ISME Communications

Abstract Candidatus Patescibacteria is a diverse bacterial phylum that is notable for members with ultrasmall cell size, reduced genomes, limited metabolic capabilities, and dependence on other prokaryotic hosts. Despite the prevalence of the name Ca. Patescibacteria in the scientific literature, it is not officially recognized under the International Code of Nomenclature of Prokaryotes and lacks a nomenclatural type. Here, we rectify this situation by describing two closely related circular metagenome-assembled genomes and by proposing one of them (ABY1TS) to serve as the nomenclatural type for the species Patescibacterium danicumTS gen. nov., sp. nov. according to the rules of the SeqCode. Rank-normalized phylogenomic inference confirmed the stable placement of P. danicumTS in the Ca. Patescibacteria class ABY1. Based on these results, we propose Patescibacterium gen. nov. to serve as the type genus for associated higher taxa, including the phylum Patescibacteriota phyl. nov. We complement our proposal with a genomic characterization, metabolic reconstruction, and biogeographical analysis of Patescibacterium. Our results confirm small genome sizes (<1 Mbp), low GC content (>36%), and the occurrence of long gene coding insertions in the 23S rRNA sequences, along with reduced metabolic potential, inferred symbiotic lifestyle, and a global distribution. In summary, our proposal will provide nomenclatural stability to the fourth-largest phylum in the bacterial domain.

Abstract The archaeal class Bathyarchaeia is widely and abundantly distributed in anoxic habitats. Metagenomic studies have suggested that they are mixotrophic, capable of CO2 fixation and heterotrophic growth, and involved in acetogenesis and lignin degradation. We analyzed 35 Bathyarchaeia metagenome-assembled genomes (MAGs), including the first complete circularized MAG (cMAG) of the Bathy-6 subgroup, from the metagenomes of three full-scale pulp and paper mill anaerobic digesters and three laboratory methanogenic enrichment cultures maintained on pre-treated poplar. Thirty-three MAGs belong to the Bathy-6, lineage while two are from the Bathy-8 lineage. In our previous analysis of the microbial community in the pulp mill digesters, Bathyarchaeia were abundant and positively correlated to hydrogenotrophic and methylotrophic methanogenesis. Several factors likely contribute to the success of the Bathy-6 lineage compared to Bathy-8 in the reactors. The Bathy-6 genomes are larger than those of Bathy-8 and have more genes involved in lignocellulose degradation, including carbohydrate-active enzymes not present in the Bathy-8. Bathy-6 also shares the Bathyarchaeal O-demethylase system recently identified in Bathy-8. All the Bathy-6 MAGs had numerous membrane-associated pyrroloquinoline quinone-domain proteins that we suggest are involved in lignin modification or degradation, together with Radical-S-adenosylmethionine (SAM) and Rieske domain proteins, and AA2, AA3, and AA6-family oxidoreductases. We also identified a complete B12 synthesis pathway and a complete nitrogenase gene locus. Finally, comparative genomic analyses revealed that Bathyarchaeia genomes are dynamic and have interacted with other organisms in their environments through gene transfer to expand their gene repertoire.

Abstract Anthropogenic influences have drastically increased nutrient concentrations in many estuaries globally, and microbial communities have adapted to the resulting hypereutrophic ecosystems. However, our knowledge of the dominant microbial taxa and their potential functions in these ecosystems has remained sparse. Here, we study prokaryotic community dynamics in a temporal–spatial dataset, from a subtropical hypereutrophic estuary. Screening 54 water samples across brackish to marine sites revealed that nutrient concentrations and salinity best explained spatial community variations, whereas temperature and dissolved oxygen likely drive seasonal shifts. By combining short and long read sequencing data, we recovered 2,459 metagenome-assembled genomes, proposed new taxon names for previously uncharacterised lineages, and created an extensive, habitat specific genome reference database. Community profiling based on this genome reference database revealed a diverse prokaryotic community comprising 61 bacterial and 18 archaeal phyla, and resulted in an improved taxonomic resolution at lower ranks down to genus level. We found that the vast majority (61 out of 73) of abundant genera (>1% average) represented unnamed and novel lineages, and that all genera could be clearly separated into brackish and marine ecotypes with inferred habitat specific functions. Applying supervised machine learning and metabolic reconstruction, we identified several microbial indicator taxa responding directly or indirectly to elevated nitrate and total phosphorus concentrations. In conclusion, our analysis highlights the importance of improved taxonomic resolution, sheds light on the role of previously uncharacterised lineages in estuarine nutrient cycling, and identifies microbial indicators for nutrient levels crucial in estuary health assessments.

Abstract Acidobacteriota are abundant in soil, peatlands, and sediments, but their ecology in freshwater environments remains understudied. UBA12189, an Acidobacteriota genus, is an uncultivated, genome-streamlined lineage with a small genome size found in aquatic environments where detailed genomic analyses are lacking. Here, we analyzed 66 MAGs of UBA12189 (including one complete genome) from freshwater lakes and rivers in Europe, North America, and Asia. UBA12189 has small genome sizes (<1.4 Mbp), low GC content, and a highly diverse pangenome. In freshwater lakes, this bacterial lineage is abundant from the surface waters (epilimnion) down to a 300-m depth (hypolimnion). UBA12189 appears to be free-living from CARD-FISH analysis. When compared to other genome-streamlined bacteria such as Nanopelagicales and Methylopumilus, genome reduction has caused UBA12189 to have a more limited metabolic repertoire in carbon, sulfur, and nitrogen metabolisms, limited numbers of membrane transporters, as well as a higher degree of auxotrophy for various amino acids, vitamins, and reduced sulfur. Despite having reduced genomes, UBA12189 encodes proteorhodopsin, complete biosynthesis pathways for heme and vitamin K2, cbb3-type cytochrome c oxidases, and heme-requiring enzymes. These genes may give a selective advantage during the genome streamlining process. We propose the new genus Acidiparvus, with two new species named “A. lacustris” and “A. fluvialis”. Acidiparvus is the first described genome-streamlined lineage under the phylum Acidobacteriota, which is a free-living, slow-growing scavenger in freshwater environments.

Abstract Nitrate leaching from agricultural soils is increasingly found in groundwater, a primary source of drinking water worldwide. This nitrate influx can potentially stimulate the biological oxidation of iron in anoxic groundwater reservoirs. Nitrate-dependent iron-oxidizing (NDFO) bacteria have been extensively studied in laboratory settings, yet their ecophysiology in natural environments remains largely unknown. To this end, we established a pilot-scale filter on nitrate-rich groundwater to elucidate the structure and metabolism of nitrate-reducing iron-oxidizing microbiomes under oligotrophic conditions mimicking natural groundwaters. The enriched community stoichiometrically removed iron and nitrate consistently with the NDFO metabolism. Genome-resolved metagenomics revealed the underlying metabolic network between the dominant iron-dependent denitrifying autotrophs and the less abundant organoheterotrophs. The most abundant genome belonged to a new Candidate order, named Siderophiliales. This new species, “Candidatus Siderophilus nitratireducens,” carries genes central genes to iron oxidation (cytochrome c cyc2), carbon fixation (rbc), and for the sole periplasmic nitrate reductase (nap). Using thermodynamics, we demonstrate that iron oxidation coupled to nap based dissimilatory reduction of nitrate to nitrite is energetically favorable under realistic Fe3+/Fe2+ and NO3−/NO2− concentration ratios. Ultimately, by bridging the gap between laboratory investigations and nitrate real-world conditions, this study provides insights into the intricate interplay between nitrate and iron in groundwater ecosystems, and expands our understanding of NDFOs taxonomic diversity and ecological role.

Abstract The order Holosporales (Alphaproteobacteria) encompasses obligate intracellular bacterial symbionts of diverse Eukaryotes. These bacteria have highly streamlined genomes and can have negative fitness effects on the host. Herein, we present a comparative analysis of the first genome sequences of ‘Ca. Hepatincola porcellionum’, a facultative symbiont occurring extracellularly in the midgut glands of terrestrial isopods. Using a combination of long-read and short-read sequencing, we obtained the complete circular genomes of two Hepatincola strains and an additional metagenome-assembled draft genome. Phylogenomic analysis validated its phylogenetic position as an early-branching family-level clade relative to all other established Holosporales families associated with protists. A 16S rRNA gene survey revealed that this new family encompasses diverse bacteria associated with both marine and terrestrial host species, which expands the host range of Holosporales bacteria from protists to several phyla of the Ecdysozoa (Arthropoda and Priapulida). Hepatincola has a highly streamlined genome with reduced metabolic and biosynthetic capacities as well as a large repertoire of transmembrane transporters. This suggests that this symbiont is rather a nutrient scavenger than a nutrient provider for the host, likely benefitting from a nutrient-rich environment to import all necessary metabolites and precursors. Hepatincola further possesses a different set of bacterial secretion systems compared to protist-associated Holosporales, suggesting different host-symbiont interactions depending on the host organism.

Abstract Visible surface films, termed slicks, can extensively cover freshwater and marine ecosystems, with coastal regions being particularly susceptible to their presence. The sea-surface microlayer (SML), the upper 1-mm at the air-water interface in slicks (herein slick SML) harbors a distinctive bacterial community, but generally little is known about SML viruses. Using flow cytometry, metagenomics, and cultivation, we characterized viruses and bacteria in a brackish slick SML in comparison to non-slick SML as well as seawater below slick and non-slick areas (subsurface water = SSW). Size-fractionated filtration of all samples distinguished viral attachment to hosts and particles. The slick SML contained higher abundances of virus-like particles, prokaryotic cells, and dissolved organic carbon compared to non-slick SML and SSW. The community of 428 viral operational taxonomic units (vOTUs), 426 predicted as lytic, distinctly differed across all size fractions in the slick SML compared to non-slick SML and SSW. Specific metabolic profiles of bacterial metagenome-assembled genomes and isolates in the slick SML included a prevalence of genes encoding motility and carbohydrate-active enzymes (CAZymes). Several vOTUs were enriched in slick SML, and many virus variants were associated with particles. Nine vOTUs were only found in slick SML, six of them being targeted by slick SML-specific clustered-regularly interspaced short palindromic repeats (CRISPR) spacers likely originating from Gammaproteobacteria. Moreover, isolation of three previously unknown lytic phages for Alishewanella sp. and Pseudoalteromonas tunicata, abundant and actively replicating slick SML bacteria, suggests that viral activity in slicks contributes to biogeochemical cycling in coastal ecosystems.

Abstract Marine bacterial lineages associated with algal blooms, such as the Roseobacter clade, have been well characterized in ecological and genomic contexts, yet such lineages have rarely been explored in freshwater blooms. This study performed phenotypic and genomic analyses of an alphaproteobacterial lineage ‘Candidatus Phycosocius’ (denoted the CaP clade), one of the few lineages ubiquitously associated with freshwater algal blooms, and described a novel species: ‘Ca. Phycosocius spiralis.’ Phylogenomic analyses indicated that the CaP clade is a deeply branching lineage in the Caulobacterales. Pangenome analyses revealed characteristic features of the CaP clade: aerobic anoxygenic photosynthesis and essential vitamin B auxotrophy. Genome size varies widely among members of the CaP clade (2.5–3.7 Mb), likely a result of independent genome reductions at each lineage. This includes a loss of tight adherence pilus genes (tad) in ‘Ca. P. spiralis’ that may reflect its adoption of a unique spiral cell shape and corkscrew-like burrowing activity at the algal surface. Notably, quorum sensing (QS) proteins showed incongruent phylogenies, suggesting that horizontal transfers of QS genes and QS-involved interactions with specific algal partners might drive CaP clade diversification. This study elucidates the ecophysiology and evolution of proteobacteria associated with freshwater algal blooms.

Abstract Marine sediments comprise one of the largest environments on the planet, and their microbial inhabitants are significant players in global carbon and nutrient cycles. Recent studies using metagenomic techniques have shown the complexity of these communities and identified novel microorganisms from the ocean floor. Here, we obtained 77 metagenome-assembled genomes (MAGs) from the bacterial phylum Armatimonadota in the Guaymas Basin, Gulf of California, and the Bohai Sea, China. These MAGs comprise two previously undescribed classes within Armatimonadota, which we propose naming Hebobacteria and Zipacnadia. They are globally distributed in hypoxic and anoxic environments and are dominant members of deep-sea sediments (up to 1.95% of metagenomic raw reads). The classes described here also have unique metabolic capabilities, possessing pathways to reduce carbon dioxide to acetate via the Wood-Ljungdahl pathway (WLP) and generating energy through the oxidative branch of glycolysis using carbon dioxide as an electron sink, maintaining the redox balance using the WLP. Hebobacteria may also be autotrophic, not previously identified in Armatimonadota. Furthermore, these Armatimonadota may play a role in sulfur and nitrogen cycling, using the intermediate compounds hydroxylamine and sulfite. Description of these MAGs enhances our understanding of diversity and metabolic potential within anoxic habitats worldwide.