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pH optimum mid1

METPO:1000456 · CLASS · REVIEWED

A pH optimum phenotype with the best-growth external pH between approximately 6 and 7, corresponding to neutrophilic physiology.

pH-optimum-mid1 neutrophile setpoint

DOI-backed graph linking minimal pH-homeostasis load at near-neutral external pH to a pH-optimum between 6 and 7 (neutrophile).

pH-optimum-mid1 neutrophile setpoint Interactive directed graph showing evidence-backed causal relationships for pH optimum mid1.

Edge evidence

  • near-neutral external pH imposes minimal pH-homeostasis load

    Near-neutral external pH imposes minimal pH-homeostasis demand on the cell.

    • DOI:10.1038/nrmicro2549 cytoplasmic pH Supports near-neutral external pH as the regime with the smallest cytoplasmic-pH gradient to maintain.
  • minimal pH-homeostasis load confers pH optimum mid1 METPO:2007700

    Minimal pH-homeostasis load yields a neutrophilic pH-optimum setpoint.

    • DOI:10.1038/nrmicro2549 pH homeostasis Supports a near-neutral optimum as the neutrophile outcome.
  • pH optimum mid1 is a pH optimum rdfs:subClassOf

    pH optimum mid1 is a quantitative bin of the pH-optimum phenotype.

    • DOI:10.1016/j.tim.2007.02.005 proton motive force Supports a near-neutral optimum as a value within the pH-optimum distribution.
  • F0F1-ATPase exports proton (H+) METPO:2007804

    The F0F1-ATPase actively exports protons to maintain cytoplasmic pH.

    • DOI:10.1371/journal.ppat.1011927 actively exporting protons via proton pumps such as the F0F1-ATPase; classic pH-homeostasis mechanism.
  • amino-acid decarboxylation pathways consumes proton (H+) biolink:consumes

    Amino-acid decarboxylation pathways consume cytoplasmic protons, supporting pH homeostasis.

    • DOI:10.1371/journal.ppat.1011927 consume protons through amino-acid decarboxylation pathways (glutamate, lysine, arginine); general acid-resistance edge.
  • glutamate decarboxylation / GABA pathway consumes proton (H+) biolink:consumes

    The glutamate decarboxylation / GABA pathway consumes intracellular H+.

    • DOI:10.1128/aem.00569-24 glutamate-based systems and the GABA metabolic pathway consume H+.
  • ammonia (NH3) binds proton (H+)

    Ammonia binds free protons, buffering against acidification.

    • DOI:10.1128/aem.00569-24 generated NH3 bind with intracellular free H+ to form NH4+; direct acid-buffering chemistry, generalizable.
  • ammonia (NH3) forms ammonium (NH4+) biolink:produces

    Ammonia combines with a proton to form ammonium, sequestering acid.

    • DOI:10.1128/aem.00569-24 generated NH3 bind with intracellular free H+ to form NH4+.
  • F0F1-ATPase contributes to minimal pH-homeostasis load RO:0002326

    Proton export by the F0F1-ATPase keeps the cytoplasmic-pH gradient small near neutral, contributing to minimal homeostasis load.

    • DOI:10.1371/journal.ppat.1011927 Proton pumping via F0F1-ATPase maintains cytosolic pH; near-neutral external pH minimizes the pumping demand.

Provenance

Source
METPO (2025-11-25)
Definition source
DOI:10.1038/nrmicro2549

Parent traits (1)

Synonyms (3)

  • Alkali Tolerant EXACT_SYNONYM · metpo.owl
  • Neutrophile EXACT_SYNONYM · metpo.owl
  • pHO_6_to_7 RELATED_SYNONYM · metpo.owl

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1000456 [-0.020, -0.772, -0.806, +1.582, …]

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/ph_optimum_mid1-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-focused research report: pH optimum mid1

## 1. Trait scope

**Trait:** pH optimum mid1  
**Identifier:** `METPO:1000456`  
**Category/kind:** ENVIRONMENT / CLASS  
**Parent:** `METPO:1000331`  
**Status:** REVIEWED

`METPO:1000456` should represent an **organism-level optimum-growth phenotype**: maximal or best growth occurs when external pH is approximately 6–7. It is therefore an assay-observed ecological/physiological preference, not merely the capacity to survive neutral pH. “Neutrophile” or `pHO_6_to_7` are appropriate synonyms. “Alkali tolerant” is potentially misleading because tolerance above pH 7 does not establish an optimum at pH 6–7.

Operational curation should require a growth-rate, biomass-yield, colony-development, or equivalent response measured across multiple controlled external-pH conditions. The pH of the medium should be measured during growth because metabolism can shift it substantially. In a 2024 *Bacillus subtilis* experiment, minimally buffered biofilms moved from approximately pH 5.5 back to 6.9, whereas standard buffering masked the dynamics (tran2024activephregulation pages 2-5).

### Boundary cases

- **Growth range is not optimum.** Growth from pH 5–9 does not by itself establish a pH 6–7 optimum.
- **Neutral-pH survival is not optimum.** Transient acid/alkali resistance and stationary-phase survival are stress phenotypes.
- **Enzyme pH optimum is not organismal pH optimum.** Purified-enzyme activity at pH 6–7 should not be annotated to this trait.
- **Biofilm microenvironment regulation is a mechanism/context**, not automatically proof that the planktonic organism has an organism-level pH optimum.
- **Mild acidophiles overlap the lower boundary.** Methanotroph examples include optima of 5.8–6.2 or 6.0–6.5, while a review groups most neutrophilic methanotrophs within a broader pH 6–8 range. Exact inclusion therefore depends on the METPO binning rule and assay uncertainty (yao2023howmethanotrophsrespond pages 4-5).
- **Alkaliphiles are outside scope.** Methanotrophs with optima around 8.5–10 are clear neighboring but distinct phenotypes (yao2023howmethanotrophsrespond pages 5-7, yao2023howmethanotrophsrespond pages 4-5).

## 2. Current mechanistic interpretation

The trait is best modeled as a **system-level set point**, not as the product of one universal “neutrophile gene.” Near-neutral external pH reduces the energetic and kinetic burden of keeping cytoplasmic pH compatible with enzyme activity, membrane energetics, and growth. Conserved homeostasis modules include respiratory proton translocation, F-type ATP synthase, cation/proton antiport, potassium uptake, membrane permeability control, and metabolism that produces or consumes acids. However, the relative causal importance of these modules varies by taxon, lifestyle, salinity, oxygen availability, and buffer capacity.

A 2023 ecophysiology review describes respiratory complexes I, III, and IV as removing cytoplasmic protons, F0F1-ATPase as using returning protons for ATP synthesis, and potassium uptake as helping generate an internally positive membrane potential. These are authoritative general mechanisms, but the reviewed evidence spans acidophilic and alkaliphilic methanotrophs and does not directly demonstrate `METPO:1000456` (yao2023howmethanotrophsrespond pages 5-7).

The strongest recent direct evidence is community-level. *B. subtilis* biofilms actively altered extracellular pH toward the neutrophile range from starting pH 6–9 through a dynamic balance of acetate and acetoin metabolism; planktonic cells lacked the same behavior (tran2024activephregulation pages 5-7, tran2024activephregulation pages 1-2).

## 3. Candidate nodes grouped by type

### Trait and environmental nodes

- pH optimum mid1 — `METPO:1000456`
- external pH 6–7 / near-neutral external pH — label-only unless the project has an approved pH-range ontology term
- acidic external pH
- alkaline external pH
- minimally buffered growth medium
- buffer capacity / MOPS concentration
- biofilm microenvironment
- planktonic growth state
- salt concentration / osmotic stress

### Chemicals and ions

- proton — `CHEBI:24636`
- sodium ion — `CHEBI:29101`
- potassium ion — `CHEBI:29103`
- acetate — `CHEBI:30089`
- acetoin — `CHEBI:15688`
- pyruvate — `CHEBI:15361`
- 2-acetolactate — use only after identifier verification
- acetyl-CoA — `CHEBI:15351`
- ATP — `CHEBI:15422`
- proton motive force — process/quality node; label-only is safer than forcing a chemical identifier

### Genes, proteins, and complexes

Showing the first 60 of 202 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_CAUSAL_GRAPH · claude

    Added DOI-backed definition and causal graph linking minimal pH-homeostasis load at near-neutral external pH to the neutrophilic pH-optimum-mid1 bin.

  3. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2000202×1, rdfs:subClassOf×1).

  4. · GROUND_CAUSAL_NODES · claude

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

  5. · FIX_NODE_GROUNDING_CURIE · claude

    Overwrote 1 pH causal-node grounding(s) to corrected PATO CURIEs (phase-2; verified vs OAK).

  6. · ENRICH_CAUSAL_GRAPH · claude

    Added 6 evidence-backed generic edges (6 new nodes) from the deep-research report.

  7. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 5 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (biolink:consumes×2, METPO:2000209×1, biolink:produces×1, RO:0002326×1).

  8. · MIGRATE_MICROBE_DOMAIN_EDGES · claude

    Re-grounded 1 causal edge(s) off microbe-domain METPO predicates (1 to confers), issue 301. 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. Edge directions are unchanged - this pass only relabels and re-grounds. RO:0002234 (has output) is used where the subject is an activity, since biolink gives it the domain 'biological process or activity'; the METPO replacements are proposed in proposals/metpo_traitmech_v8 and v9 and are placeholder ids until METPO mints them.

  9. · 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.

  10. · NORMALISE_NODE_TYPE · claude

    Under the PATHWAY-vs-BIOLOGICAL_PROCESS rule, one node_id means one thing corpus-wide (issue 356): amino_acid_decarboxylation is typed BIOLOGICAL_PROCESS. PATHWAY is a named, conventionally enumerable multi-step route; BIOLOGICAL_PROCESS is everything else. A REACTION CLASS, not a route. The corpus describes it that way in neutrophilic.yaml -- 'Enzyme-catalyzed decarboxylation reaction that consumes cytoplasmic protons' -- and the wording varies by record, so read that as the family's sense rather than as this record's own text. Named systems that implement it (Gad) would be pathways; the reaction class is not. Was 4 BIOLOGICAL_PROCESS to 2 before this tranche.