MOCAT: A Metagenomics Assembly and Gene Prediction
Jens Roat Kultima1., Shinichi Sunagawa1., Junhua Li2,3, Weineng Chen2, Hua Chen2, Daniel R. Mende1,
Manimozhiyan Arumugam1, Qi Pan2, Binghang Liu2, Junjie Qin2, Jun Wang2, Peer Bork1,4*
1Structural and Computational Biology Unit, European Molecular Biology Laboratory, Heidelberg, Germany, 2Department of Science and Technology, BGI-Shenzhen,
Shenzhen, Guangdong, China, 3School of Bioscience and Biotechnology, South China University of Technology, Guangzhou, Guangdong, China, 4Max-Delbruck-Centre
for Molecular Medicine, Berlin-Buch, Germany
MOCAT is a highly configurable, modular pipeline for fast, standardized processing of single or paired-end sequencing data
generated by the Illumina platform. The pipeline uses state-of-the-art programs to quality control, map, and assemble reads
from metagenomic samples sequenced at a depth of several billion base pairs, and predict protein-coding genes on
assembled metagenomes. Mapping against reference databases allows for read extraction or removal, as well as abundance
calculations. Relevant statistics for each processing step can be summarized into multi-sheet Excel documents and
queryable SQL databases. MOCAT runs on UNIX machines and integrates seamlessly with the SGE and PBS queuing systems,
commonly used to process large datasets. The open source code and modular architecture allow users to modify or
exchange the programs that are utilized in the various processing steps. Individual processing steps and parameters were
benchmarked and tested on artificial, real, and simulated metagenomes resulting in an improvement of selected quality
metrics. MOCAT can be freely downloaded at http://www.bork.embl.de/mocat/.
Citation: Kultima JR, Sunagawa S, Li J, Chen W, Chen H, et al. (2012) MOCAT: A Metagenomics Assembly and Gene Prediction Toolkit. PLoS ONE 7(10): e47656.
Editor: Jack Anthony Gilbert, Argonne National Laboratory, United States of America
Received June 27, 2012; Accepted September 13, 2012; Published October 17, 2012
Copyright: ? 2012 Kultima et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was funded by EMBL, the European Community’s Seventh Framework Programme via the MetaHIT (HEALTH-F4-2007-201052), International
Human Microbiome Standards (IHMS) (HEALTH-F4-2010-261376), and Cancerbiome (ERC Advanced Grant 268985) grants. The funders had no role in study design,
data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
. These authors contributed equally to this work.
The emerging field of metagenomics has enabled researchers to
study the structure, dynamics, and functionality of uncultured
microbial communities. Processing the vast amounts of metage-
nomics data usually involves quality-controlling raw sequence
reads, aligning them to reference databases, and assembling them
into longer contigs prior to predicting genes. Several packages are
available for processing and analyzing metagenomics data, either
as web- and cloud-based services or stand-alone computational
pipelines [1–7]. But currently none of them supports the assembly
and gene prediction of metagenomics data produced by the
As exemplified by recent clinical, large-scale, and on-going
studies (e.g., the Human and Earth Microbiome Projects), the
usage of high throughput sequencing (HTS) data can be
anticipated to further increase considerably in both terms of data
volume and scope of application [8–11]. Thus, there is an
imminent need for applications providing standardized methods
for processing of HTS data in the form of pipelines  to
facilitate comparative downstream analyses.
To address these issues, we have developed MOCAT, a
metagenomics assembly and gene prediction toolkit for both small
and large-scale processing of metagenomic data produced by the
Illumina sequencing technology.
Results and Discussion
The main pipeline is divided into five major steps: (i) quality
trimming and filtering of raw reads, (ii) optional mapping to
remove, extract, and/or quantify reads matching a reference
database, (iii) assembly, (iv) assembly revision, and (v) gene
prediction (Figure 1). Statistics from each step are summarized
into multi-sheet Excel documents, as well as queryable SQLite
databases. Full details of output files and statistics produced in
each processing step are given in Table S7.
The individual processing steps in MOCAT were benchmarked
using three different data sets: 124 published human gut
metagenomic samples , a mock community produced by the
Human Microbiome Project (HMP) with 22 species from 19
genera , and a simulated metagenome with 100 strains from 85
species . By using this combination of host associated,
artificial, and simulated metagenomes with different taxonomical
resolution, we show that MOCAT can efficiently process a variety
of metagenomic samples, ranging in both size (0.5–16.6 Gbp),
origin and composition owing to new developments in each of the
five major steps.
i) Quality Trimming and Filtering of Raw Reads
Read quality trimming and filtering can greatly improve the
length and accuracy of metagenomic assemblies . Therefore,
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in the first processing step, raw reads below specified quality
and length cutoffs are trimmed or removed using either the
FastX program (http://hannonlab.cshl.edu/fastx_toolkit/) or the
DynamicTrim algorithm in the SolexaQA package . The
supported FastX program removes bases from the 39 end below
a user-defined threshold, whereas the DynamicTrim algorithm
in the SolexaQA package keeps the longest contiguous read
segment in which all quality scores are above the user-defined
threshold . After quality trimming and filtering our three
test datasets, 57–79% of the reads remained as high quality
reads (Table S1).
Additionally, to reduce base composition-biases that commonly
occur in HTS data , the frequency of each base at each
position over all reads is calculated, and bases that exceed two
standard deviations of the average base frequency within a sample
are trimmed from the 59 end of all reads. Using our test data set of
124 published human gut microbial samples, on average, the
fraction of reads that could be mapped to assemblies was 1%
higher when using 59 trimmed reads, compared to non-trimmed
reads (Table S2).
MOCAT also supports the FastQC package, for evaluating raw
read quality statistics (http://www.bioinformatics.bbsrc.ac.uk/
ii) Mapping, and Removal or Extraction of Reads
Matching a Reference Database
In the second step, reads can be mapped to reference sequences
in order to extract or remove reads from the original data set as
well as to calculate base or read coverages. For example, reads
from a human fecal metagenomic sample can be mapped to the
provided human genome database (hg19, Genome Reference
Consortium Human Reference 37) to remove reads of human
origin using SOAPAligner2 , or reads containing adapters
used for sequencing library construction can be removed using
Usearch . Reads can also be mapped to any other custom
reference database to calculate base and insert coverage of
reference sequences to estimate taxonomic and/or functional
composition of a sample, for example.
Here, we estimated the taxonomic composition of the simulated
metagenome by mapping reads to the set of original reference
genomes (Table S2 in  and Table S3) and calculating genome
size-normalized base and read coverages. The Pearson and
Spearman correlation coefficients between the observed and
expected composition of the simulated metagenome were 0.95
and 0.90, respectively, for both base and read counts (Figure 2),
and only 80 out of more than 30 Million reads were not aligned.
However, the observed abundances of genomes with very high
sequence identity may deviate from the expected abundances due
to reads mapping to both the genome of origin and other highly
When estimating taxonomic composition of the HMP mock
community, reads were mapped to reference sequences of the
community (Table S4). By first removing quality filtered and
trimmed reads matching known Illumina adapter sequences
(Table S5), the percentage of bases and reads mapping to the
reference genomes increased from 94.3% to 97.3%, and 95.0% to
97.6%, respectively, indicating the usefulness of a pre-screening
step. The taxonomic composition estimated here is similar to the
values calculated by the HMP consortium (Pearson correlation
coefficient of 0.75 and 0.83 for bases and reads mapping,
respectively, Figure 3), and also to estimates of 16S sequences
using 454 sequencing presented in (Figure S14, ). Experimen-
tal errors, not applicable to estimates of computationally simulated
metagenomes, may explain the lower correlation in the mock
community, compared to the simulated metagenome.
In the assembly step, a new version (1.06) of SOAPdenovo 
is used. For paired-end sequences, the insert size of each
sequencing library is estimated at run-time by mapping reads to
either reference marker genes  prior to assembly, or assembled
contigs prior to scaffolding. Similarly, Kmer sizes used for
assemblies are calculated at run-time for each individual
metagenome. Empirical tests on a large number of samples show
that estimating a Kmer size for each sample as the closest odd
number larger or equal to half the average read length may not
yield the best possible assembly, but balances assembly throughput
The accuracy of metagenomic assemblies was assessed using
data from the simulated metagenome and the mock community.
We used the percentage of predicted complete genes aligning to
the reference sequences of origin, as a proxy for correctly
assembled scaftigs (contigs that were extended and linked using
the paired-end information of sequencing reads). For the simulated
Figure 1. The MOCAT data processing pipeline. Metagenomic
samples are collected and sequenced. The raw sequence reads are
given as input to the pipeline, which are processed by modular steps
resulting in metagenome assemblies and predicted genes. Arrows
extending to the right from boxes, indicate input to various
downstream analyses. Statistics from each step are summarized into
multi-sheet Excel documents, as well as queryable SQLite databases.
MOCAT: Metagenomics Toolkit
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metagenome this value was 95.2% (12,385 complete genes
predicted), and for the mock community 89.3% of the complete
genes aligned (1,042 complete genes predicted). The lower number
of predicted complete genes in the mock community may be
explained by the relatively low number of high quality reads used
in the assembly for this metagenome.
The effect of using variable Kmer sizes, rather than a fixed
kmer, in the assembly step, was evaluated using the 124 gut
metagenomes. Estimating Kmer sizes at run-time for each
individual metagenome, rather than using a fixed Kmer size
across all samples, improved the number and frequency of
complete gene calls as well as overall average gene length (column
1 in Table 1).
iv) Assembly Revision
In the assembly revision step, a feature independent of the
utilized assembly packages, MOCAT can revise existing paired-
end read assemblies by aligning the reads to assembled scaftigs
Figure 2. Relative abundance of each reference genome present in the simulated metagenome. The observed abundances by mapping
reads to reference genomes and the expected abundance correlate with a Pearson correlation coefficient of 0.95 (base and read counts). Circles
represent genomes with multiple strains from one species and squares represent genomes with only one strain within the species. All, but one, of the
observations deviating from the diagonal are strains from the same species. These strains are either over- or under represented because reads are
mapped to other closely related strains in addition to the strain of origin. Highlighted by dashed lines, are two examples where a high sequence
similarity between strains (99.9% and 98.7% for the Synechococcus elongatus and Escherichia coli strains, respectively) can result in deviations from
Figure 3. Relative abundance of each genus present in the even HMP mock community. The estimated abundances using qPCR and by
mapping reads to reference genomes correlate with a Pearson correlation coefficient of 0.75 (base counts) and 0.83 (read counts).
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using the gap-tolerant BWA aligner  to correct for base errors
and short indels, and the fast SOAPaligner2 to resolve chimeric
regions. Performing assembly revision on the 124 human fecal
metagenomes further improved gene prediction metrics (column 2
in Table 1).
v) Gene Prediction
Finally, protein coding genes on the metagenomes are predicted
using either the default component Prodigal  or MetaGene-
Mark . An in depth comparison of the gene prediction
software is beyond the scope of this article. However, each
software have been benchmarked by the respective authors
(http://prodigal.ornl.gov/results.php and ). An independent
comparison determined that MetaGeneMark had a higher
precision and Prodigal a higher recall rate (http://genome.jgi.
The functionality and versatility of the pipeline has been
demonstrated using an artificial mock community metagenome, a
simulated metagenome with 100 species, and 124 human gut
metagenomes. Based on parameter exploration and data driven
parameter optimization at run-time, the MOCAT pipeline can
process metagenomes in a standardized and automated way while
improving the quality of assembly and gene prediction compared
to using default parameters for the supported programs. To date,
MOCAT has additionally been used to process and assemble
hundreds of host-associated and ocean metagenomes within the
scope of the MetaHIT  and TARA Oceans projects .
Implementation, Availability, and Requirements
MOCAT is implemented in Perl and installed by extracting the
package and executing one script, which downloads the default
external software used by the pipeline and sets up the software.
This reduces the otherwise tedious process of downloading all the
individual components, a common drawback of in-house pipelines
. Optional components requiring a license, such as MetaGene-
Mark  for gene prediction, and Usearch  for removal or
extraction of reads by alignment to a FASTA-formatted sequence
file, require a manual download.
A new project is quickly setup requiring only single- or paired-
end FastQ formatted sequencing reads files  for each sample in
a separate directory. The use of a project-specific configuration
file, with suggested default settings, offers users to run all
processing steps up to gene prediction without additional setup,
while allowing experienced users to modify parameters and
programs used in MOCAT. All of the settings are described in
the MOCAT documentation.
A queuing system enables processing of a large number of
samples in parallel. If present, MOCAT seamlessly integrates all
processing steps with the SGE and PBS queuing systems.
However, if no queuing system is available, MOCAT processes
samples serially on the machine it was executed.
MOCAT runs on 64-bit UNIX systems and can be freely
downloaded at http://www.bork.embl.de/mocat/. Perl version
5.8.8 or above is required. MOCAT is also available in a
Virtual Machine package, which could be used to run MOCAT
on a PC or a cloud based system. The open source code and
modular architecture allow users to modify or exchange the
programs that are utilized in the various processing steps. There
are no minimum hardware requirements for the pipeline itself
to run, however, requirements for analyzing metagenomic
datasets vary depending on the number of samples to process
in parallel and the sequencing depth of each sample. To aid in
determining whether local computational resources are ade-
quate, we provide in Table S6 and S8 the maximum resources
required to process the datasets in this article. We recommend
at least 16 GB of RAM to process smaller metagenomes and
64 GB of RAM to process medium sized metagenomes, but
these requirements may vary depending on project settings and
systems used. The hard disk space requirements depend on the
sizeand numberof metagenomes
recommend at least 500 GB of hard disk space.
Data for the simulated metagenome is publically available at
dataset consisted of simulated paired-end raw reads and 193
reference sequences (chromosomes and plasmids) from 100
genomes used to simulate this metagenome (Table S3). Metage-
nomic data for the even HMP mock community were downloaded
from http://www.ncbi.nlm.nih.gov/bioproject/48475, and the
references sequences were downloaded from the NCBI database
(Table S4), with the exception of Candida albicans, which was
for the mock community was downloaded from http://www.
hmpdacc.org/HMMC/. Datasets for the simulated metagenome
and the mock community can optionally be downloaded auto-
matically when installing the MOCAT pipeline.
Raw reads for the 124 human gut microbiomes were
Table 1. Progressive improvement of gene prediction metrics in 124 human gut metagenomes.
Improvement compared to fixed kmer=23 (%)
No assembly revision Revised assembly
Number of complete genes 8.110.2
Number of complete genes/Mbp4.618.5
Average gene length1.7 1.8
Gene prediction metrics are improved when using an automated kmer size in SOAPdenovo and with assembly revision (correction of base errors, short indels, and
chimeric contigs), compared to a fixed kmer size of=23 in SOAPdenovo and no assembly revision. The Kmer size is estimated as the closest odd number greater than
half the average read length for a sample. Numbers reported are in percent improvement of the respective quality metric. The calculated Kmer for each sample is given
in Table S8.
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Data Processing and Software Settings
The three datasets were processed by the read_trim_ filter step in
MOCAT with length cut off set to 30 and quality cut off set to 20,
using solexaqa for the mock community and the simulated
metagenome, and fastx for the 124 gut metagenomes.
Estimated taxonomic compositions for the simulated metagen-
ome and the mock community were calculated in three steps. First,
quality trimmed and filtered reads from the mock community were
screened against a FASTA-file with Illumina adapter sequences
(Table S5), using the screen_fastafile option and e-value set to 0.01.
Second, screened reads from the mock community and quality
trimmed and filtered reads from the simulated metagenome were
mapped and filtered against the custom-made reference databases
with chromosome and plasmid sequences from the 22 mock
genomes (Table S4) and 100 genomes from the simulated
metagenome (Table S2 in  and Table S3), respectively. This
was done by executing the screen and filter commands with length
cutoff set to 30, percentage identity set to 90 and paired_end_filter-
ing set to yes for the simulated metagenome and set to no for the
mock community. Finally, the taxonomic composition was
estimated using the calculate_coverage command.
Assembly and gene prediction, on the simulated metagenome
and mock community, were performed using the assembly
(SOAPdenovo version 1.06) and gene_prediction (MetaGeneMark)
options. Quality trimmed and filtered reads from the simulated
metagenome, and adapter-screened reads from the mock com-
munity, were assembled into scaftigs 60 bp or longer. Predicted
complete genes were aligned to their respective metagenomes
using blastall v2.2.26  (program blastn, 95% sequence identity,
alignment length .=90%, and e-value 0.1) and only the best hit
The 124 human gut microbiomes were processed with and
without 59 trimming. 59 trimmed reads were assembled using
SOAPdenovo 1.05, using both the Kmer determined by MOCAT
and a fixed Kmer size set to 23. These assemblies were revised
using SOAPdenovo 1.06 using the assembly_revision options, and
genes were predicted, with MetaGeneMark as selected software, on
scaftigs from both assemblies and revised assemblies. The non 59
trimmed and 59 trimmed reads were mapped to the assembled
scaftigs using the screen option using length cutoff 30 and quality
Complete commands for processing the simulated metagenome
and mock community in MOCAT are bundled with the
installation of the pipeline.
for the three metagenomic data sets used in this study.
Raw and high quality read and base statistics
trimmed and 59 trimmed reads.
Comparison of mapping rates of 59 un-
mated abundances for the simulated metagenome.
Mapping used when summarizing the esti-
even HMP mock community were mapped.
Reference sequences to which reads from the
to known Illumina adapters.
Aligned raw reads form the mock community
cessing time required for each processing step, for each
of the datasets used in this article.
Maximum computational resources and pro-
processing steps in MOCAT.
Output files and statistics from each of the
and reads, calculated Kmer size, and the computational
resources (RAM and HDD) required to assemble the 124
fecal metagenomics samples.
Number of raw and high quality (HQ) bases
We wish to thank the MetaHIT consortium and members of the Bork
group, especially Siegfried Schloissnig, for fruitful discussions and code
Conceived and designed the experiments: JRK SS PB. Performed the
experiments: JRK SS. Analyzed the data: JRK SS. Contributed reagents/
materials/analysis tools: DRM MA JL WC HC QP BL JQ JW. Wrote the
paper: JRK SS PB.
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