Archaeal Tree Research Articles

Carbon fixation pathways across the bacterial and archaeal tree of life.

Carbon fixation is a critical process for our planet; however, its distribution across the bacterial and archaeal domains of life has not been comprehensively studied. Here, we performed an analysis of 52,515 metagenome-assembled genomes and discover carbon fixation pathways in 1,007 bacteria and archaea. We reveal the genomic potential for carbon fixation through the reverse tricarboxylic acid cycle in previously unrecognized archaeal and bacterial phyla (i.e. Thermoplasmatota and Elusimicrobiota) and show that the 3-hydroxypropionate bi-cycle is not, as previously thought, restricted to the phylum Chloroflexota. The data also substantially expand the phylogenetic breadth for autotrophy through the dicarboxylate/4-hydroxybutyrate cycle and the Calvin-Benson-Bassham cycle. Finally, the genomic potential for carbon fixation through the 3-hydroxypropionate/4-hydroxybutyrate cycle, previously exclusively found in Archaea, was also detected in the Bacteria. Carbon fixation thus appears to be much more widespread than previously known, and this study lays the foundation to better understand the role of archaea and bacteria in global primary production and how they contribute to microbial carbon sinks.

Diversity and Evolution of Methane-Related Pathways in Archaea.

Methane is one of the most important greenhouse gases on Earth and holds an important place in the global carbon cycle. Archaea are the only organisms that use methanogenesis to produce energy and rely on the methyl-coenzyme M reductase complex (Mcr). Over the last decade, new results have significantly reshaped our view of the diversity of methane-related pathways in the Archaea. Many new lineages that synthesize or use methane have been identified across the whole archaeal tree, leading to a greatly expanded diversity of substrates and mechanisms. In this review, we present the state of the art of these advances and how they challenge established scenarios of the origin and evolution of methanogenesis, and we discuss the potential trajectories that may have led to this strikingly wide range of metabolisms.

Unexpectedly high mutation rate of a deep-sea hyperthermophilic anaerobic archaeon.

Deep-sea hydrothermal vents resemble the early Earth, and thus the dominant Thermococcaceae inhabitants, which occupy an evolutionarily basal position of the archaeal tree and take an obligate anaerobic hyperthermophilic free-living lifestyle, are likely excellent models to study the evolution of early life. Here, we determined that unbiased mutation rate of a representative species, Thermococcus eurythermalis, exceeded that of all known free-living prokaryotes by 1-2 orders of magnitude, and thus rejected the long-standing hypothesis that low mutation rates were selectively favored in hyperthermophiles. We further sequenced multiple and diverse isolates of this species and calculated that T. eurythermalis has a lower effective population size than other free-living prokaryotes by 1-2 orders of magnitude. These data collectively indicate that the high mutation rate of this species is not selectively favored but instead driven by random genetic drift. The availability of these unusual data also helps explore mechanisms underlying microbial genome size evolution. We showed that genome size is negatively correlated with mutation rate and positively correlated with effective population size across 30 bacterial and archaeal lineages, suggesting that increased mutation rate and random genetic drift are likely two important mechanisms driving microbial genome reduction. Future determinations of the unbiased mutation rate of more representative lineages with highly reduced genomes such as Prochlorococcus and Pelagibacterales that dominate marine microbial communities are essential to test these hypotheses.

The growing tree of Archaea: new perspectives on their diversity, evolution and ecology.

The Archaea occupy a key position in the Tree of Life, and are a major fraction of microbial diversity. Abundant in soils, ocean sediments and the water column, they have crucial roles in processes mediating global carbon and nutrient fluxes. Moreover, they represent an important component of the human microbiome, where their role in health and disease is still unclear. The development of culture-independent sequencing techniques has provided unprecedented access to genomic data from a large number of so far inaccessible archaeal lineages. This is revolutionizing our view of the diversity and metabolic potential of the Archaea in a wide variety of environments, an important step toward understanding their ecological role. The archaeal tree is being rapidly filled up with new branches constituting phyla, classes and orders, generating novel challenges for high-rank systematics, and providing key information for dissecting the origin of this domain, the evolutionary trajectories that have shaped its current diversity, and its relationships with Bacteria and Eukarya. The present picture is that of a huge diversity of the Archaea, which we are only starting to explore.

Methanogenesis and the Wood–Ljungdahl Pathway: An Ancient, Versatile, and Fragile Association

Methanogenesis coupled to the Wood–Ljungdahl pathway is one of the most ancient metabolisms for energy generation and carbon fixation in the Archaea. Recent results are sensibly changing our view on the diversity of methane-cycling capabilities in this Domain of Life. The availability of genomic sequences from uncharted branches of the archaeal tree has highlighted the existence of novel methanogenic lineages phylogenetically distant to previously known ones, such as the Methanomassiliicoccales. At the same time, phylogenomic analyses have suggested a methanogenic ancestor for all Archaea, implying multiple independent losses of this metabolism during archaeal diversification. This prediction has been strengthened by the report of genes involved in methane cycling in members of the Bathyarchaeota (a lineage belonging to the TACK clade), representing the first indication of the presence of methanogenesis outside of the Euryarchaeota. In light of these new data, we discuss how the association between methanogenesis and the Wood–Ljungdahl pathway appears to be much more flexible than previously thought, and might provide information on the processes that led to loss of this metabolism in many archaeal lineages. The combination of environmental microbiology, experimental characterization and phylogenomics opens up exciting avenues of research to unravel the diversity and evolutionary history of fundamental metabolic pathways.

Extending the conserved phylogenetic core of archaea disentangles the evolution of the third domain of life.

Initial studies of the archaeal phylogeny relied mainly on the analysis of the RNA component of the small subunit of the ribosome (SSU rRNA). The resulting phylogenies have provided interesting but partial information on the evolutionary history of the third domain of life because SSU rRNA sequences do not contain enough phylogenetic signal to resolve all nodes of the archaeal tree. Thus, many relationships, and especially the most ancient ones, remained elusive. Moreover, SSU rRNA phylogenies can be heavily biased by tree reconstruction artifacts. The sequencing of complete genomes allows using a variety of protein markers as an alternative to SSU rRNA. Taking advantage of the recent burst of archaeal complete genome sequences, we have carried out an in-depth phylogenomic analysis of this domain. We have identified 200 new protein families that, in addition to the ribosomal proteins and the subunits of the RNA polymerase, form a conserved phylogenetic core of archaeal genes. The accurate analysis of these markers combined with desaturation approaches shed new light on the evolutionary history of Archaea and reveals that several relationships recovered in recent analyses are likely the consequence of tree reconstruction artifacts. Among others, we resolve a number of important relationships, such as those among methanogens Class I, and we propose the definition of two new superclasses within the Euryarchaeota: Methanomada and Diaforarchaea.

Rooting the domain archaea by phylogenomic analysis supports the foundation of the new kingdom Proteoarchaeota.

The first 16S rRNA-based phylogenies of the Archaea showed a deep division between two groups, the kingdoms Euryarchaeota and Crenarchaeota. This bipartite classification has been challenged by the recent discovery of new deeply branching lineages (e.g., Thaumarchaeota, Aigarchaeota, Nanoarchaeota, Korarchaeota, Parvarchaeota, Aenigmarchaeota, Diapherotrites, and Nanohaloarchaeota) which have also been given the same taxonomic status of kingdoms. However, the phylogenetic position of some of these lineages is controversial. In addition, phylogenetic analyses of the Archaea have often been carried out without outgroup sequences, making it difficult to determine if these taxa actually define lineages at the same level as the Euryarchaeota and Crenarchaeota. We have addressed the question of the position of the root of the Archaea by reconstructing rooted archaeal phylogenetic trees using bacterial sequences as outgroup. These trees were based on commonly used conserved protein markers (32 ribosomal proteins) as well as on 38 new markers identified through phylogenomic analysis. We thus gathered a total of 70 conserved markers that we analyzed as a concatenated data set. In contrast with previous analyses, our trees consistently placed the root of the archaeal tree between the Euryarchaeota (including the Nanoarchaeota and other fast-evolving lineages) and the rest of archaeal species, which we propose to class within the new kingdom Proteoarchaeota. This implies the relegation of several groups previously classified as kingdoms (e.g., Crenarchaeota, Thaumarchaeota, Aigarchaeota, and Korarchaeota) to a lower taxonomic rank. In addition to taxonomic implications, this profound reorganization of the archaeal phylogeny has also consequences on our appraisal of the nature of the last archaeal ancestor, which most likely was a complex organism with a gene-rich genome.

An integrated analysis of the genome of the hyperthermophilic archaeon Pyrococcus abyssi.

The hyperthermophilic euryarchaeon Pyrococcus abyssi and the related species Pyrococcus furiosus and Pyrococcus horikoshii, whose genomes have been completely sequenced, are presently used as model organisms in different laboratories to study archaeal DNA replication and gene expression and to develop genetic tools for hyperthermophiles. We have performed an extensive re-annotation of the genome of P. abyssi to obtain an integrated view of its phylogeny, molecular biology and physiology. Many new functions are predicted for both informational and operational proteins. Moreover, several candidate genes have been identified that might encode missing links in key metabolic pathways, some of which have unique biochemical features. The great majority of Pyrococcus proteins are typical archaeal proteins and their phylogenetic pattern agrees with its position near the root of the archaeal tree. However, proteins probably from bacterial origin, including some from mesophilic bacteria, are also present in the P. abyssi genome.

Stripped Down to the Bare Essentials

EVOLUTION Coupling the belief that early forms of life arose under inhospitable conditions (high temperatures and low oxygen) to the reasonable assumption that simpler organisms are more ancient than complex ones would place Methanopyrus kandleri near the base not only of the “black smoker” chimneys in the deep ocean but also of the archaeal tree. Nevertheless, having sequenced the genome of this organism, which is capable of growing on H2 and CO2 at temperatures above 100°C, Slesarev et al. conclude just the opposite. A substantial fraction of the M. kandleri proteins can also be found in two other archaeal methanogens, and this trio consistently forms a cluster in trees of archaeal species when analyzed on the basis of genes and operons. Unlike its relations, however, M. kandleri shares only a small percentage of its proteome with bacteria, which the authors suggest may reflect having had fewer opportunities for lateral transfer in its extreme environment, and its present impoverished state (in terms of gene number and signal transduction pathways) appears to be a consequence of life as a minimalist.— GJC Proc. Natl. Acad. Sci. U.S.A. 99 , 4644 (2002).

The complete genome of hyperthermophile Methanopyrus kandleri AV19 and monophyly of archaeal methanogens.

We have determined the complete 1,694,969-nt sequence of the GC-rich genome of Methanopyrus kandleri by using a whole direct genome sequencing approach. This approach is based on unlinking of genomic DNA with the ThermoFidelase version of M. kandleri topoisomerase V and cycle sequencing directed by 2'-modified oligonucleotides (Fimers). Sequencing redundancy (3.3x) was sufficient to assemble the genome with less than one error per 40 kb. Using a combination of sequence database searches and coding potential prediction, 1,692 protein-coding genes and 39 genes for structural RNAs were identified. M. kandleri proteins show an unusually high content of negatively charged amino acids, which might be an adaptation to the high intracellular salinity. Previous phylogenetic analysis of 16S RNA suggested that M. kandleri belonged to a very deep branch, close to the root of the archaeal tree. However, genome comparisons indicate that, in both trees constructed using concatenated alignments of ribosomal proteins and trees based on gene content, M. kandleri consistently groups with other archaeal methanogens. M. kandleri shares the set of genes implicated in methanogenesis and, in part, its operon organization with Methanococcus jannaschii and Methanothermobacter thermoautotrophicum. These findings indicate that archaeal methanogens are monophyletic. A distinctive feature of M. kandleri is the paucity of proteins involved in signaling and regulation of gene expression. Also, M. kandleri appears to have fewer genes acquired via lateral transfer than other archaea. These features might reflect the extreme habitat of this organism.

Early evolutionary relationships among known life forms inferred from elongation factor EF-2/EF-G sequences: phylogenetic coherence and structure of the archaeal domain.

Phylogenies were inferred from both the gene and the protein sequences of the translational elongation factor termed EF-2 (for Archaea and Eukarya) and EF-G (for Bacteria). All treeing methods used (distance-matrix, maximum likelihood, and parsimony), including evolutionary parsimony, support the archaeal tree and disprove the "eocyte tree" (i.e., the polyphyly and paraphyly of the Archaea). Distance-matrix trees derived from both the amino acid and the DNA sequence alignments (first and second codon positions) showed the Archaea to be a monophyletic-holophyletic grouping whose deepest bifurcation divides a Sulfolobus branch from a branch comprising Methanococcus, Halobacterium, and Thermoplasma. Bootstrapped distance-matrix treeing confirmed the monophyly-holophyly of Archaea in 100% of the samples and supported the bifurcation of Archaea into a Sulfolobus branch and a methanogen-halophile branch in 97% of the samples. Similar phylogenies were inferred by maximum likelihood and by maximum (protein and DNA) parsimony. DNA parsimony trees essentially identical to those inferred from first and second codon positions were derived from alternative DNA data sets comprising either the first or the second position of each codon. Bootstrapped DNA parsimony supported the monophyly-holophyly of Archaea in 100% of the bootstrap samples and confirmed the division of Archaea into a Sulfolobus branch and a methanogen-halophile branch in 93% of the bootstrap samples. Distance-matrix and maximum likelihood treeing under the constraint that branch lengths must be consistent with a molecular clock placed the root of the universal tree between the Bacteria and the bifurcation of Archaea and Eukarya. The results support the division of Archaea into the kingdoms Crenarchaeota (corresponding to the Sulfolobus branch and Euryarchaeota). This division was not confirmed by evolutionary parsimony, which identified Halobacterium rather than Sulfolobus as the deepest offspring within the Archaea.

Archaeal Phylogeny: Reexamination of the Phylogenetic Position of Archaeoglohus fulgidus in Light of Certain Composition-induced Artifacts

A major and too little recognized source of artifact in phylogenetic analysis of molecular sequence data is compositional difference among sequences. The problem becomes particularly acute when alignments contain ribosomal RNAs from both mesophilic and thermophilic species. Among prokaryotes the latter are considerably higher in G + C content than the former, which often results in artificial clustering of thermophilic lineages and their being placed artificially deep in phylogenetic trees. In this communication we review archaeal phylogeny in the light of this consideration, focusing in particular on the phylogenetic position of the sulfate reducing species Archaeoglobus fulgidus, using both 16S rRNA and 23S rRNA sequences. The analysis shows clearly that the previously reported deep branching of the A. fulgidus lineage (very near the base of the euryarchaeal side of the archaeal tree) is incorrect, and that the lineage actually groups with a previously recognized unit that comprises the Methanomicrobiales and extreme halophiles.

Methanopyrus kandleri: An Archaeal Methanogen Unrelated to all Other Known Methanogens

Analysis of its 16S rRNA sequence shows that the newly discovered hyperthermophilic methanogen, Methanopryus kandleri, is phylogenetically unrelated to any other known methanogen. The organism represents a separate lineage originating near the root of the archaeal tree. Although the 16S rRNA sequence of Mp. kandleri resembles euryarchaeal 16S rRNAs more than it does crenarchaeal, it shows more crenarchaeal signature features than any known euryarchaeal rRNA. Attempts to place it in relation to the root of the archaeal tree show that the Mp. kandleri lineage likely arises from the euryarchaeal branch of the tree. While the existence of so deeply branching a methanogenic lineage brings into question the thesis that methanogenesis evolved from an earlier metabolism similar to that seen in Thermococcus, it at the same time reinforces the notion that the aboriginal [correction of aborginal] archaeon was a thermophile.