temperature optimum mid2

Deep research

Generated by just research-trait; source: research/traits/environment/temperature_optimum_mid2-deep-research-falcon.md

# Curation report: temperature optimum mid2

## Executive assessment

**Target:** `METPO:1000444` — **temperature optimum mid2**  
**Category:** ENVIRONMENT; **term kind:** CLASS; **mapping:** REVIEWED  
**Parent:** `METPO:1000304`  
**Synonyms:** *Mesophilie*, `TO_27_to_30`

The trait should represent an **experimentally observed growth optimum**, not merely survival or growth somewhere between 27 and 30 °C. Operationally, an isolate qualifies when a temperature-response assay places its maximum specific growth rate, shortest doubling time, or—if rate is unavailable—maximum standardized biomass/yield at approximately 27–30 °C. The strongest general mechanistic explanation is not a single “mesophily gene,” but balance among membrane physical state, enzyme activity/stability, translation and RNA structure, and proteostasis. Current evidence most strongly supports a membrane-homeoviscosity subgraph; evidence that any individual mechanism specifically fixes an organism’s optimum within the narrow 27–30 °C interval remains limited.

## 1. Trait scope and boundaries

### Included phenotype

`METPO:1000444` denotes the location of the optimum of a microbial growth-versus-temperature response curve. Suitable observations include:

- maximum specific growth rate at 27–30 °C;
- minimum generation time in that interval;
- maximum biomass or colony-production endpoint there, provided medium, incubation duration, oxygen regime, pH, salinity, and inoculum are controlled;
- a reported optimum such as 28 °C or 30 °C, allowing the approximate interval specified by the ontology definition.

### Excluded or distinct observations

1. **Growth range:** growth at 28 °C does not establish an optimum at 28 °C.
2. **Thermotolerance or survival:** survival after heat or cold shock is not an optimum phenotype.
3. **Transient acclimation:** induction of desaturases, chaperones, or cold-shock proteins after a shift documents adaptation, not the location of the steady-state optimum.
4. **Enzyme optimum:** an isolated enzyme’s catalytic optimum is not automatically the organism’s growth optimum, although organismal growth temperature and mean enzyme optima can be strongly correlated.
5. **Host-associated performance:** infection or colonization at 26–29 °C is not equivalent to axenic growth optimum.
6. **Nearby classes:** organisms whose measured maxima fall below approximately 27 °C or above approximately 30 °C should map to the adjacent temperature-optimum class, even if broadly described as mesophiles.

The assay should ideally sample temperatures on both sides of the proposed maximum. A lone measurement at 28 or 30 °C cannot distinguish a true optimum from an assay endpoint or plateau.

## 2. Current mechanistic understanding

Cooling orders and thickens lipid bilayers. Bacteria commonly compensate by increasing unsaturated or branched-chain fatty acids, lowering lipid packing and restoring a fluid state needed for transport, respiration, division, and other membrane-associated processes. This is **homeoviscous adaptation**. In *Bacillus subtilis*, the canonical model is decreased fluidity/increased thickness → DesK kinase activity → DesR phosphorylation → `des` transcription → fatty-acid desaturation → restored fluidity. Importantly, membrane physical state rather than temperature alone can activate this circuit: increasing order at constant 37 °C also induces `des`. (mendoza2014temperaturesensingby pages 5-6, mendoza2014temperaturesensingby pages 1-2, mendoza2014temperaturesensingby pages 2-4)

Recent work qualifies that canonical model. A 2024 *B. subtilis* study found robust promoter activation after a mild 37→25 °C shift, but not after stronger shifts to 16 or 4 °C despite membrane rigidification. DesK partitioned into fluid domains, and `des`, `desK`, and `desR` deletions produced no detectable fluidity-adaptation phenotype under the tested conditions. Branched-chain fatty acids, reported as 80–96% of total fatty acids, appear to dominate fluidity control in this organism. Thus, the Des pathway is mechanistically real but should not be represented as the universal or sufficient determinant of mesophily. (sidarta2024lipidphaseseparation pages 1-2, sidarta2024lipidphaseseparation pages 12-14)

Direct *Escherichia coli* perturbation evidence strengthens the broader membrane-to-growth link. Lowering unsaturated-fatty-acid synthesis through `fadR` disruption reduced membrane fluidity and caused growth defects around 25–30 °C in sensitized genetic backgrounds. Palmitoleic acid supplementation rescued growth, whereas saturated palmitic acid did not; increased temperature also rescued growth by increasing fluidity. The alarmone (p)ppGpp buffered cell division when fluidity fell. (singh2024(p)ppgppbufferscell pages 8-11)

At the systems level, a dataset of **21,498 nonredundant microbes** found a Pearson correlation of up to **0.89** between organismal growth temperature and mean enzyme optima. It identified **319 enzyme functions** whose occurrence changed with growth temperature and **eight enriched metabolic pathways**; the data could associate growth-temperature metadata with **43% of UniProt entries** at that time. These are valuable node-discovery statistics, but they are comparative correlations rather than causal evidence for the 27–30 °C class. (engqvist2018correlatingenzymeannotations pages 1-2)

## 3. Candidate causal-graph nodes

Identifiers below are limited to high-confidence, stable CURIEs. Label-only entries are deliberately retained where gene products are species-specific or an exact cross-reference was not verified.

### Trait and environmental/experimental nodes

| Candidate node | Suggested grounding | Role |
|---|---|---|
| temperature optimum mid2 | `METPO:1000444` | Target phenotype |
| parent temperature-optimum trait | `METPO:1000304` | Ontological parent |
| ambient/growth temperature | `ENVO:01000205` (air temperature) only when air is truly the medium; otherwise label-only “incubation temperature” is safer | Experimental/environmental input |
| 27–30 °C incubation | Label-only assay condition | Defining interval |
| decreased temperature / cold shift | Label-only process | Perturbation |
| elevated temperature / heat shift | Label-only process | Perturbation |
| specific growth rate | Label-only quantitative phenotype | Preferred assay readout |
| biomass yield / optical density | Label-only assay readout | Secondary evidence |

Showing the first 60 of 227 lines of findings; the linked file also carries the run's front matter and the prompt it was given — read the full report.