Members of the archaeal order Caldarchaeales (previously considered as Aigarchaeota phylum) are poorly sampled and with relatively few number of genomes available in databases. Although they are widely distributed globally, they appear to be mostly dominant in hot springs or geothermal habitats. Due to lack of representative genomes, their status as a phylum of its own (Aigarchaeota), remains uncertain. Additional genomes of these archaea are urgently needed to better understand their placement within the archaeal tree of life and their roles in various environments. Here, we constructed five metagenome-assembled genomes (MAGs) of Aigarchaeota from two different geothermal features to investigate their metabolic potential and phylogenomic placement within the domain Archaea. Two MAGs were obtained from fumarolic lava rocks from Mauna Ulu, Hawaii, and three MAGs were obtained from El Tatio geothermal field in Chile. MAGs from Hawaii are highly complete and one of them belongs to a novel species within a new genus WAQC01, which was recently discovered from a submarine volcano near New Zealand. Analysis of their genomes revealed that they are highly versatile metabolically and may be able to perform both chemoorganotrophy and chemolithotrophy. All the MAGs also have the potential to fix carbon through reverse oxidative TCA cycle and one displays the potential to fix carbon through 3-hydroxypropionate/4-hydroxybutyrate cycle. Putative prophage regions were also identified in the MAGs and they encode several auxiliary metabolic genes (AMGs) that may confer them fitness advantage in various extreme habitats. Investigation of the diversity, distribution, and abundance of Aigarchaeota showed that although they are widespread across various environments, they are most abundant in geothermal habitats.
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