Abstract
Elucidating the diversity of the Archaea has many important ecological and evolutionary implications. The Asgard superphylum of the archaea, described recently from metagenomic data, has reignited the decades-old debate surrounding the topology of the tree of life. This review synthesizes recent findings through publicly available genomes and literature to describe the current ecological and evolutionary significance of the Asgard superphylum. Asgard archaea have been found in a diverse range of microbiomes across the globe, primarily from sedimentary environments. Within these environments, positive correlations between specific members of the Asgard archaea and Candidate Division TA06 bacteria have been observed, opening up the possibility of symbiotic interactions between the groupings. Asgard archaeal genomes encode functionally diverse metabolic pathways, including the Wood-Ljungdahl pathway as a carbon-fixation strategy, putative nucleotide salvaging pathways, and novel mechanisms of phototrophy including new rhodopsins. Asgard archaea also appear to be active in nitrogen cycling. Asgard archaea encode genes involved in both dissimilatory nitrate reduction and denitrification, and for the potential to use atmospheric nitrogen or nitrite as nitrogen sources. Asgard archaea also may be involved in the transformation of sulfur compounds, indicating a putative role in sulfur cycling. To date, all Asgard archaeal genomes identified were described as obligately anaerobic. The Asgard archaea also appear to have important evolutionary implications. The presence of eukaryotic signature proteins and the affiliation of Asgard archaea in phylogenetic analyses appears to support two-domain topologies of the tree of life with eukaryotes emerging from within the domain of archaea, as opposed to the eukaryotes being a separate domain of life. Thus far, Heimdallarchaeota appears as the closest archaeal relative of eukaryotes.
Highlights
The Archaea constitute a significant portion of the Earth’s microbial diversity, being dominant members of marine and soil environments [1,2,3]
Asgard archaea were formerly grouped as Marine Benthic Group B (MBG-B) [21], Ancient Archaeal Group (AAG) [22], Deep-Sea Archaeal Group (DSAG) [23,24], and Marine Hydrothermal Vent Group (MHVG) [22] archaea
As most Asgard archaeal genomes and 16S rDNA to date were extracted from anoxic environments [8,9,11,12,27,30], and most Asgard archaea lack a complete tricarboxylic acid cycle (TCA) cycle and are capable of anaerobic metabolisms such as WL-pathway, they were thought to be obligately anaerobic
Summary
The Archaea constitute a significant portion of the Earth’s microbial diversity, being dominant members of marine and soil environments [1,2,3]. With the emergence of cultivation-independent techniques, our understanding of the diversity of the Archaea is increasing, as highlighted by recent reviews [4,5]. Members of the Asgard archaea have been described from a diverse range of habitats, including hydrothermal sediments [6,7,8], microbial mats [9], as well as a range of freshwater [10] and marine [8,11,12,13] environments. Archaea of the TACK superphylum were found to contain proteins related to those in eukaryotes and to affiliate with eukaryotes in phylogenetic analysis, as discussed in recent reviews [20]. This review will discuss how cultivation-independent sequencing data has informed our understanding of the ecology, lifestyle, and evolutionary implications of the Asgard superphylum
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