**3. Antimicrobial potential of extremely halophilic archaea**

Halophilic archaea were the first members of archaea found to produce bacteriocins-like proteins known as halocins. The first studies date from the beginning of 1980s with experiments demonstrating the presence of antagonistic interactions between halophilic archaeal strains isolated from the Alicante salt in Spain [20]. Today, about fifteen halocins have been described and only three genes, *halC8*, *halS8* and *halH4*, coding C8, S8 and H4 halocins, respectively, have been identified [21, 22]. Yet, no three-dimensional structural data of archaeocins are available in public databases.

*Extremophilic Microbes and Metabolites - Diversity, Bioprospecting and Biotechnological...*

organisms as three separate domains: Eucarya (*Eukaryotes*), Archaea and Bacteria (bacteria) [3]. Archaea share several similarities with the other two domains of life. They are similar to size-level bacteria, organization of their chromosomes, absence of nucleus and organelles, presence of polycistronic transcription units and use of Shine-Dalgarno sequences for the initiation of the translation. In addition, it was shown that their metabolic proteins are essentially bacterial in nature following analysis of many complete genomes of Archaea [2]. Archaea also share similarities with the Eucarya domain, such as the proteins involved in key informational processes such as replication [4], transcription, translation [5, 6], DNA repair [7, 8], mRNA degradation and proteolysis. Translation in Archaea has eukaryotic initiation and elongation factors, and their transcription involves TATA binding protein

The biotopes colonized by these microorganisms, are supposed to approach to the primitive terrestrial atmosphere (high salinity or pH, devoid of O2, rich in H2 and CO2 constituting the raw materials for the production of methane) [10]. They present spectacular adaptations, especially in extreme environments. We distinguish: (i) Thermophilic Archaea: living at high temperatures (60–80°C) (ii) Hyperthermophilic Archaea: living at very high temperatures (up to 121°C); (iii) Psychrophilic Archaea: prefering low temperatures (below 15°C) [11]; (iv) Halophilic Archaea: colonizing very saline environments (3–5 M NaCl) such as the Dead Sea [12, 13]; (v) Acidophilic Archaea: living at low pH (as low as pH 1 and dying at pH 7) and Alkaliphilic Archaea: thriving at high pH (up to 9) [14].

The first phylogenetic study based on the comparison of the 16S rDNA gene sequences coding for the small subunit, separated the first founding members of Archaea into two taxa, one grouping methanogenic species and those living under conditions of extreme salinity, the other containing species living at very high temperatures and at acidic pH [15]. Ten years later, analyses on a larger taxonomic group led to the division of the Archaea kingdom into two groups: (i) *Crenarchaeota*, which is composed exclusively of microorganisms living at very high temperatures, and (ii) *Euryarchaeota*, a heterogeneous group of species with different phenotypes (methanogenic species, species living at very high temperatures, moderate temperatures or at high salt concentrations) [3]. Fifteen years later and thanks to metagenomic analyses, two phyla, *Thaumarchaeota* and *Korarchaeota* were established based on the results of genomic comparison of two uncultivable strains, *Candidatus cenarchaeum symbosium* and *Candidatus korarchaeum cryptofilum*, with genomic traits belonging to both phyla *Crenarchaeota* and *Euryarchaeota* [16]. On the other hand, the symbiont *Nanoarchaeum equitans*, occupying cells of the host *Ignicoccus hospitalis*, showed even more genomic divergence with the other members of the *Crenarchaeota* and was therefore the first member of the phyla *Nanoarchaeota*. Today, we count more of 15 phyla in the reign of Archaea, some of them having been grouped in superphylum. One distinguishes the superphylum TACK, proposed in 2011 and of which the eukaryotes would have evolved according to the theory of the eocyte, grouping *Thaumarchaeota*, *Aigarchaeota*, *Crenarchaeota* and *Korarchaeota* phyla [17]. This superphylum has been joined by recently proposed phyla: *Bathyarchaeota*, *Geoarchaeota* and *Lokiarchaeota* [18]. Another superphylum, DPANN, was proposed in 2013 and includes *Diapherotrites*, *Parvarchaeota*, *Aenigmarchaeota*, *Nanoarchaeota* and *Nanohaloarchaeota* phyla (**Figure 1**) [19]. Finally, the phyla *Woesearchaeota* and *Pacearchaeota*, described in 2016, were

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and TFIIB [9].

**2. Taxonomy of the archaeal domain**

grouped in the DPANN superphylum (**Figure 1**).
