and phosphate acetyltransferase) commonly found
in general fermentative anaerobes (Mai & Adams, 1996), in addition to glycolysis. They
may reoxidize NADH produced during glycolysis by converting pyruvate to D-lactate and
acetyl-CoA to ethanol (Fig. 4).
The KSB3 genomes possess a large complement of enzymes for conversion and
transport of amino acid and peptides (Fig. 4; COG category E in Fig. 3), including
numerous proteases and peptidases (Table 1). Peptide and amino acid degradation in
the KSB3 filaments may produce pyruvate, oxaloacetate, succinyl-CoA, and possibly
propionyl CoA (Fig. 4). Notably, a complete set of genes for the methylmalonyl CoA
pathway was identified in UASB270, suggesting a role in either amino acid degradation,
propionate oxidation and/or propionate formation as a fermentative end product in this
organism. Some fermentative anaerobes are known to produce hydrogen to scavenge
excess electrons generated during metabolism (Sieber, McInerney & Gunsalus, 2012).
In both KSB3 genomes, we identified several hydrogenase genes (Fig. 4). By examining
domain structure and gene neighbourhoods (Fig. S11), we predict that some of these genes
encode catalytic enzymes. This may permit them to engage in syntrophic interactions
with hydrogenotrophs, such as methanogens, in the sludge granules (Sieber, McInerney &
Gunsalus, 2012). However, based on FISH experiments highlighting KSB3 and archaeal
cells, we did not observe a close proximity between the two groups that would facilitate
syntrophy (data not shown). Some of the hydrogenase genes are located next to signal
transduction genes raising the possibility that they are involved in signal transduction and
chemotaxis (Fig. S11).
Sensory capabilities and motility
One of the most striking features of the KSB3 genomes is the presence of extensive
regulatory networks, including two-component signal transduction systems (Table 1;
Table S8). Signal transduction genes (COG category T) are among the highest represented
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Figure 3 Relative representation of COG categories by predicted ORFs in the UASB14 and UASB270 genomes. Global averages and standard
deviation (bars) are shown for 2,279 publicly available finished bacterial and archaeal genomes (Markowitz et al., 2014). Statistically significant
differences are indicated by percentile of scores for all the available finished bacterial and archaeal genomes.
Sekiguchi et al. (2015), PeerJ, DOI 10.7717/peerj.740 14/24

Figure 4 Composite metabolic overview of the Modulibacteria (KSB3) genomes based on identified genes and pathways. Gray indicates elements
common to both genomes, while orange and green show elements specific to UASB14 and UASB270, respectively. Both filament types have the
genes necessary to produce acetate, ethanol, lactate, and hydrogen (and possibly propionate) as fermentative end products, likely generating energy
through the glycolytic Embden-Meyerhof-Parnas (EMP) pathway and the fermentation of amino acids and sugars. Abbreviations: ETF, electron
transfer flavoprotein; Fd-ox and Fd-red, oxidized and reduced ferredoxin, respectively; UQ, ubiquinone.
categories in both genomes (Fig. 3). Two-component systems respond to a broad range of
extracellular and intracellular signals, and play a role in many cellular processes including
growth, motility, and the cell cycle (Galperin, 2004;Skerker et al., 2005;Kirby, 2009).
UASB14 and UASB270 encode 135 and 114 putative transmembrane sensor proteins likely
used for environmental signaling (Galperin, 2004), and 131 and 116 putative response
regulators containing CheY-like domains, respectively (Table S8). They each contain over
60 methyl-accepting proteins and numerous Che-like chemotaxis proteins (Table S8).
Even when compensating for their relatively large genome sizes, both KSB3 genomes
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Figure 5 Number of protein domains inferred to be involved in environmental signaling for the two
Modulibacteria (KSB3) genomes and finished bacterial and archaeal genomes. Number of protein
domains inferred to be involved in environmental signaling (Table S8) as a function of genome size
for the two Modulibacteria (KSB3) genomes (in red) and 2,279 publicly available finished bacterial and
archaeal genomes (in blue). The KSB3 filaments have among the highest proportion of signaling domains,
only surpassed by members of the Myxobacteria (open blue circles), which are capable of fruiting body
formation by contact-mediated signaling.
possess high proportions of environmental sensory networks compared to other sequenced
bacterial and archaeal genomes (Fig. 5;Figs. S12 and S13;Table S9). The high represen-
tation of sensory components in the KSB3 genomes is on par with social Myxococcales
such as Sorangium cellulosum and Stigmatella aurantiaca, both of which exhibit complex,
self-organizing behavior in response to environmental stimuli (Huntley et al., 2011).
Sensory capabilities are an important component of a bacterium’s overall social
“intelligence” or social IQ (Ben-Jacob et al., 2004), a metric recently proposed based on
the abundance of two-component systems, transcription factors, defense mechanisms and
transport systems (Sirota-Madi et al., 2010). We determined that the KSB3 filaments have
among the highest social IQ scores of any sequenced bacterial and archaeal species to date,
scoring particularly well in the two-component and transport system categories (Fig. S14).
This suggests that the filaments are sensitive to their surroundings and capable of adaptable
behavior in response to changes in their local environment. Key to this adaptability is
motility. No genes for flagella production were identified in either KSB3 genome, so to
determine if KSB3 bacteria are indeed motile, we observed filaments enriched from UASB
granules by wet mount microscopy under a range of conditions. KSB3-specific FISH of
samples taken in parallel confirmed that the majority of filaments in these samples were
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members of the KSB3 phylum (Fig. 1). Initially no motility was observed, therefore based
on the metabolic reconstruction of the KSB3 genomes, we added a range of compounds
(mostly simple sugars, see ‘Methods’ section) to the edge of the microscope slides to create
a gradient that could be sensed by the filaments to stimulate a motility response. We
observed gliding motility at rates of between 20 to 30 µm/min only when a glucose or
maltose gradient was applied under conditions mimicking the UASB reactor operation
(Movie S1). Both KSB3 genomes encode a number of the genes necessary for type IV
pili formation (pilB, pilC, pilG, pilT, pilV, and flp pilus assembly protein) that may
enable gliding via extension and retraction (Jarrell & McBride, 2008). However, the full
gene complement for pili formation (Mauriello et al., 2010) was not detected and the
mechanism for KSB3 gliding motility remains to be determined.
DISCUSSION
Despite the biotechnological significance of industrial-scale anaerobic digestion, our
understanding of the microbial ecology that underpins these processes is still rudimentary
because most microorganisms cannot be cultured and such systems are essentially man-
aged as “black boxes” (Ahring, 2003b;Rivi`
ere et al., 2009). Emerging culture-independent
molecular techniques such as differential coverage binning of metagenomic data, which
allows even low abundance population genomes to be recovered (Sharon et al., 2013;
Albertsen et al., 2013), are providing new opportunities to understand and optimize system
performance (Vanwonterghem et al., 2014).
Using this approach, we obtained the first population genomes representing candidate
bacterial phylum KSB3 (Tanner et al., 2000;Yamada et al., 2007). One of these genomes,
UASB14, belongs to a high abundance filament (∼10% of the community; Table 1;Fig. 1)
previously reported to be responsible for bulking in an industrial UASB system treating
wastewater from sugar manufacture (Yamada et al., 2007). A second genome from the
same habitat, UASB270, represents a low abundance (<0.5%) filament only moderately
related to the first, i.e., they represent different classes within the KSB3 phylum (Fig. 1).
Metabolic reconstruction indicates that both filaments are primary fermenters of sugar
and amino acid-containing compounds in the system (Fig. 4), and both have a high “social
IQ” based in part on possession of extensive regulatory networks (Table 1;Tables S8 and
S9;Fig. S14). These findings support the hypothesis that KSB3 filaments are important
primary fermenters in healthy sludge granules (Yamada et al., 2011) and further suggest
that the filaments are sensitive to their surroundings and that their cellular processes,
such as growth, may be controlled by external signals. Whether these features can be
extrapolated to the whole KSB3 phylum, or simply reflect the specialized habitat from
which the genomes were obtained, remains to be determined. Environmental surveys
suggest that the phylum has a shallow ecological footprint, having been identified in mostly
anoxic saline habitats (Fig. 1A), which may indicate that a fermentative metabolism is
universal.
The inferred capacity of the filaments to detect physicochemical gradients in their
surroundings suggests that they should be motile. Apart from an incomplete gene
Sekiguchi et al. (2015), PeerJ, DOI 10.7717/peerj.740 17/24

complement for Type IV pili, no motility mechanism could be identified. However,
microscopic observations indicated that the KSB3 filaments are capable of gliding motility
in response to applied sugar gradients (Movie S1). Gliding motility is thought to have
evolved independently in multiple bacterial lineages, and the molecular mechanisms
of gliding are only partially elucidated for a limited number of bacterial taxa (Jarrell
& McBride, 2008;Mignot & Kirby, 2008). This is the first report of gliding motility
of organisms in UASB sludge granules, which have long been considered to have an
organization driven by growth and attachment rather than motility of cells (Liu et al.,
2003;HulshoffPol et al., 2004). An enhanced sensory system is also likely the key driver of
the bulking phenomenon; that is, changes in the UASB reactor such as increases in glucose
or maltose concentration trigger outgrowth of the KSB3 filaments (Yamada et al., 2011).
It may also explain why repeated attempts to cultivate KSB3 filaments have failed to date
(Yamada et al., 2011), because they require specific and possibly complex environmental
cues to stimulate growth in axenic culture.
The inference that the KSB3 filaments sense sugars and the observation of a gliding
motility response in the presence of a glucose or maltose gradient is consistent with the
previous observation of uptake of these sugars by KSB3 filaments (Yamada et al., 2011).
Plant operators began monitoring glucose concentration in the UASB reactor influent
using a simple urine test strip. No further bulking has occurred to date since keeping
influent glucose concentration uniformly low (<200 mg/L) via adjustment of retention
times in the acidification pretreatment. A more detailed understanding of environmental
stimuli responsible for growth and bulking will be facilitated by the availability of the KSB3
genome sequences which may lead to genome-directed cultivation (Tyson et al., 2005) and
other treatment options for bulking.
We propose the names ‘Candidatus Moduliflexus flocculans’ and ‘Candidatus
Vecturithrix granuli’ for the two KSB3 filament types represented by the UASB14 and
UASB270 genomes respectively, and the phylum name, Modulibacteria, and intermediate
rank names (Table 1;Supplemental Information 1).
CONCLUSIONS
In summary, this study adds novel genomic ‘foliage’ to the tree of life by reporting the near
complete genomes of two phylogenetically diverse members of candidate bacterial phylum
KSB3 obtained from an industrial UASB system. Genome-based metabolic reconstruction
and experimental observations provide clues to the roles of the KSB3 bacteria in the
treatment system including their ability to ferment sugars and chemotactically respond
to glucose and maltose gradients, laying the foundations for a detailed understanding of
their ecophysiology and role in wastewater bulking.
ACKNOWLEDGEMENTS
We thank Jason Steen, Ben Woodcroft, Mohamed F. Haroon, and Michael Imelfort from
the University of Queensland for assistance and advice on the bioinformatic analyses and
Taeko Yokoi from AIST for assistance with ITS and FISH experiments. We also thank
Sekiguchi et al. (2015), PeerJ, DOI 10.7717/peerj.740 18/24

Satoshi Hanada from AIST and Bernhard Schink from the University of Konstanz for
etymological advice.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
The study was supported by the Biomedical Research Institute of the National Institute of
Advanced Industrial Science and Technology (AIST). Philip Hugenholtz and Donovan H.
Parks were supported by an Australian Research Council (ARC) Discovery Outstanding
Researcher Award (DORA) grant DP120103498. Gene W. Tyson was supported by an ARC
Queen Elizabeth II Fellowship, grant DP1093175. Donovan H. Parks was also supported
by the Natural Sciences and Engineering Research Council of Canada. The funders had no
role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
Grant Disclosures
The following grant information was disclosed by the authors:
National Institute of Advanced Industrial Science and Technology.
Australian Research Council.
Discovery Outstanding Researcher Award: DP120103498.
ARC Queen Elizabeth II Fellowship: DP1093175.
Natural Sciences and Engineering Research Council of Canada.
Competing Interests
Yuji Sekiguchi and Akiko Ohashi are employees of the National Institute of Advanced
Industrial Science and Technology (AIST). Toshihiro Yamauchi is an employee of Kubota
Kasui Corporation.
Author Contributions
•Yuji Sekiguchi conceived and designed the experiments, performed the experiments,
analyzed the data, contributed reagents/materials/analysis tools, wrote the paper,
prepared figures and/or tables, reviewed drafts of the paper.
•Akiko Ohashi and Toshihiro Yamauchi performed the experiments, contributed
reagents/materials/analysis tools, reviewed drafts of the paper.
•Donovan H. Parks and Gene W. Tyson analyzed the data, contributed
reagents/materials/analysis tools, reviewed drafts of the paper.
•Philip Hugenholtz analyzed the data, contributed reagents/materials/analysis tools,
wrote the paper, prepared figures and/or tables, reviewed drafts of the paper.
DNA Deposition
The following information was supplied regarding the deposition of DNA sequences:
The genome sequences of ‘Candidatus Moduliflexus flocculans’ and ‘Candidatus
Vecturithrix granuli’ have been deposited in DDBJ/EMBL/GenBank under the accession
numbers DF820455–DF820462 and DF820463–DF820483. The ITS sequences of the
Sekiguchi et al. (2015), PeerJ, DOI 10.7717/peerj.740 19/24

two genotypes have also been deposited in the databases under the accession numbers
AB933567 and AB933568.
New Species Registration
The following information was supplied regarding the registration of a newly described
species:
Not applicable.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/
10.7717/peerj.740#supplemental-information.
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