Detects and removes contamination in genome assemblies via two disjoint workflows - foreign-sequence screening of a single-organism (eukaryote/isolate) assembly with NCBI FCS-GX (GenBank-submission-mandatory), FCS-adaptor, and BlobToolKit blob plots; and MAG/bin quality assessment with CheckM2 plus GUNC (chimerism) plus GTDB-Tk taxonomy, judged against MIMAG. Covers why CheckM2 alone is blind to disjoint-marker chimeras, the FCS-GX RAM wall, organelle/NUMT triage, strain heterogeneity, and the HGT-vs-contamination (tardigrade) trap. Use when screening an assembly for foreign contamination before GenBank submission, assessing MAG completeness/contamination/chimerism, deciding which contigs to remove, or distinguishing real HGT from contaminant contigs.
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Detects and removes contamination in genome assemblies via two disjoint workflows - foreign-sequence screening of a single-organism (eukaryote/isolate) assembly with NCBI FCS-GX (GenBank-submission-mandatory), FCS-adaptor, and BlobToolKit blob plots; and MAG/bin quality assessment with CheckM2 plus GUNC (chimerism) plus GTDB-Tk taxonomy, judged against MIMAG. Covers why CheckM2 alone is blind to disjoint-marker chimeras, the FCS-GX RAM wall, organelle/NUMT triage, strain heterogeneity, and the HGT-vs-contamination (tardigrade) trap. Use when screening an assembly for foreign contamination before GenBank submission, assessing MAG completeness/contamination/chimerism, deciding which contigs to remove, or distinguishing real HGT from contaminant contigs.
Before using code patterns, verify installed versions match. If versions differ:
CLI: <tool> --version then <tool> --help to confirm flags
Python: pip show <package> then help(module.function) to check signatures
Database release drives results more than the binary version. Record: the FCS-GX GX database release (it grows each release; ~470 GiB and rising), the CheckM2 DIAMOND DB version, the (proGenomes 2.1 default vs GTDB), and the (e.g. R220) - GTDB-Tk fails loudly if the DB release does not match the binary. If code throws an error, introspect the installed tool and adapt rather than retrying.
GUNC reference
GTDB-Tk reference package release
Contamination Detection
"Is my assembly contaminated?" -> First decide which of two unrelated questions is being asked - "which contigs are not my organism?" (foreign-sequence screen) or "how complete/clean/chimeric is this bin?" (MAG quality) - then run the matching toolset; crossing them returns confidently wrong numbers.
CLI (MAG): checkm2 predict -i bins/ -o out/ AND gunc run -d bins/ -o out/, gtdbtk classify_wf
The Single Most Important Modern Insight -- "Contamination" Is Two Disjoint Problems With Disjoint Toolsets
There is no single "contamination" measurement. There are two questions that share a word and almost nothing else, and applying one toolset to the other problem runs to completion and prints a plausible, meaningless number.
Foreign sequence in a single-organism assembly (a eukaryotic nuclear genome or a cultured isolate with bacterial/human/vector contigs in it). The question is which contigs are not my target organism? Answer = a set of contigs to remove, trim, or split out. Tools: NCBI FCS-GX + FCS-adaptor (now GenBank-submission-mandatory), BlobToolKit (the GC x coverage x taxonomy blob plot). There is no "contamination %".
MAG/bin quality (a bin recovered from a metagenome). The question is how complete is this bin and how much is from other organisms? Answer = a percentage pair (completeness %, contamination %) plus a chimerism verdict, judged against MIMAG. Tools: CheckM2 + GUNC together, GTDB-Tk for taxonomy.
The cardinal category error: running CheckM2/CheckM on a eukaryotic nuclear assembly (its bacterial/archaeal marker sets are meaningless for a eukaryote - eukaryote completeness is BUSCO; see assembly-qc), or running FCS-GX on a single MAG and reading the flagged-length fraction as "the MIMAG contamination %". Both wrong applications complete silently and emit a number. The only defense is knowing which question each tool answers.
The second load-bearing fact - CheckM2 and GUNC are blind to different things, so report the pair, never the % alone. CheckM2 estimates contamination from marker-gene redundancy (a single-copy marker seen twice = contamination signal). A chimera of two organisms with disjoint marker complements - organism A contributed markers 1-60, organism B markers 61-120 - shows no duplicated markers, so CheckM2 reports low contamination while the bin is biological nonsense. GUNC catches exactly this, because it scores whether taxonomic signal is consistent across contigs (clade separation score), not marker counts. Neither is a superset of the other.
ML completeness + contamination (lineage-agnostic; handles novel/reduced lineages)
MAG (default)
CheckM (legacy)
Parks 2015 Genome Res
lineage-specific collocated markers on a placement tree; reports Strain heterogeneity
MAG (legacy; slow, ~40 GB RAM)
GUNC
Orakov 2021 Genome Biol
taxonomic-chimerism detection via clade separation score (CSS)
MAG (run WITH CheckM2)
GTDB-Tk
Chaumeil 2020 Bioinformatics
GTDB taxonomic classification of bacterial/archaeal genomes
MAG taxonomy
BlobTools (Laetsch & Blaxter 2017 F1000Res) and CheckM are the legacy predecessors; BlobToolKit and CheckM2 are the maintained defaults. CAT/BAT (contig taxonomy) and MAGpurify (reference-free MAG cleaning) exist but are secondary; RefineM is deprecated by its author (use GUNC + manual curation, or MDMcleaner for SAGs/dark-matter lineages).
Decision Tree by Scenario
Scenario
Recommended
Why
Eukaryotic nuclear assembly headed for GenBank
FCS-adaptor -> FCS-GX (mandatory) -> BlobToolKit for triage
submission bounces without a clean FCS screen
Cultured bacterial/archaeal isolate
FCS-GX + FCS-adaptor; CheckM2 as a completeness sanity check
an isolate should be ~100% complete, low contam
MAG(s) binned from a metagenome
CheckM2 AND GUNC together; score vs MIMAG; GTDB-Tk for taxonomy
the % pair plus the orthogonal chimerism verdict
Unbinned metagenome
bin first (-> metagenome-assembly), THEN CheckM2 + GUNC per bin
CheckM2/GUNC consume bins, do not make them
Eukaryotic contigs mixed into a metagenome
tiara / Whokaryote / EukRep to partition euk from prok
CheckM2 markers are prokaryote-only
Organellar (mito/chloroplast) contigs in a nuclear assembly
tiara organelle classes + coverage spike -> separate, do NOT delete
organelles are real biology; submit as own record
Foreign-looking gene embedded in a host scaffold
investigate integration (-> comparative-genomics) before removing
# Adaptor/vector screen; tiny DB, trivial RAM. --euk or --prok for the lineage.
run_fcsadaptor.sh --fasta-input assembly.fa.gz --output-dir ./adaptor_out --euk
Adaptor and vector hits are unambiguous - always trim or exclude. Running adaptor first removes the obvious junk and shrinks the input before the expensive GX pass.
FCS-GX (The GenBank-Mandatory Foreign Screen)
# Screen against the GX database; --tax-id is the NCBI taxid of the SOURCE organism.
python3 ./fcs.py screen genome --fasta assembly.fa.gz --out-dir ./gx_out/ \
--gx-db "$GXDB_LOC/gxdb" --tax-id 9606
# Apply ONLY the auto-clean actions (EXCLUDE/TRIM/FIX) to produce a cleaned FASTA.
zcat assembly.fa.gz | python3 ./fcs.py clean genome \
--action-report ./gx_out/assembly.fa.9606.fcs_gx_report.txt \
--output clean.fasta --contam-fasta-out contam.fasta
The action report (<name>.<taxid>.fcs_gx_report.txt, where <name> keeps the input name minus only .gz - e.g. assembly.fa.gz -> assembly.fa.9606.fcs_gx_report.txt) assigns each flagged region an action: EXCLUDE (drop the whole sequence), TRIM (remove a contaminated end), FIX (hard-mask an internal span), or REVIEW (flagged but NOT auto-cleaned - manual inspection required). A separate INFO action specifically marks sequence known to be integrated into host genomes (e.g. endosymbiont insertions). clean genome acts on EXCLUDE/TRIM/FIX only; it does not touch REVIEW or INFO. Those non-auto-cleaned tiers exist precisely because FCS-GX can flag legitimate HGT/endosymbiont sequence as contaminant - never bulk-delete every flagged contig without reading the report.
The RAM wall (an operational, cloud-forcing fact). The GX database is ~470 GiB on disk and the documented sweet spot is a ~512 GiB-RAM host. Underprovisioned, the run does not fail - it crawls (minutes become days when the DB spills out of RAM). Best practice copies the DB into a tmpfs RAM disk; most labs rent a high-memory cloud VM for the screen. The first question before advising FCS-GX is "is there ~1/2 TB RAM or a cloud budget?".
Each contig is plotted by GC fraction (x) vs coverage (y), sized by length, colored by best-hit taxonomy. The target organism forms one tight GC x coverage cloud; a free-living contaminant grows independently and forms its own cloud at a different GC and a different coverage. Inputs: a hit file (BLAST/DIAMOND vs nt/UniProt), a coverage BAM, and an NCBI taxdump. blobtools filter can extract or drop by taxon - but see the failure mode below before doing so.
Merge CheckM2 and GUNC
Goal: Produce the joint CheckM2 x GUNC table that is the field standard for MAG QC, so a chimera invisible to CheckM2 is not reported as clean.
Approach: Load both reports, join on genome name, and apply MIMAG thresholds AND the GUNC pass flag together; never gate on contamination % alone.
CheckM2 also runs as checkm2 predict -i bins/ -o checkm2_out --threads 16 -x fa (downloads its own DIAMOND DB), and GUNC as gunc run -d bins/ -o gunc_out -t 16 -e .fa against a downloaded reference (gunc download_db). GTDB-Tk taxonomy is gtdbtk classify_wf --genome_dir bins/ --out_dir gtdbtk_out -x fa --cpus 16.
GUNC and the RRS Trap
pass.GUNC = TRUE at CSS <= 0.45 is not a clean bill of health when reference_representation_score (RRS) is low. Low RRS means the genome is barely represented in GUNC's reference DB - there is not enough signal to detect chimerism, so "pass" degenerates into "can't tell, not clean". For genuinely novel lineages (microbial dark matter), a GUNC pass is weak evidence; trust it only when RRS > 0.5, and otherwise lean on manual contig inspection or MDMcleaner. Read the configurations: CheckM2-high-contam + GUNC-pass = duplicative contamination within one lineage; CheckM2-low-contam + GUNC-fail = the dangerous disjoint-marker chimera; both pass with high RRS = the only "clean" verdict.
The Tardigrade Lesson (HGT vs Contamination)
Boothby 2015 (PNAS 112:15976) reported ~17% of tardigrade genes arrived by horizontal gene transfer; Koutsovoulos 2016 (PNAS 113:5053) showed almost all of it was undetected bacterial contamination in the assembly, dropping real HGT to ~1-2%. The original paper was corrected, not retracted - it stands as a cautionary monument. A foreign-looking gene has two explanations the sequence alone cannot distinguish: a contaminant contig, or a real HGT/endosymbiont gene integrated into the host genome. The tell is physical integration, not taxonomy. A real HGT gene sits on a host contig (host-gene flanks, host GC, host coverage, host introns); a contaminant sits on its own contig (foreign GC, its own coverage cloud, prokaryotic gene structure). Deleting everything that looks bacterial manufactures the opposite of Boothby's error - erasing real biology. Require integration evidence (host flanks, host-typical GC/coverage/introns, long reads spanning the host->foreign junction) before keeping or stripping.
Organelles and NUMTs (A Third Category)
Mitochondrial and chloroplast sequence is legitimate biology that does not belong in the nuclear assembly record - it needs separation, not deletion. Deleting it loses the organelle genome (often the most-cited part of a non-model genome paper); leaving it inflates assembly size and creates fake duplicated content. Identify organellar contigs by tiara's plastid/mito classes plus a massive coverage spike and circular topology; assemble/submit them as their own record. The subtle trap: NUMTs/NUPTs (nuclear-integrated organellar fragments) are real nuclear sequence that looks organellar - do not strip them. Coverage discriminates: free organelle = huge coverage spike; NUMT = nuclear-level coverage.
Per-Method Failure Modes
CheckM2 on a eukaryotic nuclear assembly
Trigger: running CheckM/CheckM2 on a vertebrate/plant/insect genome. Mechanism: the marker sets are bacterial/archaeal single-copy genes; a eukaryote has none of the relevant context. Symptom: a plausible Completeness/Contamination pair that is pure noise. Fix: use BUSCO for eukaryote completeness (-> assembly-qc); CheckM2 is for prokaryotes/MAGs only.
CheckM2 contamination read as the whole story (the chimera blind spot)
Trigger: "CheckM2 says 3%, the MAG is clean." Mechanism: CheckM2 counts marker redundancy and is blind to a chimera of two organisms with disjoint marker sets. Symptom: a 50/50 chimeric bin reported as low-contamination. Fix: run GUNC too; report the pair; trust a GUNC pass only when RRS > 0.5.
FCS-GX % read as MIMAG contamination
Trigger: running FCS-GX on a single MAG and citing flagged-length as the contamination %. Mechanism: FCS-GX answers "foreign vs declared tax-id", not the intra-domain marker-redundancy MIMAG cares about. Symptom: a "contamination %" that means something different from CheckM2's. Fix: use CheckM2 for the MIMAG %; FCS-GX for foreign-sequence screening of single-organism assemblies.
Auto-stripping every FCS-GX/blob foreign hit
Trigger: running clean genome and resubmitting, or blobtools filter on a whole taxon cloud, without reading the report. Mechanism: the screen flags legitimate HGT/endosymbiont/host genes with conserved-domain hits. Symptom: a real biological finding (HGT, symbiont) deleted from the assembly. Fix: treat EXCLUDE on near-complete-bacterial-genome contigs as real contamination; treat foreign flags on host-integrated, host-coverage, host-flanked sequence as REVIEW-and-verify.
Trusting blob-plot taxonomy color over cluster geometry
Trigger: filtering out contigs by their BLAST-hit color. Mechanism: best-hit taxonomy is the noisiest axis - conserved domains (ribosomal proteins, HSP70) hit foreign taxa, and no-hit != contaminant. Symptom: real host genes deleted; large no-hit fractions panic-removed. Fix: decide on cluster geometry - off the main cloud on GC and coverage and consistent foreign taxonomy; coverage is the strongest single signal.
Strain heterogeneity misread as contamination
Trigger: high CheckM contamination on a MAG with co-binned conspecific strains. Mechanism: near-identical single-copy markers from multiple strains register as duplicated. Symptom: inflated contamination % for what is one species' population. Fix: read CheckM's Strain heterogeneity column; high strain-heterogeneity = look closer (not ignore); GUNC + the strain flag disambiguate foreign-mixing from strain-mixing from duplication.
Quantitative Thresholds
Threshold
Source
Rationale
MIMAG high-quality MAG: completeness > 90% AND contamination < 5%
Bowers 2017 Nat Biotechnol
above 5% contam, gene-content/metabolic inference is unreliable
MIMAG medium-quality: completeness >= 50% AND contamination < 10%
Bowers 2017 Nat Biotechnol
community-standard MQ floor
MIMAG HQ also requires 5S/16S/23S rRNA + >= 18 tRNAs
Bowers 2017 Nat Biotechnol
binners systematically lose rRNA; the most common reason a >90%/<5% MAG is only MQ
GUNC pass: CSS <= 0.45
Orakov 2021 Genome Biol
benchmarked chimera/non-chimera cutoff; orthogonal to the % pair
GUNC pass trustworthy only when RRS > 0.5
Orakov 2021 Genome Biol / GUNC docs
low RRS = too poorly represented to judge; pass means "can't tell"
Blob-plot contaminant call needs GC AND coverage AND taxonomy agreement
Laetsch & Blaxter 2017; field practice
any single axis alone is a false-positive generator
FCS-GX GX DB ~470 GiB, ~512 GiB RAM host
NCBI FCS wiki (grows per release)
underprovisioned = crawls, not fails; cloud-forcing
Common Errors
Error / symptom
Cause
Solution
CheckM2 prints a Completeness/Contamination on a eukaryote
wrong tool for the organism
BUSCO for eukaryotes; CheckM2 is prokaryote/MAG only
"Clean" MAG (3% contam) is actually a chimera
CheckM2 blind to disjoint-marker chimeras
run GUNC; report the pair
GenBank submission bounced
FCS-GX/FCS-adaptor not run before submission
screen with FCS-adaptor then FCS-GX first
FCS-GX takes days
GX DB spilled out of RAM
provision ~512 GiB / tmpfs RAM disk / cloud VM
GTDB-Tk crashes on startup
DB release does not match the binary
install the reference package matching the GTDB-Tk version
Real HGT/symbiont gene deleted
auto-cleaned or taxon-filtered without review
check physical integration (host flanks/GC/coverage) before removal
Organelle genome lost
deleted as "contamination"
separate organellar contigs (coverage spike); submit as own record
References
Astashyn A, et al. 2024. Rapid and sensitive detection of genome contamination at scale with FCS-GX. Genome Biol 25:60.
Chklovski A, et al. 2023. CheckM2: a rapid, scalable and accurate tool for assessing microbial genome quality using machine learning. Nat Methods 20:1203-1212.
Parks DH, et al. 2015. CheckM: assessing the quality of microbial genomes recovered from isolates, single cells, and metagenomes. Genome Res 25:1043-1055.
Orakov A, et al. 2021. GUNC: detection of chimerism and contamination in prokaryotic genomes. Genome Biol 22:178.
Bowers RM, et al. 2017. Minimum information about a single amplified genome (MISAG) and a metagenome-assembled genome (MIMAG) of bacteria and archaea. Nat Biotechnol 35:725-731.
Chaumeil PA, et al. 2020. GTDB-Tk: a toolkit to classify genomes with the Genome Taxonomy Database. Bioinformatics 36:1925-1927.
Challis R, et al. 2020. BlobToolKit - interactive quality assessment of genome assemblies. G3 (Bethesda) 10:1361-1374.
Karlicki M, Antonowicz S, Karnkowska A. 2022. Tiara: deep learning-based classification system for eukaryotic sequences. Bioinformatics 38:344-350.
Boothby TC, et al. 2015. Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. PNAS 112:15976-15981.
Koutsovoulos G, et al. 2016. No evidence for extensive horizontal gene transfer in the genome of the tardigrade Hypsibius dujardini. PNAS 113:5053-5058.
Related Skills
assembly-qc - BUSCO completeness for eukaryotes (not CheckM2) and the QC handoff
metagenome-assembly - Binning produces the bins this skill scores with CheckM2 + GUNC
hifi-assembly - False duplications inflate apparent content; distinct from contamination
long-read-assembly - Produces the contigs screened here for foreign sequence
metagenomics/kraken-classification - Read/contig taxonomic classification and pre-assembly host screening
comparative-genomics/genome-distance-and-species-delineation - ANI/species placement after decontamination
