# Sifarchaeota
**Candidatus Sifarchaeota** is a candidate phylum of [Asgard archaea](https://www.edgepedia.org/asgard-archaea) proposed in 2021 for metagenome-assembled genomes (MAGs) recovered from deep marine sediments off the [Costa Rica](https://www.edgepedia.org/costa-rica) margin, which form a sister lineage to "Candidatus Thorarchaeota" and show average amino acid identity below 50% to all other Asgard MAGs.[1](https://europepmc.org/articles/PMC8091018) No representative has been cultured, so the taxon rests entirely on genome sequences.[6](https://www.nature.com/articles/s41467-026-71534-5) Its rank has since been disputed: the Genome Taxonomy Database (GTDB) treats the lineage as a class, "Candidatus Sifarchaeia", within the phylum Asgardarchaeota,[4](https://doi.org/10.1038/s43705-021-00032-0) and the List of Prokaryotic names with Standing in [Nomenclature](https://www.edgepedia.org/nomenclature) (LPSN) records "Ca. Sifarchaeota" and the validly published name Promethearchaeota Imachi et al. 2024 as separate phyla.[2](https://lpsn.dsmz.de/phylum/sifarchaeota)
| Key fact | Detail |
|---|---|
| Proposed as | Candidate phylum, Farag, Zhao & Biddle 2021[1](https://europepmc.org/articles/PMC8091018) |
| Circumscription criterion | Average amino acid identity below 50% to other Asgard MAGs[1](https://europepmc.org/articles/PMC8091018) |
| Founding genomes | 3 medium-quality MAGs, 67–80% complete, 0.93–1.9% contamination[1](https://europepmc.org/articles/PMC8091018) |
| Type species | "Ca. Sifarchaeum subterraneum" (CR_Bin_142, GCA_016292335.1), Costa Rica margin sediment[4](https://doi.org/10.1038/s43705-021-00032-0) |
| Current rank (GTDB) | Class "Ca. Sifarchaeia" within phylum Asgardarchaeota[4](https://doi.org/10.1038/s43705-021-00032-0) |
| Validly published name | None; "Ca. Sifarchaeota" remains a Candidatus name, separate from Promethearchaeota Imachi et al. 2024[2](https://lpsn.dsmz.de/phylum/sifarchaeota) |
| Cultivated representatives | None; only two Asgard archaea are cultured, both from Lokiarchaea[6](https://www.nature.com/articles/s41467-026-71534-5) |
| Known habitats | Marine subseafloor sediment (Costa Rica margin) and lacustrine sediment (Lake Weyba, Australia)[1](https://europepmc.org/articles/PMC8091018) • [4](https://doi.org/10.1038/s43705-021-00032-0) |
## Definition and naming
The name was introduced in 2021 in Applied and Environmental Microbiology by Farag, Zhao, and Biddle for a novel Asgard lineage reconstructed from Costa Rica margin sediment.[1](https://europepmc.org/articles/PMC8091018) The circumscription rested on two genomic criteria: phylogenetic separation from described Asgard phyla in a tree of 16 ribosomal proteins, and average amino acid identity (AAI) below 50% compared to other Asgard MAGs, a level consistent with separate phylum status under conventions used for archaeal ranks.[1](https://europepmc.org/articles/PMC8091018)
The name derives from Sif, a goddess in [Norse mythology](https://www.edgepedia.org/norse-mythology), applied through the candidate genus name "Ca. Sifarchaeum" plus the Latin neuter plural suffix -ota used to denote a phylum.[2](https://lpsn.dsmz.de/phylum/sifarchaeota) LPSN lists "Candidatus Sifarchaeota" Farag et al. 2021 with type genus "Ca. Sifarchaeum" corrig. Farag et al. 2021. The name is not validly published under the [International Code of Nomenclature of Prokaryotes](https://www.edgepedia.org/international-code-of-nomenclature-of-prokaryotes) but carries pro-valid publication status; if treated as a separate phylum it is the pro-correct name.[2](https://lpsn.dsmz.de/phylum/sifarchaeota)
Spelling has been corrected downstream: LPSN records the pro-correct genus name as "Ca. Sifarchaeum", treating the originally published "Sifarchaeotum" as an inaccurate spelling, and the species combinations "Ca. Sifarchaeotum marinoarchaea" and "Ca. Sifarchaeotum subterraneus" as misspellings.[3](https://lpsn.dsmz.de/genus/sifarchaeotum) The genus is placed in the family "Ca. Sifarchaeaceae" Sun et al. 2021.[3](https://lpsn.dsmz.de/genus/sifarchaeotum)
## Taxonomic placement and rank within the Asgard archaea
In the original description, the three Costa Rica MAGs formed a sister lineage to "Ca. Thorarchaeota" in a phylogenomic tree built from 16 ribosomal proteins.[1](https://europepmc.org/articles/PMC8091018) Later phylogenomic work places the lineage among the non-Heimdallarchaeia Asgard classes, with Heimdallarchaeia, not Sifarchaeia, positioned as the sister group to Eukarya.[4](https://doi.org/10.1038/s43705-021-00032-0) A 2025 analysis of 411 Asgard genomes using 53 archaeal marker proteins confirmed ten class-level Asgard lineages, Sifarchaeia among them.[5](https://www.nature.com/articles/s41586-025-08955-7)
**Rank is contested.** Sun et al. (2021) applied GTDB rank normalization, which sets ranks by relative evolutionary divergence rather than AAI thresholds alone, and proposed the lineage as a class, "Ca. Sifarchaeia", within the phylum Asgardarchaeota.[4](https://doi.org/10.1038/s43705-021-00032-0) The same analysis synonymised the separately proposed phyla "Sifarchaeota" and "Borrarchaeota" under one class, retaining Sifarchaeota as the base name on publication priority.[4](https://doi.org/10.1038/s43705-021-00032-0) No source in this entry reports a direct test that has overturned the lineage's monophyly; the challenge is to rank, not to coherence.
## Candidate genera and species
All names in the group are "Candidatus" names. Farag et al. designated two type strains from their bins: bin 042 as "Ca. Sifarchaeotum marinoarchaea" (corrected spelling "Ca. Sifarchaeum marinoarchaeum") and bin 142 as "Ca. Sifarchaeotum subterraneus".[1](https://europepmc.org/articles/PMC8091018) • [3](https://lpsn.dsmz.de/genus/sifarchaeotum) Sun et al. fixed "Ca. Sifarchaeum subterraneum", based on genome CR_Bin_142 (GenBank assembly GCA_016292335.1) from Costa Rica margin sediment, as the type species of the class, with "Ca. Sifarchaeaceae" as the type family.[4](https://doi.org/10.1038/s43705-021-00032-0)
A second family-level lineage, "Ca. Borrarchaeaceae", sits within the class and contains "Ca. Borrarchaeum weybense", recovered from Lake Weyba sediments as MAG lw60_2018_gm2_56 with 94.08% completeness and 3.74% contamination.[4](https://doi.org/10.1038/s43705-021-00032-0) GTDB rank normalization treats the later-proposed phylum "Borrarchaeota" as the family Borrarchaeaceae within Sifarchaeia.[8](https://www.rinkelab.org/index.php/blog/11-disentangling-taxonomic-knots-asgard)
## Genomes and data quality
The phylum rests on metagenome-assembled genomes only; no Sifarchaeota strain has been cultured.[6](https://www.nature.com/articles/s41467-026-71534-5) The founding set comprised three medium-quality draft MAGs with completion of 67 to 80% and contamination of 0.93 to 1.9%.[1](https://europepmc.org/articles/PMC8091018) For context, the 71 Asgardarchaeota MAGs analyzed by Sun et al. averaged 78.7 ± 15.3% completeness and 3.8 ± 2.3% contamination, with GC content from 28.8 to 48.4% and an estimated average genome size of about 4 Mbp.[4](https://doi.org/10.1038/s43705-021-00032-0) The 2025 phylogenomic dataset combined 395 public genomes with 223 newly reconstructed Asgard MAGs, each above 70% completeness and below 10% contamination, with mean completeness of 85.3% and mean contamination of 3.6%.[5](https://www.nature.com/articles/s41586-025-08955-7) [Individual](https://www.edgepedia.org/individual) quality statistics for each Sifarchaeota MAG beyond the founding ranges are not given in the sources reviewed here.
## Comparison with other Asgard phyla
Within GTDB release 226, the Asgard archaea comprise 296 assemblies assigned to 12 class-level lineages: Asgard-, Atabey-, Baldr-, Heimdall-, Hermod-, Jord-, Loki-, Njord-, Odin-, Sif-, Thor- and Wukongarchaeia.[6](https://www.nature.com/articles/s41467-026-71534-5) Sifarchaeia therefore sits alongside Odinarchaeia as a separate class, while several sibling names proposed at phylum rank in 2021 were absorbed at lower ranks: "Kariarchaeota" and "Hodarchaeota" became the orders Kariarchaeales and Hodarchaeales within Heimdallarchaeia, and "Borrarchaeota" became the family Borrarchaeaceae inside Sifarchaeia.[8](https://www.rinkelab.org/index.php/blog/11-disentangling-taxonomic-knots-asgard) "Gerdarchaeota" and "Helarchaeota" were likewise recovered as order-level lineages within Heimdallarchaeia and Lokiarchaeia respectively.[4](https://doi.org/10.1038/s43705-021-00032-0)
Cultivation also separates the lineages: only two Asgard representatives have been cultured, "Ca. Prometheoarchaeum syntrophicum" MK-D1 and "Ca. Lokiarchaeum ossiferum" B-35, both from Lokiarchaea, so Sifarchaeia, like most Asgard classes, is known solely from sequences.[6](https://www.nature.com/articles/s41467-026-71534-5)
## Recent developments
Two developments have reshaped the taxon's standing. First, LPSN records Promethearchaeota Imachi et al. 2024 as a separate, validly published phylum alongside "Ca. Sifarchaeota".[2](https://lpsn.dsmz.de/phylum/sifarchaeota) Second, genome sampling has expanded substantially: the 2025 Nature study added 223 new Asgard MAGs from 40 salt marsh and mangrove wetland sediment samples across China, including 108 new species-level genomes in six of the ten Asgard classes, and identified four new family-level and ten new genus-level clades, some within Sifarchaeia.[5](https://www.nature.com/articles/s41586-025-08955-7) GTDB release 226 holds 296 Asgard assemblies across 12 classes.[6](https://www.nature.com/articles/s41467-026-71534-5)
This contraction in rank mirrors a broader reorganization. The number of recognized Asgard phylum-level lineages grew from one in 2015 ([Lokiarchaeota](https://www.edgepedia.org/lokiarchaeota)) to 18 by late 2021, with Liu et al. describing six new phyla from 75 MAGs;[7](https://doi.org/10.1002/mlf2.12012) GTDB rank normalization then demoted four of the newly proposed lineages, including Sifarchaeota, to classes and absorbed others at order or family rank, leaving 12 class-level lineages in current databases.[8](https://www.rinkelab.org/index.php/blog/11-disentangling-taxonomic-knots-asgard) • [6](https://www.nature.com/articles/s41467-026-71534-5)
## References
1. Farag IF, Zhao R, Biddle JF. "Sifarchaeota," a Novel Asgard Phylum from Costa Rican Sediment Capable of Polysaccharide Degradation and Anaerobic Methylotrophy. Applied and Environmental Microbiology, 2021. https://europepmc.org/articles/PMC8091018
2. LPSN: Phylum "Candidatus Sifarchaeota". https://lpsn.dsmz.de/phylum/sifarchaeota
3. LPSN: Genus "Candidatus Sifarchaeum" (recorded under Sifarchaeotum). https://lpsn.dsmz.de/genus/sifarchaeotum
4. Sun et al. Recoding of stop codons expands the metabolic potential of two novel Asgardarchaeota lineages. ISME Communications, 2021. https://doi.org/10.1038/s43705-021-00032-0
5. Deep origin of eukaryotes outside Heimdallarchaeia within Asgardarchaeota. Nature, 2025. https://www.nature.com/articles/s41586-025-08955-7
6. Phylogenomics of Asgard archaea reveals a unique blend of prokaryotic-like horizontal transfer and eukaryotic-like gene duplication. Nature Communications, 2026. https://www.nature.com/articles/s41467-026-71534-5
7. The expanding Asgard archaea and their elusive relationships with Eukarya. mLife. https://doi.org/10.1002/mlf2.12012
8. Disentangling taxonomic knots in the realm of Asgard. Rinke lab blog. https://www.rinkelab.org/index.php/blog/11-disentangling-taxonomic-knots-asgard
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Archaea › Archaeal taxonomy and diversity › Asgard archaea › Other Asgard phyla › Sifarchaeota*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026 · Edited: Sep 30, 2026; Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Sep 30, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license