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obligately acidophilic

METPO:1003006 · CLASS · REVIEWED

A pH growth preference characterized by the requirement for acidic environments (pH below 5.5) for growth, with inability to grow at neutral or alkaline pH values.

Obligate acidophily pH homeostasis mechanism

Evidence-backed causal sketch linking obligate acidophily to acidic external pH, proton stress, and constitutive pH homeostasis mechanisms.

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

Edge evidence

  • acidic external pH selects for obligately acidophilic METPO:2007401

    Acidic environments select for organisms with acidic pH growth optima.

    • DOI:10.3389/fmicb.2021.822229 acidic optimal growth pH Review supports acidophile classification by acidic optimum pH; the obligate neutral-pH exclusion remains from the METPO definition.
  • acidic external pH increases gradient of proton METPO:2007601

    Low external pH creates a steep proton gradient across the membrane.

    • DOI:10.3389/fmicb.2021.822229 proton gradient across the cytoplasmic membrane Supports external-to-internal proton gradient under low-pH growth.
  • low proton permeability membrane limits influx of proton

    Reduced membrane permeability helps obligate acidophiles maintain intracellular pH.

    • DOI:10.1016/j.tim.2007.02.005 highly impermeable cell membranes Supports proton exclusion by membrane properties in acidophiles.
  • reversed membrane potential suppresses influx of proton

    Reversed membrane potential reduces passive proton entry.

    • DOI:10.1016/j.tim.2007.02.005 reversed membrane potential Supports reversed membrane potential as a shared acidophile feature.
  • proton export pumps and antiporters contributes to cytoplasmic pH homeostasis RO:0002326

    Proton export and antiport systems contribute to intracellular pH control during acid stress.

    • DOI:10.3389/fmicb.2021.822229 proton export pumps and antiporters Supports transporter-mediated acid resistance mechanisms.
  • cytoplasmic pH homeostasis confers obligately acidophilic METPO:2007700

    Obligate acidophilic growth requires maintaining cytoplasmic pH despite acidic external pH.

    • DOI:10.1038/nrmicro2549 maintain a cytoplasmic pH of approximately 6.0 Supports intracellular pH control during extreme acidophilic growth.
  • K+ uptake system (Kdp/Kef) increases reversed membrane potential RO:0002213

    K+ uptake systems generate the inside-positive (reversed) membrane potential that forms an electrochemical barrier to protons.

    • DOI:10.3389/fmicb.2023.1149903 Review describes inside-positive membrane potential via potassium-transporting ATPases and K+ uptake systems (kdp, Kef) forming an electrochemical barrier to proton influx.
  • P-type proton-translocating ATPase increases proton efflux from cytoplasm RO:0002213

    P-type ATPases actively export protons from the cytoplasm.

    • DOI:10.3389/fmicb.2023.1149903 Dopson review lists active proton efflux via P-type ATPases as a pH homeostasis strategy in acidophiles.
  • proton efflux from cytoplasm contributes to cytoplasmic pH homeostasis RO:0002326

    Active proton efflux helps maintain near-neutral cytoplasmic pH under acidic conditions.

    • DOI:10.3389/fmicb.2023.1149903 Proton pumps/ATPases maintain a near-neutral cytoplasm during acidophilic growth.
  • Na+/H+ antiporter (Nha) exports proton METPO:2007804

    Na+/H+ antiporters exchange cytoplasmic protons for extracellular sodium, contributing to proton export.

    • DOI:10.3389/fmicb.2023.1149903 Dopson lists Na+/H+ exchange (nhaA sodium/proton antiporter) among acidophile pH-homeostasis systems.
  • hopanoid-containing membrane decreases low proton permeability membrane RO:0002212

    Hopanoid membrane lipids reduce membrane proton permeability, reinforcing proton exclusion.

    • DOI:10.1111/1758-2229.70019 Acid resistance is linked to hopanoid membrane lipids and reduced proton permeability as a core proton-exclusion strategy.
  • cyclopropane fatty acids decreases low proton permeability membrane RO:0002212

    Cyclopropane fatty acid formation reduces membrane proton permeability.

    • DOI:10.3389/fmicb.2023.1149903 Review includes cyclopropane-fatty-acyl-phospholipid synthase among membrane adaptations that reduce proton permeability.
  • glutamate decarboxylase system (Gad) consumes proton biolink:consumes

    Glutamate decarboxylase consumes cytoplasmic protons during decarboxylation, buffering intracellular pH.

    • DOI:10.3389/fmicb.2023.1149903 Review lists decarboxylases (Adi, Gad) as cytoplasmic proton-consuming systems used in acidophile pH homeostasis.

Provenance

Source
METPO (2025-11-25)
Author
Jed Dongjin Kim-Ozaeta
Definition source
DOI:10.3389/fmicb.2021.822229

Parent traits (1)

Synonyms (1)

  • obligate acidophile EXACT_SYNONYM · metpo.owl

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1003006 [-2.557, -2.409, -2.459, -0.781, …]

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/obligately_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: obligately acidophilic

**Trait:** obligately acidophilic  
**Identifier:** **METPO:1003006**  
**Category / kind / status:** ENVIRONMENT / CLASS / REVIEWED  
**Parent:** METPO:1003000  
**Synonym:** obligate acidophile

## 1. Scope summary

This trait should represent a **growth requirement**, not merely survival under acid stress: the organism grows only in acidic medium—under the supplied definition, below pH 5.5—and cannot grow at neutral or alkaline pH. The causal endpoint should therefore be **growth under an obligately acidic pH range**, supported by a measured growth curve or explicit minimum/maximum-growth-pH assay.

Important distinctions are:

- **Acid-tolerant / acid-resistant:** survives transient low-pH challenge but may grow optimally near neutrality; insufficient for this trait.
- **Acidophilic:** prefers acidic conditions, but published usage does not always establish failure to grow at neutral pH.
- **Moderate acidophile:** commonly reported growth range pH 3–7.5 and optimum pH 4–5; this category can include organisms that grow near neutrality and therefore is not automatically obligate acidophily. (dopson2023eurypsychrophilicacidophilesfrom pages 2-4)
- **Extreme acidophile:** usually defined by optimum pH ≤3 (some literature uses growth at ≤3.5). This describes degree of acid preference, not logically the same property as obligacy. (vergara2020evolutionofpredicted pages 1-3, gonzalezrosales2022integrativegenomicssheds pages 1-2)
- **Polyextremophile:** acidophily combined with temperature, salinity, metal, or other adaptations; these additional traits should remain separate graph branches.

The mechanistic core is maintenance of a cytoplasm near pH 6–7 despite a strongly proton-rich exterior. Extreme acidophiles may face proton gradients of 10⁴–10⁵ fold. Direct measurements in *Methylacidiphilum* sp. RTK17.1 found intracellular pH 6.55 ± 0.05 over extracellular pH 1.5–3.0, illustrating the physiological endpoint but not proving that every obligate acidophile uses every proposed mechanism. (vergara2020evolutionofpredicted pages 1-3, gonzalezrosales2022integrativegenomicssheds pages 1-2, carere2021growthonformic pages 3-4)

**Curation recommendation:** retain the supplied pH <5.5 definition for METPO consistency, but require evidence of **absence of growth at neutral/alkaline pH**. Do not infer “obligately acidophilic” solely from an acidic optimum, habitat metadata, genome content, or the label “extreme acidophile.”

## 2. Current mechanistic model

Authoritative recent reviews divide acid homeostasis into two interacting defenses. A first line limits proton entry through low-permeability membranes, envelope proteins, and an inside-positive electrical potential. A second line removes or consumes protons that enter, through respiratory pumping, antiport, decarboxylation, and buffering. Comparative genomics indicates that acidophilic Acidithiobacillia gained hopanoid synthesis and redundant systems for generating positive membrane potential relative to inferred neutrophilic ancestors, but much of that evidence remains predictive because these organisms are difficult to manipulate genetically. (gonzalezrosales2022integrativegenomicssheds pages 1-2)

Direct perturbation evidence is strongest in *Methylacidiphilum* sp. RTK17.1. At external pH 2.5 it maintained intracellular pH 6.52 ± 0.04; growth occurred over pH 1–6 with optimum pH 2.5 and μmax 0.015 h⁻¹. Nigericin/valinomycin treatment acidified the cytoplasm, while formic acid lowered intracellular pH from 6.52 to 6.05 at 1 mM and inhibited batch growth. These observations causally connect intact ion gradients and cytoplasmic pH homeostasis to growth in acid. (carere2021growthonformic pages 4-5, carere2021growthonformic pages 3-4)

## 3. Candidate nodes grouped by type

Identifiers below are supplied only where they are well-established and unambiguous. Gene-family labels are preferable to invented or strain-unspecified UniProt accessions.

### Trait and environmental nodes

- **obligately acidophilic** — METPO:1003006.
- **acidic environment / acidic growth medium** — candidate ENVO grounding should be selected according to the assayed habitat; retain label-only for generic culture pH.
- **extracellular pH below 5.5** — experimental-factor node; represent the numeric condition in evidence metadata.
- **high extracellular proton activity / proton gradient** — chemical/process node; proton: CHEBI:15378.
- **neutral or alkaline growth condition** — negative assay condition needed to establish obligacy.
- **growth**, **no growth**, **specific growth rate**, and **intracellular pH** — assay/output nodes.

### Cellular structures and locations

- cytoplasm — GO:0005737.
- plasma membrane — GO:0005886.
- outer membrane — GO:0019867, applicable to Gram-negative taxa only.
- cell envelope — GO:0030313.
- respiratory chain / membrane respiratory complexes — label or appropriate taxon-specific GO terms.

### Ions, chemicals, and metabolites

- proton — CHEBI:15378.
- potassium cation — CHEBI:29103.
- sodium cation — CHEBI:29101.
- spermidine — CHEBI:16610.
- glutamate — use the charge-state-specific CHEBI entity matching the reaction.
- γ-aminobutyrate/GABA — CHEBI:16865.
- arginine — use the charge-state-specific CHEBI entity matching the reaction.

Showing the first 60 of 241 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 obligately acidophilic trait and added DOI-backed evidence and causal graph for acidic pH homeostasis. The graph does not assert a specific molecular cause of the obligate neutral-pH growth exclusion because that varies by lineage.

  3. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002326×1, 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 1 causal-node grounding field(s) via mappings/node_grounding.tsv (PATO:0001428×1).

  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 1 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 7 evidence-backed generic edges (7 new nodes) from the deep-research report.

  12. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 7 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002213×2, RO:0002212×2, RO:0002326×1, METPO:2000209×1, biolink:consumes×1).

  13. · 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.

  14. · MIGRATE_MICROBE_DOMAIN_EDGES_PART2 · claude

    Re-grounded 1 causal edge(s) off microbe-domain METPO predicates onto their causal-graph counterparts (1 to exports), 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.