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Abstract:
The order Thermoplasmatales (Euryarchaeota) is represented by the most acidophilic organisms known
so far that are poorly amenable to cultivation. Earlier culture-independent studies in Iron Mountain
(California) pointed at an abundant archaeal group, dubbed ‘G-plasma’. We examined the genomes and
physiology of two cultured representatives of a Family Cuniculiplasmataceae, recently isolated from
acidic (pH 1–1.5) sites in Spain and UK that are 16S rRNA gene sequence-identical with ‘G-plasma’.
Organisms had largest genomes among Thermoplasmatales (1.87–1.94 Mbp), that shared 98.7–98.8%
average nucleotide identities between themselves and ‘G-plasma’ and exhibited a high genome
conservation even within their genomic islands, despite their remote geographical localisations.
Facultatively anaerobic heterotrophs, they possess an ancestral form of A-type terminal oxygen
reductase from a distinct parental clade. The lack of complete pathways for biosynthesis of histidine,
valine, leucine, isoleucine, lysine and proline pre-determines the reliance on external sources of amino
acids and hence the lifestyle of these organisms as scavengers of proteinaceous compounds from
surrounding microbial community members. In contrast to earlier metagenomics-based assumptions,
isolates were S-layer-deficient, non-motile, non-methylotrophic and devoid of iron-oxidation despite
the abundance of methylotrophy substrates and ferrous iron in situ, which underlines the essentiality of experimental validation of bioinformatic predictions.
