A perturbed indigenous microbial community from vanadium-titanium magnetite mine tailings in Panzhihua, Sichuan, China, the world's largest V-Ti magnetite deposit and largest vanadium production city. This community comprises nitrogen-fixing rhizobia (Bradyrhizobium pachyrhizi, Rhizobium species), metal-reducing bacteria, and indigenous microbiota that facilitate phytoremediation of polymetallic tailings containing vanadium (340 mg/kg), titanium, iron, copper, nickel, manganese, and zinc. The Panzhihua deposit has generated approximately 570 million tons of tailing slurries from mining activities, creating a massive contamination challenge requiring sustainable bioremediation. The indigenous microbial community has adapted to extreme metal stress through nitrogen fixation coupled with plant growth promotion, enabling successful legume-based phytoremediation with Pongamia pinnata and Leucaena leucocephala. Bradyrhizobium species demonstrate exceptional heavy metal tolerance while maintaining nitrogen-fixing capacity (increasing plant nitrogen content by 10-145%), promoting plant growth through IAA production, and facilitating metal phytoextraction (enhancing Cu and Ni uptake by 600%). Metal-reducing bacteria contribute to vanadium detoxification through dissimilatory V(V) reduction to less soluble V(IV), decreasing vanadium bioavailability and mobilization risk. The community drives transformation of soil organic carbon, increasing active carbon components and carbon transformation enzymes during revegetation, improving overall tailings soil quality. This system represents a critical model for coupling nitrogen fixation, metal tolerance, and phytoremediation in polymetallic mine tailings, addressing both resource recovery and environmental remediation challenges in the world's largest vanadium-titanium deposit.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Bradyrhizobium pachyrhizi | NCBITaxon:280333 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
SYNTROPHIC_PARTNER
NITROGEN_FIXING_SYMBIONT
|
ABUNDANT |
|
|||
| Rhizobium selenitireducens — ontology label: Paenirhizobium selenitireducens ATCC BAA-1503 | NCBITaxon:1336235 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
CROSS_FEEDER
NITROGEN_FIXING_SYMBIONT
|
COMMON |
|
|||
| Rhizobium pisi | NCBITaxon:574561 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
NITROGEN_FIXING_SYMBIONT
|
RARE |
|
|||
| Bacillus species — ontology label: Bacillus <firmicutes> | NCBITaxon:1386 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
CROSS_FEEDER
|
DOMINANT |
|
|||
| Ochrobactrum intermedium — ontology label: Brucella intermedia | NCBITaxon:94625 · TaxonMech · TraitMech |
SECONDARY_FERMENTER
CROSS_FEEDER
|
COMMON |
|
|||
| Polaromonas species — ontology label: Polaromonas | NCBITaxon:52972 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
SYNTROPHIC_PARTNER
|
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.
Pongamia Nodulation and Nitrogen-Enrichment Phenotypes
Bradyrhizobium isolates from Pongamia pinnata nodules in Panzhihua tailings formed nodules and increased plant nitrogen and growth in separate nitrogen-free vermiculite pot assays. Results identify PP76 and other Bradyrhizobium isolates, while PP1 belongs to Rhizobium in the Results and must not be assigned to B. pachyrhizi. The abstract assigns twelve isolates to B. pachyrhizi, but species-level trees and labels are inconsistent. The canonical species endpoint therefore represents the authors provisional group assignment, not a verified PP1 identity. These are positive inoculation phenotypes with nifH evidence, not a measured ammonium-transfer flux, V-Fe nitrogenase demonstration, or mutualism between bacterial species. The same study also reports separate metal-tolerance phenotypes for PP76 and other isolates; internal Cd/Zn inconsistencies preclude unqualified metal-specific numerical assignments here.
Source Taxon: Bradyrhizobium pachyrhizi
Evidence
-
doi:10.3389/fmicb.2023.1078333 - SUPPORT (IN_VIVO)"only 11 strains built symbiotic relationships with P. pinnata."
-
doi:10.3389/fmicb.2023.1078333 - PARTIAL (IN_VIVO)"the nitrogen content of the plants’ aboveground parts and roots was 2.4–1.8 times that of the non-inoculation control."
-
doi:10.3389/fmicb.2023.1078333 - PARTIAL (OTHER)"twelve isolates were identified as Bradyrhizobium pachyrhizi"
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doi:10.3389/fmicb.2023.1078333 - SUPPORT (IN_VIVO)"the biomass of the inoculated P. pinnata plants was significantly (p < 0.05) higher than that of the control plants."
Metal Tolerance of Cultured Tailings Bacillus
The 2014 Panzhihua bulk-soil isolate screen found multiple-metal tolerance among Bacillus isolates, including five strains tolerant to all six tested metals. These were isolate growth assays, not a demonstration that Bacillus transfers metabolites to rhizobia or enhances plant metal uptake. Keep this result separate from the 2023 nodule-isolate screen and the 2018 S. saheli YH1/Leucaena system, which reduced plant metal uptake. The previously assigned B. liaoningense phytoextraction percentages do not belong to B. pachyrhizi or to the cited YH1 experiment. Efflux, sequestration, ACC degradation and metal-redox mechanisms were not isolated by the tolerance assay.
Source Taxon: Bacillus species
Evidence
-
doi:10.1371/journal.pone.0106618 - SUPPORT (IN_VITRO)"Five strains, the B. licheniformis KT-87 and KT-88 and the Bacillus sp. KT-72, KT-74 and KT-76 were tolerant against all the tested heavy metals."
Ochrobactrum Isolate Growth-Promoter Production
The 2014 bulk-soil collection included thirteen isolates identified as Ochrobactrum intermedium. In that collection, six Ochrobactrum isolates produced more than 60 micrograms/mL IAA, and KT80 had the highest reported IAA concentration, 83.05 micrograms/mL; two Ochrobactrum isolates were among the siderophore producers. The main-text wording does not unambiguously allocate IAA/siderophore co-production to both isolates. These are in-vitro production phenotypes, not demonstrated metabolite delivery to Bradyrhizobium, ACC deaminase activity, metal reduction or a community-resilience mechanism. The 2023 Ochrobactrum nodule isolates are a different collection with different species assignments; they cannot establish O. intermedium symbiotic nitrogen fixation.
Source Taxon: Ochrobactrum intermedium
Evidence
-
doi:10.1371/journal.pone.0106618 - SUPPORT (IN_VITRO)"Ochrobactrum intermedium (13 isolates)"
-
doi:10.1371/journal.pone.0106618 - SUPPORT (IN_VITRO)"Ochrobactrum sp. KT80 produced highest concentration"
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doi:10.1371/journal.pone.0106618 - SUPPORT (IN_VITRO)"two Ochrobactrum sp. siderophore producers"
Knowledge gaps & discussions (2)
KNOWLEDGE_GAP OPEN Which sampled community and verified isolates support each Panzhihua graph claim?
The record combines bulk-soil isolates, individually tested nodule isolates and separate plant-remediation communities. Positive nodulation, nitrogen enrichment, isolate tolerance and IAA production remain. Co-isolation does not establish functional redundancy, mutualism or compensatory resilience. Polaromonas occurrence in this exact Panzhihua system is unverified; PMID33125214 concerns its own vanadium-tailings system, already represented separately as CommunityMech:000055. Removing that node exposes an unresolved canonical-membership problem rather than concealing it through all-member community-level credit. The 2023 paper also conflicts internally on PP1 identity and Cd/Zn assays. Canonical identity, top-level environment and trophic roles require separate review.
Attaches to: ecological_interactions#Pongamia Nodulation and Nitrogen-Enrichment Phenotypes, ecological_interactions#Metal Tolerance of Cultured Tailings Bacillus, ecological_interactions#Ochrobactrum Isolate Growth-Promoter Production
2 evidence item(s)
twelve isolates were identified as Bradyrhizobium pachyrhizi
Abstract species assignment; Results tree comparisons and strain names are inconsistent, so exact strain-to-species mapping remains unresolved.
V V -reducing enrichments indicated that bacteria associated with Polaromonas
External-system context only; no evidence of occurrence or vanadium reduction at this record’s Panzhihua sampling site.
KNOWLEDGE_GAP OPEN How should the positive Pongamia soil-carbon response be represented with the correct community identity?
PMID38744211 reports significant increases in ROC, POC, DOC and MBC, three carbon-transformation enzyme measures, and carbon degradation/fixation/methane-oxidation gene abundance after long-term Pongamia remediation. This positive aggregate result is retained here, not discarded as merely hypothetical. SEM supports an inferred community-level pathway, not Bacillus-to-Bradyrhizobium commensalism or a reciprocal nitrogen/carbon feedback connecting different studies. The former DOC metabolite was incorrectly grounded as carbon dioxide. A graph node needs a correctly scoped community and source-resolved participants; converting this composite record to implicit all-member activity would falsely include Polaromonas. Authorized full text and identity repair precede new topology.
Attaches to: ecological_interactions#Pongamia Nodulation and Nitrogen-Enrichment Phenotypes, ecological_interactions#Metal Tolerance of Cultured Tailings Bacillus
2 evidence item(s)
four soil active organic carbon components (ROC, POC, DOC, and MBC)
Positive aggregate carbon result in a separate remediation study; does not resolve the proposed bacterial pair.
The abundance of carbon transformation functional genes such as carbon degradation, carbon fixation, and methane oxidation were also significantly
Gene-abundance changes are not measured pathway flux or a demonstrated nitrogen-to-carbon feedback.
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| pH | 6.5-7.5 | pH units |
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| Total Vanadium Concentration | 340 | mg/kg |
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| Polymetallic Contamination | Cu 127 mg/kg, Ni 60 mg/kg, Mn 1200 mg/kg, Zn 155 mg/kg | mg/kg |
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| Temperature | 20-30 | °C |
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| Revegetation and Phytoremediation Setup | Pongamia pinnata and Leucaena leucocephala legume plantings | N/A |
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| Tailings Scale and Origin | 570 million tons accumulated | metric tons |
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| Indigenous Microbiome Diversity | 136 culturable isolates; 58% Bacillus, 21% Rhizobium, 10% Ochrobactrum | N/A |
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Growth Media
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