A natural tripartite anaerobic symbiosis inside a single ciliate cell, found in freshwater rainforest stream sediment near Libreville, Gabon. The host, Tropidoatractus magnetotacticus sp. nov., produces H2 in hydrogenosomal mitochondrion-related organelles. Two intracellular prokaryotic partners consume it: a magnetotactic sulfate-reducing bacterial population in the Thermodesulfobacteriota that encodes and expresses a magnetosome gene cluster, and an endosymbiotic hydrogenotrophic methanogen affiliated with Methanoregula. The bacterial partner biomineralizes chains of ferrimagnetic magnetite nanoparticles inside the host, so the eukaryote's magnetotaxis is a trait acquired from a symbiont rather than one it encodes itself. "Tripartite" names three functional guilds, not three genomes. The sulfate reducer bin splits by GC content into a magnetosome-bearing and a magnetosome-free population, and refined binning yielded six MAGs for the former, two for the methanogen, and four sub-bins for the ciliate. The whole syntrophic network is built from gene presence and transcript expression. No metabolite concentration or flux was measured, no FISH confirmation succeeded, and the source microcosm died after two years, so nothing here rests on a cultivated or re-samplable system.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Tropidoatractus magnetotacticus — ontology label: Tropidoatractus sp. MC-2025a | NCBITaxon:3469242 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
ELECTRON_DONOR
|
DOMINANT |
|
|||
| magnetotactic sulfate-reducing bacterial endosymbiont — ontology label: Thermodesulfobacteriota | NCBITaxon:200940 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
CROSS_FEEDER
|
N/A |
|
|||
| endosymbiotic hydrogenotrophic methanogen — ontology label: Methanoregula | NCBITaxon:395331 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
CROSS_FEEDER
|
COMMON |
|
|||
Ecological Interactions
The graph shows only explicitly linked interaction participants. Community members without an asserted participant link are omitted; membership alone does not imply an interaction.
Proposed host hydrogen transfer to the magnetotactic sulfate reducer
SYNTROPHYHYPOTHESIZED: host hydrogenosomal Fe-hydrogenase expression and bacterial metabolic reconstruction support possible delivery of host H2 or other fermentation products to the magnetotactic sulfate reducer. Retained SYNTROPHY denotes the authors proposed relationship, not measured hydrogen transfer, reciprocal dependence or a quantified host fitness benefit.
Source Taxon: Tropidoatractus magnetotacticus
Target Taxon: magnetotactic sulfate-reducing bacterial endosymbiont
Metabolites: hydrogen (CHEBI:18276 · MediaIngredientMech · CMMMech)
Biological Processes:
- hydrogenosomal fermentation (GO:0006113 · PathwayMech)
PARTIAL: proposed host-product consumption by the sulfate reducer could lower hydrogen availability and thereby improve host fermentation energetics. This is the authors mechanistic model, not a measured transfer-to-fitness causal effect.
Evidence
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"The Magneto_SRB_bin syntrophic partner could potentially utilize H2 (and or other fermentation products) released from the ciliate MROs"
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"The SRB and methanogen populations likely use the H2 produced by the MRO hydrogenosomal Fe-hydrogenase as an electron donor"
Proposed host hydrogen transfer to the endosymbiotic methanogen
SYNTROPHYHYPOTHESIZED: a complete methanogenesis pathway and expressed Methanoregula mcrA support the proposed hydrogenotrophic partner, alongside host hydrogenosomal Fe-hydrogenase expression. This is stronger pathway evidence than for the sulfate reducer but not direct measurement of host-to-methanogen H2 flux. SYNTROPHY remains a proposed relationship.
Source Taxon: Tropidoatractus magnetotacticus
Target Taxon: endosymbiotic hydrogenotrophic methanogen
Metabolites: hydrogen (CHEBI:18276 · MediaIngredientMech · CMMMech), methane (CHEBI:16183 · MediaIngredientMech · CMMMech)
Biological Processes:
- methanogenesis from carbon dioxide (GO:0019386 · PathwayMech)
PARTIAL: proposed host-product consumption by the methanogen could lower hydrogen availability and thereby improve host fermentation energetics. This is the authors mechanistic model, not a measured transfer-to-fitness causal effect.
Evidence
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"endosymbiotic Methanoregula cells efficiently consume host-produced H2"
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"encoded a complete methanogenesis pathway"
Symbiont-conferred magnetotaxis
Observed field-reversal responses establish active host magnetotaxis, and microscopy identifies internal magnetite chains. The bacterial population encodes and expresses magnetosome genes, supporting the authors attribution of the magnetic capability to internal bacteria. Genome-to-cell identity remains inferred because FISH failed at low cell numbers. Native swimming polarity and adaptive benefit are unresolved: detection followed six months of storage in Munich. Capability provision does not itself measure reciprocal fitness, so no MUTUALISM type is assigned (consistent with issue #749).
Source Taxon: magnetotactic sulfate-reducing bacterial endosymbiont
Target Taxon: Tropidoatractus magnetotacticus
Metabolites: magnetite (CHEBI:46726 · MediaIngredientMech · CMMMech)
Biological Processes:
- magnetosome assembly (GO:0140923 · PathwayMech)
Evidence
-
PMID:42475569 - SUPPORT (IN_VIVO)"T. magnetotacticus exhibits magnetotaxis due to the magnetic moment of internal ferrimagnetic magnetite (Fe3O4) nanoparticles forming ellipsoidal"
-
PMID:42475569 - SUPPORT (COMPUTATIONAL)"a genomic population of MTB (Thermodesulfobacteriota) in magnetically sorted T. magnetotacticus cells was found that encoded and expressed a magnetosome gene cluster responsible for magnetosome Fe3O4 biomineralization"
-
PMID:42475569 - PARTIAL (IN_VIVO)"All attempts at fluorescent in situ hybridization (FISH) to independently verify the identity of the internal MTB also failed because of the low cell numbers"
Proposed tripartite hydrogen sink and host energetic benefit
HYPOTHESIZED: internal sulfate reducers and methanogens could consume host-derived H2 or other fermentation products, potentially improving host fermentation energetics. Molecular evidence supports this tripartite model, but hydrogen removal, energetic improvement and mutual survival dependence were not experimentally established for this association. It is not a measured reciprocal-benefit outcome.
Participating Taxa: Tropidoatractus magnetotacticus, magnetotactic sulfate-reducing bacterial endosymbiont, endosymbiotic hydrogenotrophic methanogen
Metabolites: hydrogen (CHEBI:18276 · MediaIngredientMech · CMMMech)
Evidence
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"likely act as sinks for H2 (and potentially other fermentation products), thereby improving the efficiency of the host"
-
PMID:42475569 - PARTIAL (COMPUTATIONAL)"a tripartite syntrophic network whereby anaerobic fermentation products from T. magnetotacticus are consumed by two syntrophic partners: MTB producing the magnetosome chains and hydrogenotrophic methanogens"
Associated Datasets
| Dataset | Type | Repository | Accession |
|---|---|---|---|
|
Magnetically sorted ciliate metagenomes and metatranscriptomes
Raw reads for the magnetically sorted T. magnetotacticus cells, registered under the provisional organism name Tropidoatractus sp. MC-2025a. Four metagenomes and four metatranscriptomes, from two magnetic sorts with two technical replicates each. |
METAGENOMICS | NCBI_BIOPROJECT | PRJNA1363731 |
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Do partner hydrogen uptake and magnetotaxis confer measurable host or symbiont fitness benefits?
The source proposes metabolic syntrophy from gene content and expression and reports magnetically guided behavior. It also states that survival dependence could not be shown and FISH identity confirmation failed. Native polarity after Munich storage remains unknown. These limitations do not erase observed magnetotaxis. Environmental polarity metadata is tracked separately in issue #1911.
Attaches to: ecological_interactions#Proposed host hydrogen transfer to the magnetotactic sulfate reducer, ecological_interactions#Proposed host hydrogen transfer to the endosymbiotic methanogen, ecological_interactions#Symbiont-conferred magnetotaxis, ecological_interactions#Proposed tripartite hydrogen sink and host energetic benefit
1 evidence item(s)
likely act as sinks for H2 (and potentially other fermentation products), thereby improving the efficiency of the host
Molecular reconstruction or author interpretation supports the proposed metabolic relationship; it does not measure interpartner hydrogen flux or host energetic benefit.
External Resources
| Name | Repository | Resource ID |
|---|---|---|
|
Primary publication - magnetotaxis via tripartite syntrophy
Open-access PNAS description of the magnetotactic ciliate, its two internal partners, and the syntrophic network inferred from their expressed metabolism. |
OTHER | PMID:42475569 |
|
||
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Geomagnetic inclination at collection | -27.9 degrees | N/A |
|
||
| Collection site | freshwater rainforest stream, Libreville, Gabon | N/A |
|
||
Cross-Mech Suite Links
Explore related entities across the CultureBotAI X-Mech knowledge bases: