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NaCl optimum

METPO:1000333 · CLASS · REVIEWED

A salinity phenotype with numerical limits that supports the most efficient growth and reproduction of an organism.

Trait evidence (2)

  • DOI:10.1093/femsre/fuy009
    optimal NaCl

    Osmoadaptation review supports the NaCl concentration at which growth rate is maximal as a standard halophily descriptor.

  • DOI:10.1186/1746-1448-4-2
    ways they cope with the high salt concentrations

    Saline-Systems review supports osmotic balance at the optimal NaCl as the mechanistic basis of the NaCl-optimum phenotype.

NaCl optimum with salt-in, salt-out, and transporter support

DOI-backed nonmechanistic graph connecting ambient NaCl, osmotic balance, salt-in K+ accumulation, acidic-proteome support, compatible-solute accumulation and uptake, cation transport, and maximal growth rate for the NaCl-optimum phenotype.

NONMECHANISTIC · This record is a quantitative measurement, interval, or bin in the environmental phenotype hierarchy; a token protein example would misrepresent the measured value as one inherited molecular mechanism.

NaCl optimum with salt-in, salt-out, and transporter support Interactive directed graph showing evidence-backed causal relationships for NaCl optimum.

Edge evidence

  • ambient NaCl concentration causes osmotic balance biolink:causes

    External NaCl level engages the osmoadaptive balance machinery.

  • compatible solutes regulates osmotic balance RO:0002211

    Compatible-solute accumulation maintains osmotic balance at the optimal NaCl.

  • Na+/H+ antiporters enables osmotic balance RO:0002327

    Na+/H+ antiporters support ion homeostasis underlying osmotic balance.

  • osmotic balance promotes maximal growth rate RO:0002213

    Maintained osmotic balance promotes peak growth at the optimal NaCl concentration.

  • maximal growth rate manifests as NaCl optimum METPO:2007400

    The NaCl concentration at which peak growth is achieved manifests the NaCl-optimum phenotype.

  • ambient NaCl concentration causes osmotic stress biolink:causes

    External NaCl creates the osmotic imbalance that requires osmoadaptive compensation.

    • DOI:10.1186/1746-1448-4-2 cytoplasm has to be at least isoosmotic Verified against the open Oren review; halophilic cells must osmotically balance the cytoplasm against high-salt medium.
  • salt-in strategy increases intracellular K+ accumulation RO:0002213

    The salt-in strategy accumulates molar potassium and chloride concentrations in the cytoplasm.

    • DOI:10.1186/1746-1448-4-2 accumulation of molar concentrations of potassium and chloride Verified against the open Oren review; high-salt-in organisms balance external salt primarily with intracellular KCl.
  • acidic proteome enables protein stability at high salt RO:0002327

    An acidic proteome enables protein folding and enzymatic function under near-saturating intracellular salt.

    • DOI:10.1186/1746-1448-4-2 extensive adaptation of the intracellular enzymatic machinery Verified against the open Oren review; high-salt-in strategists require intracellular enzymatic machinery adapted to molar salt.
  • compatible-solute accumulation regulates osmotic balance RO:0002211

    Organic compatible-solute accumulation regulates osmotic balance while limiting inorganic salt in the cytoplasm.

    • DOI:10.1186/1746-1448-4-2 exclude salt from their cytoplasm as much as possible Verified against the open Oren review; the salt-out strategy excludes salt from the cytoplasm and accumulates organic solutes that minimally perturb enzymatic activity.
  • compatible solute uptake associated with lower energetic cost than de novo synthesis biolink:associated_with

    Compatible-solute uptake is associated with lower energetic cost than de novo compatible-solute synthesis.

    • DOI:10.3389/fmicb.2022.846677 uptake of compatible solute from the medium is preferred Verified against the open Halomonas elongata review; external compatible-solute uptake is energetically preferred over de novo synthesis.
  • Na+/K+/H+ transporter activity regulates cellular ion homeostasis RO:0002211

    Na+/K+/H+ transporter activity contributes to cellular ion homeostasis under varying salinities.

    • DOI:10.1128/aem.00145-24 transport systems that regulate intracellular Na+/K+/H+ concentration Verified against the Xing et al. long-term salinity-stress study; the edge was broadened from Na+ homeostasis to match the reported Na+/K+/H+ transporter and intracellular K+ response.
  • intracellular K+ accumulation contributes to osmotic balance RO:0002326

    Intracellular KCl accumulation is a salt-in osmoadaptation route that contributes to balancing external salt.

    • DOI:10.3389/fmicb.2013.00315 Extremely halophilic microorganisms that accumulate KCl for osmotic balance Verified against the open Oren minireview; the connector links KCl accumulation to osmotic balance without making the salt-in strategy universal for every NaCl optimum.
  • salt-in strategy associated with acidic proteome biolink:associated_with

    Salt-in osmoadaptation is associated with proteome-scale acidic adaptation for molar intracellular KCl.

    • DOI:10.3389/fmicb.2013.00315 have a large excess of acidic amino acids in their proteins Verified against the open Oren minireview; the connector keeps acidic-proteome composition scoped to salt-in strategists.
  • compatible solute uptake associated with compatible-solute accumulation biolink:associated_with

    External compatible-solute uptake is associated with the salt-out branch that accumulates organic osmolytes for osmoadaptation.

    • DOI:10.3389/fmicb.2022.846677 Uptake of ectoine from the medium is facilitated by the osmoregulated TRAP transporter TeaABC Verified against the open Halomonas elongata review; the connector treats uptake as an accumulation branch with a lower energetic cost than de novo synthesis.
  • cellular ion homeostasis associated with osmotic balance biolink:associated_with

    Na+/K+/H+ transporter control of cellular ion homeostasis is associated with osmotic balancing during salinity change.

    • DOI:10.1128/aem.00145-24 ensuring cellular ion homeostasis under varying salinities Verified against the Xing et al. long-term salinity-stress study; the connector places the species-specific transporter response inside the osmotic-balance context.

Provenance

Identifier source
METPO (2026-06-12)
Author
Luke Wang
Definition source
DOI:10.1093/femsre/fuy009

Parent traits (2)

Children (4)

kg-microbe context

Matched 1 kg-microbe node via direct_metpo.

  • METPO:1000333 [-3.125, -1.042, -1.721, +1.474, …]

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/nacl_optimum-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: NaCl optimum

**Trait:** NaCl optimum
**Identifier:** `METPO:1000333`
**Category / kind / status:** ENVIRONMENT / CLASS / REVIEWED
**Definition supplied:** “A salinity phenotype with numerical limits that supports the most efficient growth and reproduction of an organism.”

## 1. Scope and current interpretation

`METPO:1000333` should represent the **assay-conditional NaCl concentration, or narrow concentration plateau, at which a microbial growth-performance endpoint is maximal**. Preferred endpoints are maximum specific growth rate during exponential growth, minimum doubling time, or—if those are unavailable—maximum biomass yield or colony radial-growth rate. In *Wallemia ichthyophaga*, for example, growth rates were calculated from exponential-phase doubling times; its optimum was 15–20% NaCl, whereas its full growth range extended from 10% to saturated NaCl (32%). Thus, optimum and range are distinct observations. (zajc2014osmoadaptationstrategyof pages 6-7, zajc2014osmoadaptationstrategyof pages 7-8, zajc2014osmoadaptationstrategyof pages 1-2, zajc2014osmoadaptationstrategyof pages 2-3)

The trait is conditional on medium composition, temperature, pH, oxygen regime, carbon source, incubation duration, inoculum, and measurement endpoint. Oren explicitly separates “the minimum salt concentration required for growth, the salinity optimum, and the upper salt limit tolerated,” while warning that all three depend on medium and temperature. (oren2008microbiallifeat pages 1-2)

### Boundary cases

- **Not NaCl growth range:** the interval permitting detectable growth.
- **Not maximum NaCl tolerated:** the upper concentration allowing any growth or survival.
- **Not minimum NaCl requirement:** especially important for obligate halophiles.
- **Not halotolerance:** halotolerant organisms can grow without an absolute salt requirement; halophily denotes preferential or required growth at elevated salinity. A traditional operational classification places moderate-halophile optima at 0.5–2.5 M salt and extreme-halophile optima at 2.5–5.2 M, but these categories are conventions rather than mechanistic boundaries. (oren2008microbiallifeat pages 2-4, oren2008microbiallifeat pages 1-2)
- **Not acute salt-shock tolerance:** short-term survival, transcription, or metabolite release after hyperosmotic shock does not establish a steady-state growth optimum.
- **Not generic salinity optimum:** NaCl concentration must not be silently equated with total dissolved salts, conductivity, or water activity. The Dead Sea, for example, contains over 35% total dissolved salts with substantial divalent-ion content; such exposure is chemically different from an NaCl-defined medium. (ionescu2024extremefluctuationsin pages 1-2)
- **Not merely water-activity optimum:** NaCl changes both water activity and ion composition. *W. ichthyophaga* grew across reported water activities of approximately 0.959–0.771, but this does not make water activity and NaCl interchangeable traits. (zajc2014osmoadaptationstrategyof pages 1-2)

**Recommended assay representation:** retain concentration value, unit and basis (`% w/v`, `g L−1`, molarity, or total Na+), medium, temperature, pH, atmosphere, endpoint, time point, and whether the result is a single optimum or plateau. Avoid converting percentages to molarity unless the original concentration basis is explicit.

## 2. Mechanistic model

External NaCl raises extracellular osmotic pressure and lowers water availability. Uncompensated cells lose water and turgor and experience ionic and macromolecular dysfunction. Efficient growth therefore occurs where osmotic and ionic homeostasis are restored without excessive energetic, transport, biosynthetic, or protein-folding costs. The optimum is an **emergent system-level outcome**, not the product of one universal “NaCl-optimum gene.”

Two canonical strategies dominate authoritative interpretations:

1. **Salt-in:** cells accumulate mainly KCl to balance external osmotic pressure. This requires a proteome adapted to molar salt—typically enriched in acidic proteins—and can impose a lower-salt growth defect because many proteins lose stability in dilute conditions.
2. **Salt-out/compatible-solute strategy:** cells limit cytoplasmic inorganic salt and synthesize or import compatible solutes such as ectoine, glycine betaine, proline, glutamate, sugars, and polyols. This generally supports a broader salinity range but consumes metabolic energy and/or transport capacity. (oren2008microbiallifeat pages 1-2, ionescu2024extremefluctuationsin pages 1-2)

Recent evidence shows that this dichotomy is not absolute. *Natranaerobius thermophilus* and organisms from fluctuating Dead Sea spring biofilms appear to use hybrid strategies combining compatible-solute and inorganic-ion mechanisms. The Dead Sea study’s conclusion is genomic and ecological—selection for hybrid capacity remains a hypothesis rather than a demonstrated determinant of a species-level NaCl optimum. (xing2024thepolyextremophilenatranaerobius pages 1-2, ionescu2024extremefluctuationsin pages 1-2, ionescu2024extremefluctuationsin pages 4-6)

## 3. Candidate graph nodes

### Trait and assay nodes

- NaCl optimum — `METPO:1000333`
- NaCl concentration — candidate chemical grounding: `CHEBI:26710` (sodium chloride); concentration itself should be represented as a measurement with value/unit
- maximal specific growth rate; doubling time; biomass yield; colony radial-growth rate — retain as label-only assay nodes unless the project’s measurement ontology supplies mappings
- growth range; minimum NaCl requirement; maximum tolerated NaCl — neighboring traits, not synonyms
- water activity — label-only candidate
- total dissolved salts / environmental salinity — label-only candidate; do not merge with NaCl concentration

### Environmental and physicochemical nodes

- extracellular osmotic pressure
- hyperosmotic environment / osmotic stress
- water availability and cellular water loss
- medium composition, temperature, pH, oxygen availability, incubation duration
- saline or hypersaline environment — ENVO grounding should be selected only after confirming the exact ENVO term required by the curation schema

### Ions and compatible solutes

- sodium ion — `CHEBI:29101`
- potassium ion — `CHEBI:29103`
- chloride — `CHEBI:17996`

Showing the first 60 of 215 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.

Canonical examples (1)

Organisms cited as exemplars of this trait. Taxon ids are NCBITaxon and link out to the NCBI record.

  • Paraliobacillus ryukyuensis NCBITaxon:200904 DOI:10.2323/jgam.48.269 Strain O15-7T had a maximum-specific-growth-rate optimum spanning 0.75-3.0% (w/v) NaCl. Because that interval touches the low/mid1 boundary, #591 keeps the complete claim on the parent rather than duplicating it onto a bin.

Curation history

  1. · SEEDED_FROM_METPO · seed_from_metpo

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

  2. · CURATED_CAUSAL_GRAPH · claude

    Added DOI-backed causal graph linking ambient NaCl, osmotic balance via compatible solutes and Na+/H+ antiporters, and maximal growth rate to the NaCl-optimum phenotype.

  3. · GROUND_CAUSAL_PREDICATES · claude

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

  4. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 1 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (METPO:2007400×1).

  5. · RENAME_PREDICATE_LABELS · claude

    Renamed 2 causal-edge predicate label(s) to align with existing groundings: maintains → regulates ×1; supports → enables ×1.

  6. · GROUND_CAUSAL_PREDICATES · claude

    Grounded 2 causal-edge predicate_id field(s) via mappings/predicate_grounding.tsv (RO:0002211×1, RO:0002327×1).

  7. · GROUND_CAUSAL_NODES · claude

    Grounded 1 causal-node grounding field(s) via mappings/node_grounding.tsv (CHEBI:65015×1).

  8. · RENAME_PREDICATE_LABELS · claude

    Renamed 1 causal-edge predicate label(s) to align with existing groundings: triggers → causes ×1.

  9. · GROUND_CAUSAL_PREDICATES · claude

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

  10. · FIX_NODE_GROUNDING_CURIE · claude

    Overwrote 1 causal-node grounding(s) to corrected CURIEs (phase-2 id-label fix; verified vs OAK).

  11. · ENRICH_CAUSAL_GRAPH · claude

    Added 6 evidence-backed generic edges (10 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 (RO:0002211×2, RO:0002213×1, RO:0002327×1, biolink:associated_with×1).

  13. · GROUND_CAUSAL_NODES · claude

    Grounded 2 causal-node grounding field(s) via mappings/node_grounding.tsv (GO:0006970×1, METPO:1007720×1).

  14. · NORMALISE_NODE_TYPE · claude

    Under the PATHWAY-vs-BIOLOGICAL_PROCESS rule, one node_id means one thing corpus-wide (issue 356): salt_in_strategy is typed BIOLOGICAL_PROCESS. PATHWAY is a named, conventionally enumerable multi-step route; BIOLOGICAL_PROCESS is everything else. A STRATEGY, not a route. The family is described as osmoadaptation by accumulating intracellular inorganic ions -- haloalkaliphilic.yaml puts it as 'Osmoadaptation by intracellular accumulation of inorganic ions (e.g. K+)', and the wording varies by record. There is no step list to enumerate, which is exactly the distinction this rule draws. Was 7 BIOLOGICAL_PROCESS to 1 before this tranche.

  15. · BACKFILL_CANONICAL_EXAMPLES · claude

    Added one exemplar taxon, the only organism in this trait's deep-research artifact for which that artifact cites a measured NaCl growth optimum. Taxon id resolved and label-checked against the local NCBITaxon build. Three sets of organisms named in the same artifact were deliberately NOT included, each because the artifact itself says the evidence stops short of a NaCl optimum: Halomonas elongata (the engineered proline strains shift an IC50, and the artifact warns "an IC50 or improved tolerance is not automatically equivalent to a rigorously re-estimated NaCl optimum"); Natranaerobius thermophilus (its reported 3.3-3.9 M optimum is total Na+ at pH 9.5, and the artifact warns "Do not merge NaCl, Na+, total salts, conductivity, osmolarity, and water activity"); and the Dead Sea spring MAG taxa Prosthecochloris, Flexistipes, Izemoplasma, Halomonas and Halanaerobiales (the artifact notes "MAG content predicts potential; it does not establish expression, flux, or the growth optimum of each organism").

  16. · NORMALISE_NODE_TYPE · codex

    Tranche 5 of issue 356 settles the process/quality families and merges ids that meant the same sense: maximal_growth_rate is QUALITY. A maximal growth rate is a measured upper bound or peak specific rate. Its edges associate, enable, or manifest that value; none describe the process of cell growth itself. An edge cannot enable a QUALITY under RO:0002327's biological-process range, so edges that causally support the rate use promotes (RO:0002213) instead.

  17. · REVIEW_GRAPH_PROTEIN_TAXON · claude

    Backfilled provenance (review issue 517) for the codex protein-taxon review tranche of 2026-08-24/25, which shipped without a per-record event. In this record the tranche: set graph scope nacl_optimum_balanced_osmoadaptation=NONMECHANISTIC with scope_notes; marked 1 GENE_OR_PROTEIN node(s) REVIEWED_LABEL_ONLY with grounding_notes (na_h_antiporters).

  18. · MOVE_CANONICAL_EXAMPLE_TO_SPECIFIC_BIN · codex

    Moved Wallemia ichthyophaga (NCBITaxon:245174; DOI:10.1128/aem.02702-13) off the NaCl-optimum parent under the #478 most-specific-placement policy. Its measured 15-20% (w/v) optimum belongs wholly in METPO:1000468; the same claim is not duplicated on parent and bin.

  19. · ADD_CANONICAL_EXAMPLES · codex

    Resolved issue #444 after the #591 source/bin policy with 1 direct source-backed canonical example(s): Paraliobacillus ryukyuensis (NCBITaxon:200904; DOI:10.2323/jgam.48.269). The note retains the measured value or scopes broad-class examples to the cited branch; no paid research was used.

  20. · REVIEW_CAUSAL_EVIDENCE · codex

    Reviewed the broad nacl_optimum_balanced_osmoadaptation graph for issue #183: added snippets to 6 edge-level evidence items, regrounded the ambient-NaCl osmotic-stress edge to biolink:causes, and broadened one transporter edge from Na+ homeostasis to Na+/K+/H+ transporter control of cellular ion homeostasis. No paid research service was called.

  21. · CONNECT_CAUSAL_GRAPH_COMPONENTS · codex

    Resolved issue #183 graph fragmentation (5 components to 1) by adding 4 source- and verbatim-snippet-backed connectors among the salt-in KCl, acidic proteome, compatible-solute uptake, cation homeostasis, and osmotic-balance branches. No paid research service was called.

  22. · ADVERSARIAL_REVIEW_REPAIR · codex

    Addressed PR #664 adversarial review: replaced copied nonmechanistic bridge snippets with independent exact source snippets while preserving the existing connector edge scope.