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facultatively acidophilic

METPO:1003007 · CLASS · REVIEWED

A pH growth preference characterized by optimal growth in acidic environments (pH below 5.5) with the capacity to also grow at near-neutral pH values.

Facultative acidophily pH homeostasis mechanism

Evidence-backed causal sketch linking facultative acidophily to acidic growth, near-neutral growth capacity, and inducible pH homeostasis.

Facultative acidophily pH homeostasis mechanism Interactive directed graph showing evidence-backed causal relationships for facultatively acidophilic.

Edge evidence

  • acidic external pH selects for facultatively acidophilic METPO:2007401

    Acidic pH selects for acidophilic growth capacity.

    • DOI:10.3389/fmicb.2021.822229 acidic optimal growth pH Supports acidophilic classification by acidic growth optimum.
  • near-neutral external pH is compatible with facultatively acidophilic

    Facultatively acidophilic growth can extend into near-neutral pH.

    • DOI:10.1099/ijs.0.066175-0 capable of growth at pH 4.0-7.2 Species-level evidence supports acidic-to-near-neutral growth range; this edge is qualified rather than generalized to all taxa.
  • acidic external pH increases gradient of proton METPO:2007601

    Acidic pH imposes an external-to-internal proton gradient.

    • DOI:10.3389/fmicb.2021.822229 external to internal proton gradient Supports proton-gradient stress during low-pH growth.
  • proton export pumps and antiporters contributes to cytoplasmic pH homeostasis RO:0002326

    Proton export and antiport systems contribute to acid-stress homeostasis.

    • DOI:10.3389/fmicb.2021.822229 proton export pumps and antiporters Supports transporter-mediated pH homeostasis mechanisms.
  • cytoplasmic buffering contributes to cytoplasmic pH homeostasis RO:0002326

    Buffering and proton-consuming reactions reduce cytoplasmic acidification.

    • DOI:10.3389/fmicb.2021.822229 cytoplasmic buffering Supports buffering as a proposed acid resistance mechanism.
  • cytoplasmic pH homeostasis confers facultatively acidophilic METPO:2007700

    Growth across acidic and near-neutral pH requires intracellular pH control.

    • DOI:10.1038/nrmicro2549 robust mechanisms for cytoplasmic pH homeostasis Supports cytoplasmic pH homeostasis for growth outside the preferred cytoplasmic pH range.
  • acidic external pH necessitates maintenance of near-neutral cytoplasm

    Acidophilic growth requires preserving near-neutral cytoplasm against a steep proton gradient.

    • DOI:10.3389/fmicb.2021.822229 maintain a near-neutral cytoplasm despite an external-to-internal proton gradient up to 10^5-fold.
  • potassium ion generates inside-positive membrane potential biolink:produces

    Intracellular K+ accumulation generates an inside-positive potential that opposes proton influx.

    • DOI:10.3389/fmicb.2021.822229 internal positive membrane potential thought to be generated by potassium ions.
  • inside-positive membrane potential contributes to cytoplasmic pH homeostasis RO:0002326

    An inside-positive membrane potential is a first-line defense reducing proton entry, supporting pH homeostasis.

    • DOI:10.3389/fmicb.2021.822229 Inside-positive potential generated by K+ opposes proton influx as a first-line defense.
  • hopanoid biosynthetic process decreases membrane proton permeability RO:0002212

    Hopanoids stiffen the membrane and reduce proton permeability under low pH.

    • DOI:10.3389/fmicb.2021.822229 membrane alterations via inclusion of hopanoids linked to acidophilic lifestyle.
  • rigid impermeable membrane limits proton RO:0002212

    A rigid, impermeable membrane acts as a barrier limiting proton entry into the cell.

    • DOI:10.3389/fmicb.2023.1149903 acidophiles use a rigid and impermeable membrane that resists proton entry.
  • proton export pumps and antiporters expels proton

    Proton export pumps and antiporters directly remove protons that enter the cytoplasm.

    • DOI:10.3389/fmicb.2021.822229 proton export pumps and antiporters listed among mechanisms maintaining pH homeostasis.
  • glutamate decarboxylase system consumes proton biolink:consumes

    Glutamate decarboxylation consumes intracellular protons as a second-line acid resistance route.

    • DOI:10.3389/fmicb.2021.822229 proton consuming reactions such as glutamate decarboxylase; gadABC among acid-adaptation genes.
  • cytoplasmic buffering stabilizes intracellular pH

    Cytoplasmic buffering dampens pH fluctuations to stabilize intracellular pH.

    • DOI:10.3389/fmicb.2023.1149903 shared mechanisms include cytoplasmic buffering that stabilizes intracellular pH.

Provenance

Source
METPO (2025-11-25)
Author
Jed Dongjin Kim-Ozaeta
Definition source
DOI:10.1099/ijs.0.066175-0

Parent traits (1)

Synonyms (1)

  • facultative acidophile EXACT_SYNONYM · metpo.owl

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1003007 [-2.661, -2.047, -2.231, -0.786, …]

512-dim DeepWalkSkipGramEnsmallen embedding from kg-microbe (2026-04-25).

Nearest neighbors in embedding space

Top-8 cosine-similar METPO traits from the 2026-04-25 deepwalk (512-D).

Deep research

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

Unreviewed literature output — not curated TraitMech content Ontology identifiers suggested below have not been resolved against their ontologies, and some are known to be wrong. Check any CURIE against the source before using it. 
# Curation report: facultatively acidophilic

## 1. Scope summary

**Target:** `METPO:1003007` — **facultatively acidophilic**  
**Category:** ENVIRONMENT · **Term kind:** CLASS · **Status:** REVIEWED  
**Parent:** `METPO:1003000`  
**Operational definition:** optimal growth in acidic conditions, below pH 5.5, together with reproducible growth at near-neutral pH.

This is an **organism-level growth-preference phenotype**, not merely survival after acid shock. For curation, evidence should ideally report growth rate or biomass across a controlled pH series and demonstrate both (i) an acidic optimum and (ii) growth near neutrality. A recent review places moderate acidophiles at approximately pH 3–5 optimum and extreme acidophiles at pH ≤3 optimum; another uses a broader moderate-acidophile range of pH 3–7.5 with optima near pH 4–5. These conventions overlap the target but do not themselves establish facultative acidophily. (gonzalez2024acidophilicheterotrophsbasic pages 1-2, dopson2023eurypsychrophilicacidophilesfrom pages 2-4)

A useful phenotype example is the mildly acidophilic methanotroph class with optima at pH 5.0–5.5 and reported ranges of pH 4.2–7.2. This directly spans acidic to near-neutral conditions and is closer to the target than an extreme acidophile that cannot grow near neutrality. (yao2023howmethanotrophsrespond pages 4-5)

### Boundary cases

* **Acid tolerant but not acidophilic:** survives low pH, but optimum remains near neutral. Do not annotate as `METPO:1003007` without an acidic growth optimum.
* **Obligate/extreme acidophile:** optimum below pH 3 but no demonstrated near-neutral growth. Relevant mechanistic evidence may be imported cautiously, but the organism does not establish the target phenotype.
* **Broad-pH environmental occurrence:** metagenomic detection at acidic and alkaline sites is not equivalent to cultured growth. For example, *Ca.* Eremiobacteria occurred predominantly below pH 6 but also in 19 alkaline samples; this is ecological association, not proof that individual organisms grow across that range. (ji2021candidatuseremiobacterotaa pages 7-9)
* **Acid resistance or acid-shock response:** transient survival, stationary-phase persistence, or induced stress genes do not establish a growth preference.
* **“Facultative” metabolism:** facultative anaerobiosis, autotrophy, or methanotrophy is unrelated to facultative acidophily.
* **Assay dependence:** medium composition, organic acids, chloride, temperature, oxygen, and growth phase can shift the apparent pH range and must be captured as experimental context.

## 2. Current mechanistic model

The best-supported general model is layered pH homeostasis. Acidic extracellular conditions create a large inward proton gradient. A relatively proton-impermeable envelope and an inside-positive membrane potential reduce proton entry; antiporters and other proton-removal systems expel protons that enter; decarboxylation and ammonia-generating reactions consume or buffer cytoplasmic protons. Together these processes preserve a near-neutral cytoplasm and permit growth at low external pH. Comparative genomics indicates that acidophilic Acidithiobacillia acquired or expanded many such systems relative to neutrophilic relatives, but much of that evidence remains predictive rather than perturbational. (gonzalezrosales2022integrativegenomicssheds pages 1-2, gonzalezrosales2022integrativegenomicssheds pages 9-12)

Direct physiology supports the central role of this architecture. *Methylacidiphilum* sp. RTK17.1 maintained intracellular pH 6.52 ± 0.04 across external pH 1.5–3.0. A measured inside-positive potential of 5.86 mV at external pH 2–3 opposed proton entry. Although this organism is an extreme thermoacidophile rather than a demonstrated facultative acidophile, the experiment strongly supports the core pH-homeostasis edges. (carere2021growthonformic pages 4-5, carere2021growthonformic pages 1-2)

Recent work also reinforces the general importance of membrane potential and antiporters in bacterial pH homeostasis, but extrapolation from neutrophiles to facultative acidophiles should be explicit rather than treated as trait-specific proof.

## 3. Candidate nodes grouped by type

### Trait and environmental nodes

* `METPO:1003007` — facultatively acidophilic
* `METPO:1003000` — supplied parent trait
* acidic external pH / acidic environment — label-level environmental condition; verify the exact ENVO term during implementation
* near-neutral external pH — label-only candidate
* extracellular-to-cytoplasmic proton gradient — label-only candidate
* pH growth-range assay — experimental-factor node
* medium composition, temperature, oxygen availability, growth phase — experimental modifiers

### Chemicals and ions

* proton — `CHEBI:15378`
* potassium ion — `CHEBI:29103`
* sodium ion — `CHEBI:29101`
* chloride — `CHEBI:17996`
* ammonia — `CHEBI:16134`
* formic acid — `CHEBI:30751`
* amino-acid substrates for decarboxylation — ground separately only when the specific system is demonstrated

### Structures and cellular states

* cytoplasm — `GO:0005737`
* plasma membrane — `GO:0005886`
* outer membrane — `GO:0019867`, where applicable
* proton-impermeable/low-permeability membrane — label-only state
* inside-positive membrane potential — label-only electrophysiological state
* intracellular pH homeostasis — `GO:0030003`

Showing the first 60 of 234 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.

Curation history

  1. · SEEDED_FROM_METPO · seed_from_metpo

    imported from data/raw/metpo.owl (CLASS)

  2. · CURATED_WITH_LITERATURE · codex

    Reviewed facultatively acidophilic trait and added DOI-backed evidence and causal graph for acidic and near-neutral pH growth capacity. Near-neutral growth evidence is species-level and should be treated as qualified.

  3. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 3 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002326×2, RO:0002327×1).

  4. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007401×1).

  5. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (GO:0030641×1).

  6. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (PATO:0001428×1, PATO:0001432×1).

  7. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007601×1).

  8. · FIX_NODE_GROUNDING_CURIE · claude

    Overwrote 1 causal-node grounding(s) (obsolete/wrong GO -> corrected, verified vs OAK).

  9. · FIX_NODE_GROUNDING_CURIE · claude

    Overwrote 2 pH causal-node grounding(s) to corrected PATO CURIEs (phase-2; verified vs OAK).

  10. · REMOVE_REDUNDANT_SYNONYM · claude

    Removed 1 synonym(s) whose text duplicated the label (seeder redundancy; no information lost).

  11. · ENRICH_CAUSAL_GRAPH · claude

    Added 8 evidence-backed generic edges (8 new nodes) from the deep-research report.

  12. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 5 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:produces×1, RO:0002326×1, METPO:2000017×1, RO:0002212×1, biolink:consumes×1).

  13. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:29103×1, GO:0019746×1).

  14. · MIGRATE_ENABLES_TRAIT_EDGES · claude

    Migrated 1 causal edge(s) off enables/RO:0002327 with a TRAIT object (1 to confers), issue 302. RO:0002327 has range 'biological process or activity', which a trait (a disposition) cannot satisfy, so the previous form entailed trait is-a BiologicalProcessOrActivity. The replacements are proposed in proposals/metpo_traitmech_v8 and are placeholder ids until METPO mints them.

  15. · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude

    Re-grounded 1 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (1 to reduces), issue 301 part 2. The previous predicates are transitively rdfs:subPropertyOf METPO:2000001, whose rdfs:domain is METPO:1000525 (microbe), so a causal-graph subject entailed that the subject IS a microbe; CausalNodeTypeEnum has no organism member, so no such edge could ever satisfy the domain. Each replacement is a 1:1 mirror of its source predicate that changes only the domain, so the claim each edge makes is unchanged and directions are unchanged. The replacements are proposed in proposals/metpo_traitmech_v9 and are placeholder ids until METPO mints them.

  16. · REGROUND_CAUSAL_EDGE · claude

    Relabelled 1 causal edge from `reduces` to `decreases` and re-grounded it from METPO:2007802 to RO:0002212 (negatively regulates), issue 330. The corpus wrote two senses under the single label `reduces` - genuine electron donation, and a lessens/decreases sense - and METPO:2007802 is defined as donating electrons to the object and lowering its oxidation state, which this edge does not assert. The two senses could not be separated mechanically because the label was identical, so they migrated together in issue 329 and were split here by reading each edge. RO:0002212 declares no rdfs:domain or rdfs:range, so this introduces no entailment of the kind issue 301 removed.