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Chemosynthetic ecosystems powered by microbial primary production are rare 'hot spots' of biological activity in the deep‐sea characterized by dense aggregations of specially adapted animal species. Among settings where such systems have been found, serpentinite‐hosted seep systems supported by alkaline geofluid are particularly understudied with just a few known sites worldwide. Mariana Forearc hosts the world's only series of active serpentinite mud volcanoes, but seep communities have only been reported from South Chamorro Seamount where large bathymodioline mussels dominate. Here, we report the discovery of a serpentinite‐hosted seep on the conical summit of Asùt Tesoru Seamount, Mariana Forearc. Named the 'Big Blue Seep', this field features white, likely carbonate crusts inhabited by animals, under which fluid seepage could be seen. We confirm 16 animal species, including typical seep‐associated fauna such as Desbruyeresia gastropods and Acharax awning‐clams. This is surprising as previous research expeditions did not notice any sign of chemosynthesis‐based ecosystems on this seamount, although the community is indeed difficult to spot due to the lack of large‐bodied epifauna such as mussels. The Big Blue Seep is adjacent to three drill holes made by the International Ocean Discovery Program expedition 366 (Holes U1496A‐C), which may have impact on seepage. Our findings represent the second chemosynthesis‐based ecosystem associated with serpentinite mud volcanism, suggestive of further such communities on other Mariana Forearc mud volcanoes. [ABSTRACT FROM AUTHOR]

Lakes of molten sulfur are features sometimes found in seafloor hydrothermal vent systems. Daikoku of the northern Mariana Arc is notable for being home to one of such features inside its summit caldera, the "Sulfur Cauldron" discovered in 2006. A number of oceanographic research cruises since then have revealed significant volcanic activities on Daikoku Seamount, including an eruption event in 2014 leading to the formation of a new basin‐like crater. How this event impacted the sulfur lake on Daikoku Seamount remained unclear. Here, we revisited Daikoku Seamount with a remotely operated vehicle to show that the new crater is currently home to a much larger molten sulfur lake than the Sulfur Cauldron, which we name the "Rengoku" sulfur lake. Our samples provided new insights on the structure of submarine sulfur lakes, and contribute to the time‐series observation of volcanic and hydrothermal activities on Daikoku Seamount. [ABSTRACT FROM AUTHOR]

Prebiotic chemical evolution and the emergence of life in the seafloor hydrothermal systems of Hadean Earth is among the most plausible and popular hypotheses for the origin of earthly life. In contrast, many studies pointed out that this hypothesis intrinsically harbors a critical unsolved problem called the "water paradox": Abundant water limits dehydration synthesis and instead facilitates hydrolysis of organic molecules during the early stage of chemical evolution. However, many of these criticisms have not referred to the abundant liquid/supercritical CO2 (L/SC-CO2) fluids and pools in modern hydrothermal systems, which not only create dry environments but also behave as hydrophobic solvents at and beneath the seafloor. In this paper, we theorize the generation and preservation of a L/SC-CO2 pool in modern seafloor hydrothermal systems and reinterpret the fossil hydrothermal systems preserved in early Archean seafloor basalts. The theoretical estimation of subseafloor phase separation and phase segregation of CO2-rich hydrothermal fluids suggests the presence of L/SC-CO2 fluids and pools in Hadean seafloor hydrothermal systems. Because they behave as hydrophobic organic solvents, L/SC-CO2 can potentially initiate the dehydration synthesis of organic molecules in seafloor hydrothermal systems. Moreover, at the interface between L/SC-CO2 and H2O-rich fluid (seawater or hydrothermal fluid), amphiphilic molecules might be generated and triggered into self-assembled growth. Based on the abundant occurrence and physicochemical properties of L/SC-CO2 fluids, we propose a new stepwise concept for the origin of life, whereby prebiotic chemical evolution was co-hosted and facilitated by L/SC-CO2 in Hadean water-rich seafloor hydrothermal systems. This "liquid/supercritical CO2 hypothesis" potentially overcomes the water paradox and strengthens the idea that earthly life was hatched in deep-sea hydrothermal systems. [ABSTRACT FROM AUTHOR]

A novel mesophilic bacterium, strain SS33 T , was isolated from a deep-sea hydrothermal vent chimney at Suiyo Seamount, Izu-Bonin Arc, Western Pacific Ocean. The cells of strain SS33 T were motile short rods with a single polar flagellum. The growth of strain SS33 T was observed at the temperature range between 33 and 55 °C (optimum growth at 45 °C), at the pH range between 5.0 and 7.1 (optimum growth at pH 6.0) and in the presence of between 2.0 and 4.5% (w/v) NaCl [optimum growth at 3.5% (w/v)]. Strain SS33 T was a facultative anaerobic chemolithoautotroph using molecular hydrogen and elemental sulphur as the sole electron donor. Nitrate, nitrous oxide, sulphate, elemental sulphur and molecular oxygen were capable of serving as the sole electron acceptor. Phylogenetic analysis based on 16S rRNA gene sequences placed strain SS33 T in the genus Hydrogenimonas belonging to the class Epsilonproteobacteria . The closely related species of strain SS33 T were Hydrogenimonas urashimensis SSM-Sur55 T (95.96%), Hydrogenimonas thermophila EP1-55-1% T (95.75%) and Hydrogenimonas cancrithermarum ISO32 T (95.24%). According to the taxonomic and physiological characteristics, it is proposed that strain SS33 T was classified into a novel species of genus Hydrogenimonas , Hydrogenimonas leucolamina sp. nov., with SS33 T (=JCM 39184 T =KCTC 25253 T ) as the type strain. Furthermore, the genome comparison of Epsilonproteobacteria revealed that their [NiFe] hydrogenase genes belonging to Group 1b could be divided into two phylogenetic lineages and suggested that the reverse gyrase gene has been lost after division to the genus Hydrogenimonas .

Members of the kingdom Nanobdellati , previously known as DPANN archaea, are characterized by ultrasmall cell sizes and reduced genomes. They primarily thrive through ectosymbiotic interactions with specific hosts in diverse environments. Recent successful cultivations have emphasized the importance of adhesion to host cells for understanding the ecophysiology of Nanobdellati . Cell adhesion is often mediated by cell surface carbohydrates, and in archaea, this may be facilitated by the glycosylated S-layer protein that typically coats their cell surface. In this study, we conducted glycoproteomic analyses on two co-cultures of Nanobdellati with their host archaea, as well as on pure cultures of both host and non-host archaea. Nanobdellati exhibited various glycoproteins, including archaellins and hypothetical proteins, with glycans that were structurally distinct from those of their hosts. This indicated that Nanobdellati autonomously synthesize their glycans for protein modifications probably using host-derived substrates, despite the high energy cost. Glycan modifications on Nanobdellati proteins consistently occurred on asparagine residues within the N-X-S/T sequon, consistent with patterns observed across archaea, bacteria, and eukaryotes. In both host and non-host archaea, S-layer proteins were commonly modified with hexose, N -acetylhexosamine, and sulfonated deoxyhexose. However, the N -glycan structures of host archaea, characterized by distinct sugars such as deoxyhexose, nonulosonate sugar, and pentose at the nonreducing ends, were implicated in enabling Nanobdellati to differentiate between host and non-host cells. Interestingly, the specific sugar, xylose, was eliminated from the N -glycan in a host archaeon when co-cultured with Nanobdella . These findings enhance our understanding of the role of protein glycosylation in archaeal interactions.IMPORTANCE Nanobdellati archaea, formerly known as DPANN, are phylogenetically diverse, widely distributed, and obligately ectosymbiotic. The molecular mechanisms by which Nanobdellati recognize and adhere to their specific hosts remain largely unexplored. Protein glycosylation, a fundamental biological mechanism observed across all domains of life, is often crucial for various cell-cell interactions. This study provides the first insights into the glycoproteome of Nanobdellati and their host and non-host archaea. We discovered that Nanobdellati autonomously synthesize glycans for protein modifications, probably utilizing substrates derived from their hosts. Additionally, we identified distinctive glycosylation patterns that suggest mechanisms through which Nanobdellati differentiate between host and non-host cells. This research significantly advances our understanding of the molecular basis of microbial interactions in extreme environments., Competing Interests: The authors declare no conflict of interest.

An anaerobic, mesophilic, syntrophic, archaeon strain MK-D1 T , was isolated as a pure co-culture with Methanogenium sp. strain MK-MG from deep-sea methane seep sediment. This organism is, to our knowledge, the first cultured representative of 'Asgard' archaea, an archaeal group closely related to eukaryotes. Here, we describe the detailed physiology and phylogeny of MK-D1 T and propose Promethearchaeum syntrophicum gen. nov., sp. nov. to accommodate this strain. Cells were non-motile, small cocci, approximately 300-750 nm in diameter and produced membrane vesicles, chains of blebs and membrane-based protrusions. MK-D1 T grew at 4-30 °C with optimum growth at 20 °C. The strain grew chemoorganotrophically with amino acids, peptides and yeast extract with obligate dependence on syntrophy with H 2 -/formate-utilizing organisms. MK-D1 T showed the fastest growth and highest maximum cell yield when grown with yeast extract as the substrate: approximately 3 months to full growth, reaching up to 6.7×10 6 16S rRNA gene copies ml -1 . MK-D1 T had a circular 4.32 Mb chromosome with a DNA G+C content of 31.1 mol%. The results of phylogenetic analyses of the 16S rRNA gene and conserved marker proteins indicated that the strain is affiliated with 'Asgard' archaea and more specifically DHVC1/DSAG/MBG-B and 'Lokiarchaeota'/'Lokiarchaeia'. On the basis of the results of 16S rRNA gene sequence analysis, the most closely related isolated relatives were Infirmifilum lucidum 3507LT T (76.09 %) and Methanothermobacter tenebrarum RMAS T (77.45 %) and the closest relative in enrichment culture was Candidatus 'Lokiarchaeum ossiferum' (95.39 %). The type strain of the type species is MK-D1 T (JCM 39240 T and JAMSTEC no. 115508). We propose the associated family, order, class, phylum, and kingdom as Promethearchaeaceae fam. nov., Promethearchaeales ord. nov., Promethearchaeia class. nov., Promethearchaeota phyl. nov., and Promethearchaeati regn. nov., respectively. These are in accordance with ICNP Rules 8 and 22 for nomenclature, Rule 30(3)(b) for validation and maintenance of the type strain, and Rule 31a for description as a member of an unambiguous syntrophic association.

Post-mega-earthquake geochemical and microbiological properties in subseafloor sediments of the Japan Trench accretionary wedge were investigated using core samples from Hole C0019E, which was drilled down to 851‍ ‍m below seafloor (mbsf) at a water depth of 6,890 m. Methane was abundant throughout accretionary prism sediments; however, its concentration decreased close to the plate boundary decollement. Methane isotope systematics indicated a biogenic origin. The content of mole-cular hydrogen (H 2 ) was low throughout core samples, but markedly increased at specific depths that were close to potential faults predicted by logging-while-drilling ana-lyses. Based on isotopic systematics, H 2 appeared to have been abundantly produced via a low-temperature interaction between pore water and the fresh surface of crushed rock induced by earthquakes. Subseafloor microbial cell density remained constant at approximately 10 5 ‍ ‍cells‍ ‍mL -1 . Amplicon sequences revealed that predominant members at the phylum level were common throughout the units tested, which also included members frequently found in anoxic subseafloor sediments. Metabolic potential assays using radioactive isotopes as tracers revealed homoacetogenic activity in H 2 -enriched core samples collected near the fault. Furthermore, homoacetogenic bacteria, including Acetobacterium carbinolicum, were isolated from similar samples. Therefore, post-earthquake subseafloor microbial communities in the Japan Trench accretionary prism appear to be episodically dominated by homoacetogenic populations and potentially function due to the earthquake-induced low-temperature generation of H 2 . These post-earthquake microbial communities may eventually return to the steady-state communities dominated by oligotrophic heterotrophs and hydrogenotrophic and methylotrophic methanogens that are dependent on refractory organic matter in the sediment.