A defined anaerobic rumen-derived coculture in which the xylan-degrading fungus Neocallimastix frontalis PNK2 is grown with the hydrogenotrophic archaeon Methanobrevibacter smithii on xylan. The methanogen removes fermentation products through interspecies hydrogen transfer, shifting fungal metabolism toward acetate formation and increasing xylan utilization and extracellular hemicellulase activities.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Neocallimastix frontalis | NCBITaxon:4757 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
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COMMON |
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| Methanobrevibacter smithii | NCBITaxon:2173 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
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COMMON |
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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.
Interspecies Hydrogen Transfer During Xylan Fermentation
SYNTROPHYIn the N. frontalis PNK2-M. smithii coculture, hydrogen and formate did not accumulate, acetate formation increased and lactate decreased, consistent with the reported interspecies hydrogen transfer and methanogenesis. M. smithii alone did not utilize xylan or supply the assayed extracellular hydrolase activities. These results support metabolic coupling without establishing obligate dependence of fungal growth on this partner.
Source Taxon: Neocallimastix frontalis
Target Taxon: Methanobrevibacter smithii
Metabolites: xylan (CHEBI:37166 · MediaIngredientMech · CMMMech), dihydrogen (CHEBI:18276 · MediaIngredientMech · CMMMech), acetate (CHEBI:30089 · MediaIngredientMech · CMMMech)
Biological Processes:
- xylan catabolic process (GO:0045493 · PathwayMech)
- biological process involved in interspecies interaction between organisms (GO:0044419 · PathwayMech)
PARTIAL - Methanogenic removal of fermentation products shifts fungal metabolism toward acetate and is proposed to support the observed increase in xylan-derived sugar utilization. The coculture comparison supports the positive effect, but does not isolate hydrogen removal from growth and other partner-associated changes; utilization is distinct from polymer depolymerization.
HYPOTHESIZED - Methanogenic product removal is proposed to improve fungal sugar uptake, reducing extracellular xylose accumulation and catabolite repression of hemicellulases. This supplies a mechanistic path to the measured enzyme increase, not merely a shared coculture context; the authors leave control of sugar uptake unresolved.
Evidence
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PMID:16348244 - SUPPORT (IN_VITRO)"metabolite profiles became acetogenic as a result of interspecies hydrogen transfer"
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PMID:16348244 - SUPPORT (IN_VITRO)"more acetate and less lactate were formed, while formate and hydrogen did not accumulate"
Methanogen-Stimulated Xylan Utilization
SYNTROPHYCoculture with M. smithii increased the rate and extent of xylan utilization by N. frontalis PNK2 relative to fungal monoculture, with stimulation maintained during an 18-day experiment. Increased utilization does not imply a corresponding increase in polymer depolymerization: N. frontalis already degraded most xylan in both conditions, and the source distinguishes degradation from uptake and fermentation of released sugars.
Source Taxon: Methanobrevibacter smithii
Target Taxon: Neocallimastix frontalis
Metabolites: xylan (CHEBI:37166 · MediaIngredientMech · CMMMech)
Biological Processes:
- xylan catabolic process (GO:0045493 · PathwayMech)
Evidence
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PMID:16348244 - SUPPORT (IN_VITRO)"Both the rate and extent of xylan utilization were increased by coculturing"
Enhanced Extracellular Hemicellulase Activities
SYNTROPHYCoculture increased extracellular xylanase, alpha-L-arabinofuranosidase and beta-D-xylosidase specific activities, normalized to fungal protein rather than explained solely by greater biomass. Magnitudes depended on fungus, enzyme, substrate and sampling time; the abstract maxima summarize a comparison panel, not uniform fold changes for N. frontalis PNK2. Cell-bound endopeptidase was not increased and alpha-D-glucosidase was little affected.
Source Taxon: Methanobrevibacter smithii
Target Taxon: Neocallimastix frontalis
Metabolites: xylan (CHEBI:37166 · MediaIngredientMech · CMMMech)
Biological Processes:
- xylan catabolism (GO:0045493 · PathwayMech)
Evidence
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PMID:16348244 - SUPPORT (IN_VITRO)"marked increases in the specific activities of extracellular xylanase (up to fivefold)"
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PMID:16348244 - SUPPORT (IN_VITRO)"alpha-l-arabinofuranosidase (up to fivefold), and beta-d-xylosidase (up to sevenfold)"
Associated Datasets
| Dataset | Type | Repository | Accession |
|---|---|---|---|
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Ruminal fungus-methanogen xylanolytic activity phenotypes
Primary study reporting xylan utilization, extracellular enzyme activities, and metabolite profiles for anaerobic ruminal fungi grown with and without M. smithii. |
PHENOTYPE | OTHER | PMID:16348244 |
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which measurements establish the proposed causal mechanism in this exact system?
Preserve the directly measured coculture utilization and enzyme gains, as well as hydrogen/formate and fermentation-product changes. Distinguish polymer degradation from utilization and protein-normalized enzyme activity from biomass. The primary main text proposes sugar-uptake and catabolite-repression mediation but calls for further work on uptake control. No selective hydrogen-removal or repression-rescue experiment is inferred from the coculture comparison. The three fungal systems in the comparison panel are not additional members of this two-member record.
Attaches to: ecological_interactions#Interspecies Hydrogen Transfer During Xylan Fermentation, ecological_interactions#Methanogen-Stimulated Xylan Utilization, ecological_interactions#Enhanced Extracellular Hemicellulase Activities
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| xylan substrate | xylan as the growth substrate | N/A |
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| anaerobic incubation with methanogenic partner | anaerobic coculture conditions | N/A |
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Growth Media
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