facultatively anaerobic

Deep research

Generated by just research-trait; source: research/traits/environment/facultatively_anaerobic-deep-research-falcon.md

# Curation report: facultatively anaerobic

## 1. Scope summary

**Target:** `METPO:1000605` — **facultatively anaerobic**  
**Category:** ENVIRONMENT; **term kind:** CLASS; **mapping:** REVIEWED  
**Definition:** growth can occur both with and without molecular oxygen (O₂).

The trait is a **growth phenotype/capacity**, not a single pathway. A positive assignment requires reproducible growth under both oxic and anoxic conditions. Under oxic conditions, many facultative anaerobes use O₂ respiration; under anoxia they may use alternative-electron-acceptor respiration, fermentation, or both. Thus, nitrate respiration is neither necessary nor sufficient by itself, and “survives oxygen exposure” is weaker than “grows with oxygen.” Facultative anaerobes differ from strict aerobes, which require O₂; strict anaerobes, whose growth is inhibited by O₂; and aerotolerant anaerobes, which tolerate O₂ but characteristically retain anaerobic energy metabolism. A 2021 expert review uses this growth-based distinction and reports that facultative anaerobes represented 8 of 12 organisms on the cited WHO antibiotic-resistant priority-pathogen list. (andre2021theselectiveadvantage pages 1-2)

The most defensible graph is therefore a **taxon-qualified E. coli/Enterobacterales mechanism** explaining metabolic switching, not a universal molecular definition. FNR, ArcBA, NarXL, electron-acceptor repertoires, terminal oxidases, and fermentation products vary substantially among taxa. Reviews explicitly caution that respiratory regulatory systems outside E. coli remain incompletely characterized. (price2021bacterialapproachesto pages 11-12)

## 2. Recommended graph architecture

Use `METPO:1000605` as the terminal phenotype node and represent two experimentally demonstrated branches:

1. **O₂ available → aerobic respiratory growth**.
2. **O₂ absent → anaerobic growth**, supported by either:
   - alternative-acceptor respiration, or
   - fermentation/redox balancing when suitable external acceptors are unavailable.

For the existing eight-node graph, the highest-value expansion is an E. coli-centered regulatory layer comprising **FNR**, **ArcB–ArcA**, **NarX–NarL**, the **quinone redox pool**, and explicit separation of **anaerobic respiration** from **fermentation**.

## 3. Candidate nodes grouped by type

### Trait and assay states

| Candidate node | Type | Suggested grounding | Curation note |
|---|---|---|---|
| facultatively anaerobic | trait | `METPO:1000605` | Preserve identifier verbatim. |
| oxygen preference | parent trait | `METPO:1000601` | Supplied parent. |
| growth in presence of O₂ | assay phenotype | Label only | Must denote growth, not survival. |
| growth in absence of O₂ | assay phenotype | Label only | Record medium, acceptors, redox conditions, and duration. |
| anoxia / oxygen limitation | environmental state | ENVO candidate; verify locally | Do not equate microaerobiosis with complete anoxia. |

### Chemicals and electron acceptors

| Node | Suggested grounding | Role |
|---|---|---|
| molecular oxygen | `CHEBI:15379` | Terminal electron acceptor in aerobic respiration; also the direct signal that destroys active FNR’s anaerobic Fe–S state. |
| nitrate | `CHEBI:17632` | Alternative terminal acceptor and NarX/NarL signal in E. coli-like systems. |
| nitrite | `CHEBI:16301` | Related Nar-system signal/intermediate; effects differ between NarX/NarQ. |
| fumarate | `CHEBI:18012` | Alternative acceptor through fumarate reductase in applicable taxa. |
| dimethyl sulfoxide | `CHEBI:28262` | Alternative acceptor through DMSO reductase in applicable taxa. |
| trimethylamine N-oxide | CHEBI candidate; verify | Alternative acceptor; taxon-specific. |
| ubiquinone, menaquinone, demethylmenaquinone pools | CHEBI candidates; verify species | Electron carriers whose oxidation state modulates ArcB; avoid collapsing them into one chemically identical node. |
| [4Fe–4S] cluster | CHEBI candidate; verify exact oxidation state | Cofactor supporting active, dimeric FNR under anoxia. |
| [2Fe–2S] cluster | CHEBI candidate; verify exact oxidation state | O₂-induced FNR conversion product associated with inactive monomer. |

### Regulators, proteins, and complexes

| Node | Type | Grounding recommendation | Scope |
|---|---|---|---|
| FNR | O₂-responsive transcription factor | UniProt/NCBI Gene for the curated strain | Direct Fe–S O₂ sensor in E. coli; not universal. |
| ArcB | membrane-associated sensor kinase | Strain-specific UniProt | Responds primarily to respiratory/quinone redox state rather than simply binding O₂. |
| ArcA | response regulator | Strain-specific UniProt | ArcA-P is the active transcriptional regulatory state. |
| NarX | nitrate/nitrite sensor kinase | Strain-specific UniProt | E. coli NarXL paradigm. |
| NarL | response regulator | Strain-specific UniProt | Cooperates with FNR at `narGHJI`. |
| NarGHIJ nitrate reductase | respiratory enzyme complex | Strain-specific UniProt subunits; EC/Rhea where verified | Enables nitrate respiration; neither universal nor required for the trait. |
| FrdABCD fumarate reductase | respiratory complex | Strain-specific UniProt | Candidate alternative-acceptor module. |

Showing the first 60 of 218 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.