Ecological features and global distribution of Asgard archaea

Potential for mercury methylation by Asgard archaea in mangrove sediments

Methylmercury (MeHg) is a potent neurotoxin that bioaccumulates along food chains. The conversion of MeHg from mercury (Hg) is mediated by a variety of anaerobic microorganisms carrying hgcAB genes. Mangrove sediments are potential hotspots of microbial Hg methylation; however, the microorganisms responsible for Hg methylation are poorly understood. Here, we conducted metagenomic and metatranscriptomic analyses to investigate the diversity and distribution of putative microbial Hg-methylators in mangrove ecosystems. The highest hgcA abundance and expression occurred in surface sediments in Shenzhen, where the highest MeHg concentration was also observed. We reconstructed 157 metagenome-assembled genomes (MAGs) carrying hgcA and identified several putative novel Hg-methylators, including one Asgard archaea (Lokiarchaeota). Further analysis of MAGs revealed that Deltaproteobacteria, Euryarchaeota, Bacteroidetes, Chloroflexi, and Lokiarchaeota were the most abundant and active Hg-methylating groups, implying their crucial role in MeHg production. By screening publicly available MAGs, 104 additional Asgard MAGs carrying hgcA genes were identified from a wide range of coast, marine, permafrost, and lake sediments. Protein homology modelling predicts that Lokiarchaeota HgcAB proteins contained the highly conserved amino acid sequences and folding structures required for Hg methylation. Phylogenetic tree revealed that hgcA genes from Asgard clustered with fused hgcAB genes, indicating a transitional stage of Asgard hgcA genes. Our findings thus suggest that Asgard archaea are potential novel Hg-methylating microorganisms and play an important role in hgcA evolution.

Actin cytoskeleton and complex cell architecture in an Asgard archaeon

Asgard archaea are considered to be the closest known relatives of eukaryotes. Their genomes contain hundreds of eukaryotic signature proteins (ESPs), which inspired hypotheses on the evolution of the eukaryotic cell^1-3. A role of ESPs in the formation of an elaborate cytoskeleton and complex cellular structures has been postulated^4-6, but never visualized. Here we describe a highly enriched culture of 'Candidatus Lokiarchaeum ossiferum', a member of the Asgard phylum, which thrives anaerobically at 20 °C on organic carbon sources. It divides every 7-14 days, reaches cell densities of up to 5 × 10⁷ cells per ml and has a significantly larger genome compared with the single previously cultivated Asgard strain⁷. ESPs represent 5% of its protein-coding genes, including four actin homologues. We imaged the enrichment culture using cryo-electron tomography, identifying 'Ca. L. ossiferum' cells on the basis of characteristic expansion segments of their ribosomes. Cells exhibited coccoid cell bodies and a network of branched protrusions with frequent constrictions. The cell envelope consists of a single membrane and complex surface structures. A long-range cytoskeleton extends throughout the cell bodies, protrusions and constrictions. The twisted double-stranded architecture of the filaments is consistent with F-actin. Immunostaining indicates that the filaments comprise Lokiactin-one of the most highly conserved ESPs in Asgard archaea. We propose that a complex actin-based cytoskeleton predated the emergence of the first eukaryotes and was a crucial feature in the evolution of the Asgard phylum by scaffolding elaborate cellular structures.

Unearthing terrestrial extreme microbiomes for searching terrestrial-like life in the Solar System

The possibility of life elsewhere in the universe has fascinated humankind for ages. To the best of our knowledge, life, as we know it, is limited to planet Earth; yet current investigation suggests that life might be more common than previously thought. In this review, we explore extreme terrestrial analogue environments in the search for some notable examples of extreme organisms, including overlooked microbial groups such as viruses, fungi, and protists, associated with limits of life on Earth. This knowledge is integral to provide the foundational principles needed to predict what sort of Earth-like organisms we might find in the Solar System and beyond, and to understand the future and origins of life on Earth.

Asgard archaea in saline environments

Members of candidate Asgardarchaeota superphylum appear to share numerous eukaryotic-like attributes thus being broadly explored for their relevance to eukaryogenesis. On the contrast, the ecological roles of Asgard archaea remains understudied. Asgard archaea have been frequently associated to low-oxygen aquatic sedimentary environments worldwide spanning a broad but not extreme salinity range. To date, the available information on diversity and potential biogeochemical roles of Asgardarchaeota mostly sourced from marine habitats and to a much lesser extend from true saline environments (i.e., > 3% w/v total salinity). Here, we provide an overview on diversity and ecological implications of Asgard archaea distributed across saline environments and briefly explore their metagenome-resolved potential for osmoadaptation. Loki-, Thor- and Heimdallarchaeota are the dominant Asgard clades in saline habitats where they might employ anaerobic/microaerophilic organic matter degradation and autotrophic carbon fixation. Homologs of primary solute uptake ABC transporters seemingly prevail in Thorarchaeota, whereas those putatively involved in trehalose and ectoine biosynthesis were mostly inferred in Lokiarchaeota. We speculate that Asgardarchaeota might adopt compatible solute-accumulating ('salt-out') strategy as response to salt stress. Our current understanding on the distribution, ecology and salt-adaptive strategies of Asgardarchaeota in saline environments are, however, limited by insufficient sampling and incompleteness of the available metagenome-assembled genomes. Extensive sampling combined with 'omics'- and cultivation-based approaches seem, therefore, crucial to gain deeper knowledge on this particularly intriguing archaeal lineage.

Genomic and transcriptomic dissection of Theionarchaea in marine ecosystem

Theionarchaea is a recently described archaeal class within the Euryarchaeota. While it is widely distributed in sediment ecosystems, little is known about its metabolic potential and ecological features. Here, we used metagenomics and metatranscriptomics to characterize 12 theionarchaeal metagenome-assembled genomes, which were further divided into two subgroups, from coastal mangrove sediments of China and seawater columns of the Yap Trench. Genomic analysis revealed that apart from the canonical sulfhydrogenase, Theionarchaea harbor genes encoding heliorhodopsin, group 4 [NiFe]-hydrogenase, and flagellin, in which genes for heliorhodopsin and group 4 [NiFe]-hydrogenase were transcribed in mangrove sediment. Further, the theionarchaeal substrate spectrum may be broader than previously reported as revealed by metagenomics and metatranscriptomics, and the potential carbon substrates include detrital proteins, hemicellulose, ethanol, and CO2. The genes for organic substrate metabolism (mainly detrital protein and amino acid metabolism genes) have relatively higher transcripts in the top sediment layers in mangrove wetlands. In addition, co-occurrence analysis suggested that the degradation of these organic compounds by Theionarchaea might be processed in syntrophy with fermenters (e.g., Chloroflexi) and methanogens. Collectively, these observations expand the current knowledge of the metabolic potential of Theionarchaea, and shed light on the metabolic strategies and roles of these archaea in the marine ecosystems.

Genomic and metabolic adaptations of biofilms to ecological windows of opportunity in glacier-fed streams

In glacier-fed streams, ecological windows of opportunity allow complex microbial biofilms to develop and transiently form the basis of the food web, thereby controlling key ecosystem processes. Using metagenome-assembled genomes, we unravel strategies that allow biofilms to seize this opportunity in an ecosystem otherwise characterized by harsh environmental conditions. We observe a diverse microbiome spanning the entire tree of life including a rich virome. Various co-existing energy acquisition pathways point to diverse niches and the exploitation of available resources, likely fostering the establishment of complex biofilms during windows of opportunity. The wide occurrence of rhodopsins, besides chlorophyll, highlights the role of solar energy capture in these biofilms while internal carbon and nutrient cycling between photoautotrophs and heterotrophs may help overcome constraints imposed by oligotrophy in these habitats. Mechanisms potentially protecting bacteria against low temperatures and high UV-radiation are also revealed and the selective pressure of this environment is further highlighted by a phylogenomic analysis differentiating important components of the glacier-fed stream microbiome from other ecosystems. Our findings reveal key genomic underpinnings of adaptive traits contributing to the success of complex biofilms to exploit environmental opportunities in glacier-fed streams, which are now rapidly changing owing to global warming.

The expanding Asgard archaea and their elusive relationships with Eukarya

The discovery of Asgard archaea and the exploration of their diversity over the last 6 years have deeply impacted the scientific community working on eukaryogenesis, rejuvenating an intense debate on the topology of the universal tree of life (uTol). Here, we discuss how this debate is impacted by two recent publications that expand the number of Asgard lineages and eukaryotic signature proteins (ESPs). We discuss some of the main difficulties that can impair the phylogenetic reconstructions of the uTol and suggest that the debate about its topology is not settled. We notably hypothesize the existence of horizontal gene transfers between ancestral Asgards and proto-eukaryotes that could result in the observed abnormal behaviors of some Asgard ESPs and universal marker proteins. This hypothesis is relevant regardless of the scenario considered regarding eukaryogenesis. It implies that the Asgards were already diversified before the last eukaryotic common ancestor and shared the same biotopes with proto-eukaryotes. We suggest that some Asgards might be still living in symbiosis today with modern Eukarya.

Newly discovered Asgard archaea Hermodarchaeota potentially degrade alkanes and aromatics via alkyl/benzyl-succinate synthase and benzoyl-CoA pathway

Asgard archaea are widely distributed in anaerobic environments. Previous studies revealed the potential capability of Asgard archaea to utilize various organic substrates including proteins, carbohydrates, fatty acids, amino acids and hydrocarbons, suggesting that Asgard archaea play an important role in sediment carbon cycling. Here, we describe a previously unrecognized archaeal phylum, Hermodarchaeota, affiliated with the Asgard superphylum. The genomes of these archaea were recovered from metagenomes generated from mangrove sediments, and were found to encode alkyl/benzyl-succinate synthases and their activating enzymes that are similar to those identified in alkane-degrading sulfate-reducing bacteria. Hermodarchaeota also encode enzymes potentially involved in alkyl-coenzyme A and benzoyl-coenzyme A oxidation, the Wood–Ljungdahl pathway and nitrate reduction. These results indicate that members of this phylum have the potential to strictly anaerobically degrade alkanes and aromatic compounds, coupling the reduction of nitrate. By screening Sequence Read Archive, additional genes encoding 16S rRNA and alkyl/benzyl-succinate synthases analogous to those in Hermodarchaeota were identified in metagenomic datasets from a wide range of marine and freshwater sediments. These findings suggest that Asgard archaea capable of degrading alkanes and aromatics via formation of alkyl/benzyl-substituted succinates are ubiquitous in sediments.