A structured freshwater phototrophic bacterial consortium in which one central motile heterotrophic beta-proteobacterium, Candidatus Symbiobacter mobilis, is surrounded by approximately 15 green sulfur bacterial Chlorobium chlorochromatii epibionts. Genome analyses support a highly specialized symbiosis involving metabolite exchange, central-bacterium sensing and motility toward light and sulfide, cell-cell adhesion, and possible quinone-mediated electron exchange.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Chlorobium chlorochromatii CaD3 epibiont — ontology label: Chlorobium chlorochromatii CaD3 | NCBITaxon:340177 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
SYNTROPHIC_PARTNER
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N/A |
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| Candidatus Symbiobacter mobilis central bacterium — ontology label: Candidatus Symbiobacter mobilis | NCBITaxon:1436290 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
CROSS_FEEDER
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N/A |
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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.
Phototroph To Central Bacterium Metabolite Exchange
SYNTROPHYHYPOTHESIZED - genome reconstruction and prior physiology suggest transfer of fixed-carbon/nitrogen compounds and cofactors from Chlorobium chlorochromatii to the central bacterium. The epibiont can grow axenically, whereas independent central-bacterium growth has not been obtained. Exact donor-to-recipient fluxes are unresolved, and the intact-consortium physiology assay required external 2-oxoglutarate. Photosynthetic CO2 and sulfide inputs are not identified exchanged products.
Source Taxon: Chlorobium chlorochromatii CaD3 epibiont
Target Taxon: Candidatus Symbiobacter mobilis central bacterium
Biological Processes:
- photosynthesis (GO:0015979 · PathwayMech)
HYPOTHESIZED - provision of epibiont-derived menaquinone-7 or a soluble intermediate could supply the quinone pool required by the proposed bidirectional electron shuttle. Neither quinone transfer nor shared protonmotive force was demonstrated; bulk/fraction quinone detection is not flux evidence.
Evidence
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PMID:24267588 - PARTIAL (COMPUTATIONAL)"extensive metabolic exchange, involving carbon, nitrogen, and sulfur sources as well as vitamins"
Central Bacterium Navigation For Epibiont Resources
PARTIAL - the central bacterium supplies the consortium flagellum, and intact consortia exhibit chemotaxis to sulfide and a scotophobic response. The 2013 study demonstrated recombinant central-bacterium photosensor activity in E. coli, supporting a direct-sensing model, but did not establish its necessity in intact consortia. Sulfide sensors remain unidentified and indirect epibiont signaling is not excluded. Navigation can position attached phototrophs at resources; colonization facilitation and its effect on metabolite transfer were not measured.
Source Taxon: Candidatus Symbiobacter mobilis central bacterium
Target Taxon: Chlorobium chlorochromatii CaD3 epibiont
Metabolites: hydrogen sulfide (CHEBI:16136 · MediaIngredientMech · CMMMech)
HYPOTHESIZED - navigation toward light and sulfide could improve resource access for the attached phototroph and thereby support production of metabolites available to the central bacterium. Taxis is observed, but its causal effect on partner-resolved metabolite flux was not isolated.
Evidence
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PMID:24267588 - PARTIAL (COMPUTATIONAL)"can sense and move towards light and sulfide"
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PMID:24267588 - PARTIAL (IN_VITRO)"Recombinant bacteriophytochromes produced in these three strains were purified and characterized by absorbance spectroscopy."
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PMID:9446684 - SUPPORT (IN_VITRO)"sulfide, thiosulfate, 2-oxoglutarate, and citrate served as a chemoattractant"
Possible Quinone-Mediated Electron Exchange
SYNTROPHYHYPOTHESIZED - quinone or intermediate transfer and bidirectional quinone/quinol cycling could couple central-bacterium energy conservation to epibiont photosynthesis, potentially sharing protonmotive force through cell contacts. HPLC detected menaquinone-7 in intact consortia and central-bacterium-enriched fractions, not purified central cells or traced interspecies transfer. The authors explicitly require experimental tests of this electron-shuttling model.
Source Taxon: Chlorobium chlorochromatii CaD3 epibiont
Target Taxon: Candidatus Symbiobacter mobilis central bacterium
Evidence
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PMID:24267588 - PARTIAL (COMPUTATIONAL)"electron cycling mechanisms, particularly those mediated by quinones"
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PMID:24267588 - PARTIAL (COMPUTATIONAL)"we hypothesize that menaquinone-7, or a soluble quinone intermediate synthesized by Chl. chlorochromatii"
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PMID:24267588 - PARTIAL (IN_VITRO)"Only menaquinone-7 was detected in extracts prepared from intact consortia and fractions enriched in the central bacterium."
Associated Datasets
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which metabolite and electron fluxes, and which sensory proteins, are necessary in the intact consortium?
The genomic model, recombinant photosensor assays and fraction HPLC do not resolve partner-specific exchange, electron flow or shared protonmotive force. Sulfide sensing and indirect signaling remain unresolved. The 1998 growth assay required external 2-oxoglutarate; epibiont axenic growth excludes mutual obligate growth. Direct transfer, mediator perturbation/rescue and intact-consortium sensory tests are needed. Main-text quinone figure numbering differs from the supplement list; no unseen supplemental panel is certified.
Attaches to: ecological_interactions#Phototroph To Central Bacterium Metabolite Exchange, ecological_interactions#Central Bacterium Navigation For Epibiont Resources, ecological_interactions#Possible Quinone-Mediated Electron Exchange
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| anoxic sulfide-reduced medium | Anaerobic sulfide-containing culture conditions | N/A |
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| light and sulfide gradients | Light and sulfide are sensed by the central bacterium and used by the epibiont. | N/A |
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Growth Media
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