# Pyrodictium
**Pyrodictium** is a genus of anaerobic, disc-shaped hyperthermophilic archaea that live in submarine volcanic environments and grow optimally at 97 to 105 °C, with a maximum growth temperature of 110 °C, among the highest known for any organism. Its cells are joined into a network by hollow protein tubules called cannulae, a feature unique to the genus and the source of its name, from the Greek *pyr* (fire) and *diktyon* (network), meaning "fire-loving network".[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395)
| Key fact | Detail |
|---|---|
| Classification | Archaea, Thermoproteota, Desulfurococcales, Pyrodictiaceae; type species *Pyrodictium occultum*[2](https://registry.seqco.de/names/17449) |
| Growth temperatures | Optimum 97–105 °C; maximum 110 °C; minimum 80–82 °C[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395) |
| Cell shape | Disk- to dish-shaped, 0.3 to 2.5 µm in diameter, with ultraflat areas about 0.1–0.2 µm thick[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395) |
| Cannulae | Hollow tubes 25 nm in outside diameter connecting cells into a network; built from three homologous glycoprotein subunits of 20, 22, and 24 kDa[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/) |
| Metabolism | Strictly anaerobic; chemolithotrophic growth on H₂ + CO₂ + S⁰ producing H₂S; DNA G+C content 60–62 mol%[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395) |
| Heat resistance | Members of *Pyrodictium* and *Pyrolobus* survive at least 1 hour of autoclaving[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1664684/) |
| Species | Four named species: *P. occultum*, *P. brockii*, *P. abyssi*, and *P. delaneyi*[2](https://registry.seqco.de/names/17449) |
## Discovery and taxonomy
The genus was described in 1983 by [Karl O. Stetter](https://www.edgepedia.org/karl-o-stetter), H. König, and E. Stackebrandt from six isolates taken from a submarine solfataric field off Vulcano, Italy, with the type species *Pyrodictium occultum* validly published in 1984.[5](https://pubmed.ncbi.nlm.nih.gov/23194811/) Stetter had reported the organisms in 1982 as ultrathin, mycelia-forming microbes from shallow water near a beach of Vulcano Island with an optimum growth temperature of 105 °C, the first organism shown to grow optimally above 100 °C.[6](https://academic.oup.com/femsre/article/20/1-2/13/512138) A second species, *P. brockii*, was distinguished in the same 1983 paper by a lower G+C content (51.5 to 56.6 mol% versus 62 mol% for *P. occultum*), a smaller major cell-envelope glycoprotein, and a growth yield greatly increased by yeast extract.[5](https://pubmed.ncbi.nlm.nih.gov/23194811/)
**Current classification.** The genus sits in the family Pyrodictiaceae, order [Desulfurococcales](https://www.edgepedia.org/desulfurococcales), within the phylum [Thermoproteota](https://www.edgepedia.org/thermoproteota) (Crenarchaeota).[2](https://registry.seqco.de/names/17449) The family also contains *Pyrolobus* and *Hyperthermus*; the autotrophic *Pyrodictium* and *Pyrolobus* species grow optimally at 105 to 108 °C, while the peptide-fermenting *P. abyssi* and *Hyperthermus butylicus* grow optimally near 97 °C.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC6092747/) Together with *Methanopyrus* and *Pyrolobus*, *Pyrodictium* represents one of the deepest and shortest lineages in the archaeal phylogenetic tree, and these genera hold the highest growth temperatures known.[6](https://academic.oup.com/femsre/article/20/1-2/13/512138)
## Morphology and the cannulae network
Cells are highly irregular disks or dishes, frequently with ultraflat areas, and carry no flagella; no motility has been observed.[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395) The cell envelope is a paracrystalline surface layer (S-layer) of glycoprotein subunits arranged in a hexagonal array, and the membrane contains isopranoid di- and tetraether lipids typical of archaea.[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395)
**Cannulae.** The genus's defining structure is a network of extracellular hollow tubules, the cannulae, which connect the periplasmic spaces of neighboring cells. Cannulae have an outside diameter of 25 nm and appear empty when cross-fractured, and they are built from at least three homologous glycoprotein subunits with identical N termini and molecular masses of 20, 22, and 24 kDa.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/) The structure is extraordinarily stable: no morphological change occurs after 60 minutes at 140 °C or 10 minutes at 100 °C in 2% sodium dodecyl sulfate.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/) During cell division of *P. abyssi* TAG11, the two daughter cells remain connected by the growing cannulae, so that cultivation ends in a dense network of cells and tubules.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/) Spontaneous cannulae-free mutants have not been observed in laboratory cultures, suggesting the network is necessary for growth.[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC4187195/) The main cannulae-forming protein of *P. abyssi*, CanA, forms hollow fibers in the presence of divalent ions, with ion binding proposed as the first step of polymerization.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC12326000/)
## Metabolism
*Pyrodictium* is a strict anaerobe. In its chemolithoautotrophic mode it oxidizes molecular hydrogen with carbon dioxide as the carbon source and elemental sulfur as the electron acceptor, producing hydrogen sulfide.[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395) In *P. occultum*, growth on H₂ + CO₂ + S⁰ produces H₂S, and in the presence of yeast extract the organism can also grow by reducing thiosulfate.[10](https://journals.asm.org/doi/full/10.1128/mmbr.65.1.1-43.2001?doi=10.1128%2Fmmbr.65.1.1-43.2001) H₂ oxidation is coupled directly to sulfur or polysulfide reduction in a membrane-bound multienzyme complex carrying both hydrogenase and sulfur reductase activities; the complex contains Fe, Ni, Cu, acid-labile sulfur, and hemes b and c, and quinones were required for activity in the *P. brockii* complex but not in the *P. abyssi* complex.[11](https://www.sciencedirect.com/topics/immunology-and-microbiology/aeropyrum) Growth occurs at pH 5 to 7 with an optimum near 5.5, and at an optimum NaCl concentration of about 1.5%; vigorous shaking prevents cell growth.[1](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395)
## Species and habitats
Members of the genus have been isolated from hydrothermal marine environments around the world, with growth temperatures ranging from 80 up to 110 °C.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/) *P. abyssi* was isolated from the wall of a black smoker chimney at a depth of 2,005 m in the [Guaymas Basin](https://www.edgepedia.org/guaymas-basin), Gulf of California, and grows at up to 110 °C as a heterotroph.[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC12326000/)
**Pyrodictium delaneyi.** A fourth species was proposed in 2018 on the basis of strain Su06ᵀ, isolated from an active deep-sea hydrothermal vent chimney on the Endeavour Segment of the north-eastern [Pacific Ocean](https://www.edgepedia.org/pacific-ocean). The strain is an obligately anaerobic, hydrogenotrophic autotroph that reduces Fe(III) oxide to magnetite and nitrate to N₂. Its average nucleotide identity against *P. occultum* PL-19ᵀ was 72%, well below the 96% species cutoff, supporting its description as a novel species; the type strain is Su06ᵀ (=DSM 28599ᵀ =ATCC BAA-2559ᵀ).[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC6092747/)
## Extreme heat tolerance
*Pyrodictium* belongs to the small group of organisms that define the upper temperature border of life. Its maximum growth temperature of 110 °C is matched among crenarchaeotes only by its relative *Pyrolobus fumarii*, which grows at 113 °C.[12](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/pyrodictium) For species of *Methanopyrus*, *Pyrodictium*, and *Pyrolobus*, temperatures of 80 °C and below are too low to support growth.[13](https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793%2899%2900663-8) Vegetative cultures of *Pyrodictium* and *Pyrolobus* survive autoclaving, and hyperthermophiles of these groups also survive deep-freezing at −140 °C, properties that have been discussed in connection with the survival of early life and with transfer of microbes between planets in impact ejecta.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC1664684/) Heat tolerance extends to the extracellular matrix: the cannulae withstand conditions far harsher than the cells' own growth range.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/)
## References
1. [Stetter, K.O. (2015). Pyrodictium. Bergey's Manual of Systematics of Archaea and Bacteria, Wiley](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118960608.gbm00395)
2. [Pyrodictium, SeqCode Registry](https://registry.seqco.de/names/17449)
3. [Cell Surface Structures of Archaea, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2546794/)
4. [Hyperthermophiles in the history of life, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1664684/)
5. [Stetter, König & Stackebrandt (1983). Pyrodictium gen. nov., PubMed](https://pubmed.ncbi.nlm.nih.gov/23194811/)
6. [Rachel, Rieger & Stetter. Ultrastructure of hyperthermophilic Archaea, FEMS Microbiology Reviews](https://academic.oup.com/femsre/article/20/1-2/13/512138)
7. [Pyrodictium delaneyi sp. nov., PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6092747/)
8. [Surface Appendages of Archaea, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4187195/)
9. [Biophysical characterization of CanA from Pyrodictium abyssi, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12326000/)
10. [Hyperthermophilic Enzymes: Sources, Uses, and Molecular Mechanisms for Thermostability, MMBR](https://journals.asm.org/doi/full/10.1128/mmbr.65.1.1-43.2001?doi=10.1128%2Fmmbr.65.1.1-43.2001)
11. [Aeropyrum, ScienceDirect Topics](https://www.sciencedirect.com/topics/immunology-and-microbiology/aeropyrum)
12. [Pyrodictium, ScienceDirect Topics](https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/pyrodictium)
13. [Stetter (1999). Extremophiles and their adaptation to hot environments, FEBS Letters](https://febs.onlinelibrary.wiley.com/doi/10.1016/S0014-5793%2899%2900663-8)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Archaea › Archaeal taxonomy and diversity › Crenarchaeota and TACK superphylum › Crenarchaeal orders (Thermoprotei) › Desulfurococcales*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license