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Abstract Members of the candidate phylum Hyd24-12 are globally distributed, but no genomic information or knowledge about their morphology, physiology or ecology is available. In this study, members of the Hyd24-12 lineage were shown to be present and abundant in full-scale mesophilic anaerobic digesters at Danish wastewater treatment facilities. In some samples, a member of the Hyd24-12 lineage was one of the most abundant genus-level bacterial taxa, accounting for up to 8% of the bacterial biomass. Three closely related and near-complete genomes were retrieved using metagenome sequencing of full-scale anaerobic digesters. Genome annotation and metabolic reconstruction showed that they are Gram-negative bacteria likely involved in acidogenesis, producing acetate and hydrogen from fermentation of sugars, and may play a role in the cycling of sulphur in the digesters. Fluorescence in situ hybridization revealed single rod-shaped cells dispersed within the flocs. The genomic information forms a foundation for a more detailed understanding of their role in anaerobic digestion and provides the first insight into a hitherto undescribed branch in the tree of life.

Abstract New and emerging environmental pathogens pose some of the greatest threats to modern aquaculture, a critical source of food protein globally. As with other intensive farming practices, increasing our understanding of the biology of infections is important to improve animal welfare and husbandry. The gill infection epitheliocystis is increasingly problematic in gilthead seabream (Sparus aurata), a major Mediterranean aquaculture species. Epitheliocystis is generally associated with chlamydial bacteria, yet we were not able to localise chlamydial targets within the major gilthead seabream lesions. Two previously unidentified species within a novel β-proteobacterial genus were instead identified. These co-infecting intracellular bacteria have been characterised using high-resolution imaging and genomics, presenting the most comprehensive study on epitheliocystis agents to date. Draft genomes of the two uncultured species, Ca. Ichthyocystis hellenicum and Ca. Ichthyocystis sparus, have been de novo sequenced and annotated from preserved material. Analysis of the genomes shows a compact core indicating a metabolic dependency on the host, and an accessory genome with an unprecedented number of tandemly arrayed gene families. This study represents a critical insight into novel, emerging fish pathogens and will be used to underpin future investigations into the bacterial origins, and to develop diagnostic and treatment strategies.

Abstract Microorganisms in biological wastewater treatment plants require adaptive strategies to deal with rapidly fluctuating environmental conditions. At the population level, the filamentous bacterium Candidatus Microthrix parvicella (Ca. M. parvicella) has been found to fine-tune its gene expression for optimized substrate assimilation. Here we investigated in situ substrate assimilation by single cells of Ca. M. parvicella using nano-scale secondary-ion mass spectrometry (nanoSIMS). NanoSIMS imaging highlighted phenotypic heterogeneity among Ca. M. parvicella cells of the same filament, whereby 13C-oleic acid and 13C-glycerol-3-phosphate assimilation occurred in ≈21–55% of cells, despite non-assimilating cells being intact and alive. In response to alternating aerobic–anoxic regimes, 13C-oleic acid assimilation occurred among subpopulations of Ca. M. parvicella cells (≈3–28% of cells). Furthermore, Ca. M. parvicella cells exhibited two temperature optima for 13C-oleic acid assimilation and associated growth rates. These results suggest that phenotypic heterogeneity among Ca. M. parvicella cells allows the population to adapt rapidly to fluctuating environmental conditions facilitating its widespread occurrence in biological wastewater treatment plants.

Abstract The candidate archaeal phylum ‘Aigarchaeota’ contains microorganisms from terrestrial and subsurface geothermal ecosystems. The phylogeny and metabolic potential of Aigarchaeota has been deduced from several recent single-cell amplified genomes; however, a detailed description of their metabolic potential and in situ transcriptional activity is absent. Here, we report a comprehensive metatranscriptome-based reconstruction of the in situ metabolism of Aigarchaeota in an oxic, hot spring filamentous ‘streamer’ community. Fluorescence in situ hybridization showed that these newly discovered Aigarchaeota are filamentous, which is consistent with the presence and transcription of an actin-encoding gene. Aigarchaeota filaments are intricately associated with other community members, which include both bacteria (for example, filamentous Thermocrinis spp.) and archaea. Metabolic reconstruction of genomic and metatranscriptomic data suggests that this aigarchaeon is an aerobic, chemoorganoheterotroph with autotrophic potential. A heme copper oxidase complex was identified in the environmental genome assembly and highly transcribed in situ. Potential electron donors include acetate, fatty acids, amino acids, sugars and aromatic compounds, which may originate from extracellular polymeric substances produced by other microorganisms shown to exist in close proximity and/or autochthonous dissolved organic carbon (OC). Transcripts related to genes specific to each of these potential electron donors were identified, indicating that this aigarchaeon likely utilizes several OC substrates. Characterized members of this lineage cannot synthesize heme, and other cofactors and vitamins de novo, which suggests auxotrophy. We propose the name Candidatus ‘Calditenuis aerorheumensis’ for this aigarchaeon, which describes its filamentous morphology and its primary electron acceptor, oxygen.

A Gram-stain-negative, non-motile, non-spore-forming, long rod-shaped bacterium, designated strain GYP-15T, was isolated from the culture broth of a marine microalga,Picochlorumasp. 122. Phylogenetic analyses revealed that strain GYP-15Tshared 90.6 % 16S rRNA gene sequence similarity with its closest relative,Kangiella aquimarinaKCTC 12183T, and represents a distinct phylogenetic lineage in a robust clade consisting of GYP-15Tand members of the generaKangiellaandPleioneain the orderOceanospirillales. Chemotaxonomic and physiological characteristics, including major cellular fatty acids, NaCl tolerance and pattern of carbon source utilization, could also readily distinguish strain GYP-15Tfrom all established genera and species. Thus, it is concluded that strain GYP-15Trepresents a novel species of a new genus, for which the nameAliikangiella marinagen. nov., sp. nov. is proposed. The type strain ofAliikangiella marinais GYP-15T( = MCCC 1K01163T = KCTC 42667T). Based on phylogenetic results, 16S rRNA gene signature nucleotide pattern and some physiological characteristics, the three generaKangiella,PleioneaandAliikangiellaare proposed to make up a novel family,Kangiellaceaefam. nov., in the orderOceanospirillales.

Abstract Methylotrophic planktonic bacteria fulfill a particular role in the carbon cycle of lakes via the turnover of single-carbon compounds. We studied two planktonic freshwater lineages (LD28 and PRD01a001B) affiliated with Methylophilaceae (Betaproteobacteria) in Lake Zurich, Switzerland, by a combination of molecular and cultivation-based approaches. Their spatio-temporal distribution was monitored at high resolution (n=992 samples) for 4 consecutive years. LD28 methylotrophs constituted up to 11 × 107 cells l−1 with pronounced peaks in spring and autumn–winter, concomitant with blooms of primary producers. They were rare in the warm water layers during summer but abundant in the cold hypolimnion, hinting at psychrophilic growth. Members of the PRD01a001B lineage were generally less abundant but also had maxima in spring. More than 120 axenic strains from these so far uncultivated lineages were isolated from the pelagic zone by dilution to extinction. Phylogenetic analysis separated isolates into two distinct genotypes. Isolates grew slowly (μmax=0.4 d−1), were of conspicuously small size, and were indeed psychrophilic, with higher growth yield at low temperatures. Growth was enhanced upon addition of methanol and methylamine to sterile lake water. Genomic analyses of two strains confirmed a methylotrophic lifestyle with a reduced set of genes involved in C1 metabolism. The very small and streamlined genomes (1.36 and 1.75 Mb) shared several pathways with the marine OM43 lineage. As the closest described taxa (Methylotenera sp.) are only distantly related to either set of isolates, we propose a new genus with two species, that is, ‘Candidatus Methylopumilus planktonicus’ (LD28) and ‘Candidatus Methylopumilus turicensis’ (PRD01a001B).

Abstract Me.tha.no.bre.vi.bac'ter. M.L. neut. n. methanum methane; L. masc. adj. brevis short; M.L. masc. n. bacter equivalent of Gr. neut. n. bakterion rod, staff; M.L. masc. n. Methanobrevibacter short methane (‐producing) rod. Oval rods or cocci to short rods , usually occurring in pairs or chains; about 0.5–0.7 µm in width and 0.8–1.4 µm in length. Rarely, filaments are formed. Nonsporing, Gram‐positive. Cell walls are composed of pseudomurein . Nonmotile. Strict anaerobes. Optimum temperature, 37–40°C; maximum, ~45°C; minimum, ~30°C. The mol % G + C of the DNA is : 27.5–31.6. Type species : Methanobrevibacter ruminantium (Smith and Hungate 1958) Balch and Wolfe 1981, 216 (Effective publication: Balch and Wolfe in Balch, Fox, Magrum, Woese and Wolfe, 1979, 284) ( Methanobacterium ruminantium Smith and Hungate 1958, 717). Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Methanobrevibacter is: correct name (last update, February 2025) * . LPSN classification: Archaea / Methanobacteriati / Methanobacteriota / Methanobacteria / Methanobacteriales / Methanobacteriaceae / Methanobrevibacter The genus Methanobrevibacter can also be recovered in the Genome Taxonomy Database (GTDB) as g__Methanobrevibacter (version v220) ** . GTDB classification: d__Archaea / p__Methanobacteriota / c__Methanobacteria / o__Methanobacteriales / f__Methanobacteriaceae / g__Methanobrevibacter * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Xi.phi.ne.ma.to.bac'ter. Gr. neut. n. Xiphinema, ‐atos the genus name of the host organism; N.L. masc. n. bacter the equivalent of Gr. neut. n. baktron a rod; N.L. masc. n. Xiphinematobacter the rod‐shaped microbe associated with Xiphinema . Full‐grown cells are rod‐shaped with rounded ends, 0.7–1.0 × 2.1–3.2 μm ; however, cells in the J 1 (first juvenile) stage of nematode development have a wrinkled, pleomorphic shape. The longer entities usually consist of a mother cell from which a daughter cell is budding, giving rise to serial pairs typical of this bacterial genus. In thin sections cells have two or three membranes consisting of, from inside to outside, a cytoplasmic membrane, an electron‐dense outer membrane, and, in many individuals, a vacuolar membrane which is probably derived from the host cell membrane and which often shows discontinuities. No peptidoglycan layer is evident ; however, a periplasmic hexagonally arrayed monolayer of 10 nm protein units is sometimes present. Gram‐stain‐negative, nonmotile, and nonsporulating. DNA is often condensed at cell poles . Bacteria live as obligate cytoplasmic symbionts with maternal transmission in nematodes of the Xiphinema americanum group ( Nematoda, Longidoridae ), in which they are presumed to induce thelytokous ( mother‐to‐daughter ) parthenogenesis . DNA G + C content ( mol %): not determined. Type species : “ Candidatus Xiphinematobacter brevicolli ” Vandekerckhove, Willems, Gillis and Coomans 2000, 2203. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Xiphinematobacter is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Verrucomicrobiota / Terrimicrobiia / Terrimicrobiales / Chthoniobacteraceae / Candidatus Xiphinematobacter Candidatus Xiphinematobacter could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Cla.vi.chla.my'di.a. L. n. clava a knotty branch, rough stick, cudgel, club; N.L. fem. n. Chlamydia taxonomic name of a bacterial genus; N.L. fem. n. Clavichlamydia a club Chlamydia (the morphology of the bacteria includes the characteristic head‐and‐tail cells that have been described earlier from salmonid fish suffering from epitheliocystis). Pleomorphic or elongated , nonmotile , obligate intracellular bacteria , up to 2 μ m in length . Cells show a developmental cycle with morphologically distinct stages and thrive inside a host vacuole . Organisms were found within gill lesions of fish and are differentiated from all other chlamydiae by the morphology of the proposed elementary bodies , which show a characteristic head‐and‐tail form (Figure 1). Members have not yet been obtained in cell culture. Type species : “ Candidatus Clavichlamydia salmonicola ” corrig. Karlsen, Nylund, Watanabe, Helvik, Nylund and Plarre 2008. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Clavichlamydia is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Chlamydiota / Chlamydiia / Chlamydiales / Chlamydiaceae / Candidatus Clavichlamydia The genus Candidatus Clavichlamydia can also be recovered in the Genome Taxonomy Database (GTDB) as g__Clavichlamydia (version v220) ** . GTDB classification: d__Bacteria / p__Chlamydiota / c__Chlamydiia / o__Chlamydiales / f__Chlamydiaceae / g__Clavichlamydia * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Mag.ne'to.bac.te'ri.um. Gr. n. magnes magnet, comb. form magneto‐; Gr. n. bakterion a small rod; M.L. neut. n. Magnetobacterium a magnetic rod. Rod‐shaped, large, magnetic cells occurring in freshwater sediments. The original description is based on bacteria enriched from the littoral sediment of a freshwater lake in Southern Germany (Chiemsee); similar types of bacteria were also observed in sediments of other freshwater lakes and ponds in Southern Germany and Brazil. Counts of active bacteria in different vertical layers of Chiemsee sediment indicated that “ Candidatus M. bavaricum” is a typical gradient organism, particularly adapted to zones characterized by low levels of oxygen, where it reaches the highest abundance (Spring et al., 1993). A minor fraction of these bacteria was also found in the anoxic zone, whereas high concentrations of oxygen can apparently not be tolerated by this bacterium over longer periods of time. Type species : “ Candidatus Magnetobacterium bavaricum ” Spring, Amann, Ludwig, Schleifer, van Gemerden and Petersen 1993, 2398. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Magnetobacterium is: synonym (and no standing) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Nitrospirota / Nitrospiria / Nitrospirales / Nitrospiraceae / Candidatus Magnetobacterium The genus Candidatus Magnetobacterium can also be recovered in the Genome Taxonomy Database (GTDB) as g__Magnetobacterium (version v220) ** . GTDB classification: d__Bacteria / p__Nitrospirota / c__Thermodesulfovibrionia / o__Thermodesulfovibrionales / f__Magnetobacteriaceae / g__Magnetobacterium * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776