An engineered acidophilic iron-oxidizing culture adapted for recovering valuable base metals from spent printed circuit boards (PCBs). This culture was dominated by Leptospirillum ferriphilum with Sulfobacillus benefaciens as an additional FeOB member. The community operates at extremely low pH and moderate temperature in PCB-enriched leachates, oxidizing ferrous iron to generate ferric iron (Fe³⁺) that dissolves base metals from PCB components. PCB leachates are toxic to the consortium, delaying the onset of exponential iron oxidation, but successive subculturing adapts the consortium to PCB toxicity and reduces that delay by up to 2.6-fold. This biotechnological approach offers an environmentally friendly alternative to pyrometallurgical e-waste processing by using an acidophilic community to regenerate the ferric iron lixiviant needed for metal recovery.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Leptospirillum ferriphilum | NCBITaxon:178606 · TaxonMech · TraitMech |
PRIMARY_PRODUCER
PRIMARY_DEGRADER
|
DOMINANT |
|
|||
| Sulfobacillus benefaciens | NCBITaxon:453960 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
|
ABUNDANT |
|
|||
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.
Ferrous Iron Oxidation for Metal Dissolution
The Leptospirillum ferriphilum-dominated FeOB culture, with Sulfobacillus benefaciens as the other named member, regenerates ferric iron from ferrous iron in acidic PCB systems. Redox potential is used to infer iron oxidation; this is not an iron-transporter or carbon-fixation assay. Culture-level activity does not establish obligate syntrophy or resolve each member's contribution. Faster oxidation after subculture did not accelerate batch metal dissolution in this experiment.
Participating Taxa: Leptospirillum ferriphilum, Sulfobacillus benefaciens
Metabolites: Fe(II) (CHEBI:29033 · MediaIngredientMech · CMMMech), Fe(III) (CHEBI:29034 · MediaIngredientMech · CMMMech)
Regenerated Fe(III) can oxidatively dissolve PCB metals and is reduced to Fe(II), exposing the culture to dissolved metals. This is an oxidant/substrate feedback, not a claim that faster biological regeneration increased the measured batch dissolution rate: initial ferric consumption and acidic dissolution dominated those batches.
Evidence
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"This study aimed to investigate the capacity of an acidophilic iron-oxidizing culture, mainly composed of Leptospirillum ferriphilum, to oxidize iron in PCB-enriched environments"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"This culture was mainly made up of L. ferriphilum and Sulfobacillus benefaciens, with L. ferriphilum being the predominant species"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"FeOB uses the resulting Fe(II) as a source of energy to regenerate the ferric iron"
Base Metal Exposure in PCB Leachate
Ferric reduction and acidic dissolution released metals from solid PCBs in the 2.2-L stirred-tank assays. The listed Cu, Zn and Ni concentrations belong to the 1% and 2% solid-PCB treatments, not the separate equivalent-leachate flask series. Metal dissolution supplies Fe(II) for microbial reoxidation while creating mixed-metal exposure. Dissolution reached its maximum after seven days regardless of subculture; the shorter iron-oxidation lag did not accelerate this batch endpoint. These are chemical/culture observations, not interspecies competition.
Participating Taxa: Leptospirillum ferriphilum, Sulfobacillus benefaciens
Metabolites: copper(2+) (CHEBI:29036 · MediaIngredientMech · CMMMech), zinc(2+) (CHEBI:29105 · MediaIngredientMech · CMMMech), nickel(2+) (CHEBI:49786 · MediaIngredientMech · CMMMech)
Oxidative metal dissolution reduces Fe(III) to Fe(II), replenishing the substrate that the FeOB culture can reoxidize. This chemically coupled return path is supported by the reaction scheme and the immediate redox drop after PCB addition, not merely by transferring a leachate to another subculture.
Evidence
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"The culture pre-adapted to 1% (w/v) PCB content successfully thrived in leachates with the equivalent of 6% of PCBs, containing 8.5 g L–1 Cu, 8 g L–1 Fe, 1 g L–1 Zn, 92 mg L–1 Ni, 12.6 mg L–1 Pb, and 4.4 mg L–1 Co, among other metals"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"Leachates of 1% (w/v) PCBs reached a maximum of 2.6 g L–1 Cu, 30 mg L–1 Ni, and 303 mg L–1 Zn, whereas leachates of 2% (w/v) PCBs contained up to 4.2 g L–1 Cu, 52 mg L–1 Ni, and 543 mg L–1 Zn."
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"indicating that Fe(III) was being reduced"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"maximum dissolution yield after 7 days as in the previous experiments, regardless of the subculture"
PCB Toxicity and Microbial Adaptation
PCB exposure delayed culture iron oxidation. In solid-PCB stirred tanks, the adapted culture had pre-oxidation lags of 11 days at 1% and 18 days at 2%. Serial 2% subcultures shortened the lag from 18 to 13 to 7 days; after a ten-day storage pause it lengthened to 11 days before shortening again. Separately, no growth or microbial iron-oxidation activity was observed at 8% equivalent chemical leachate during the 25-day flask assay; this is not a universal above-8% threshold. Operational adaptation does not identify genetic resistance, altered membranes or a specific toxic compound. Planktonic counts omit attached biomass and are affected by solids.
Participating Taxa: Leptospirillum ferriphilum, Sulfobacillus benefaciens
Biological Processes:
- response to toxic substance (GO:0009636 · PathwayMech)
PCB exposure delays iron-oxidizing activity, while serial transfer under the tested 2% solid-PCB conditions shortens the pre-oxidation lag. This operational history changes the onset of culture activity; the molecular adaptation mechanism is unresolved and the faster onset did not accelerate batch metal dissolution.
Evidence
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"However, the inhibiting effect of PCBs limited the microbial activity by delaying the onset of the exponential iron oxidation"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"thrived in leachates with the equivalent of 6% of PCBs"
-
doi:10.3389/fmicb.2021.669738 - SUPPORT (IN_VITRO)"Successive subcultures boosted the activity of the culture by reducing this delay by up to 2.6 times"
Knowledge gaps & discussions (1)
KNOWLEDGE_GAP OPEN Which toxic components and physiological changes determine the iron-oxidation lag?
Retain the FeIII/FeII chemical-biological feedback rather than a subculture-workflow cycle. Retain the operational adaptation-to-oxidation link without claiming measured genetic detoxification, membrane changes or obligatory syntrophy. In these batches metal dissolution reached maximum after seven days regardless of subculture, despite faster iron oxidation. Distinguish equivalent-leachate flask toxicity from solid-PCB stirred-tank concentrations and continuous-startup results. Non-graph acidity, assay-regime and organism-role claims remain #1448; preserve the prior #596 vessel evidence correction. No new nodes or arrows are required.
Attaches to: ecological_interactions#Ferrous Iron Oxidation for Metal Dissolution, ecological_interactions#Base Metal Exposure in PCB Leachate, ecological_interactions#PCB Toxicity and Microbial Adaptation
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Extreme Acidity | 1.2 ± 0.1 | pH |
|
||
| Temperature | 35 | °C |
|
||
| Aeration with CO2 Enrichment | 60 L/h with 1% CO2 | L/h |
|
||
| PCB Leachate Load | 2-8% equivalent PCB leachate tested; growth through 6% | % w/v |
|
||
| PCB-Delayed Iron Oxidation | delayed onset of exponential iron oxidation | N/A |
|
||
Growth Media
Cross-Mech Suite Links
Explore related entities across the CultureBotAI X-Mech knowledge bases: