Publications

4874

Abstract Environmental genomics has led to the discovery of many new lineages of archaea, including “DPANN” (or Nanobdellati), comprising organisms with small genomes, reduced gene content, and potentially symbiotic or parasitic lifestyles. DPANN live in various environments, and several lineages have been identified that are adapted to extremely high-salt concentrations, including the Nanohaloarchaeota. Since it was long thought that the Haloarchaea (within “Euryarchaeota”) were the only high salt-adapted archaea, the origins of these genome-reduced halophiles have been debated. Here, we used phylogenetic, comparative genomic, and gene tree-species tree reconciliation approaches to resolve the evolution of halophily within DPANN, making use of recently published genomes that help to inform the phylogenetic placement and genome evolution of salt-adapted lineages. Phylogenetic analysis placed Nanohaloarchaeota sister to a previously uncharacterized lineage, which we here refer to as Terrarchaeota. Terrarchaeota appear to be predominantly anaerobic thermophiles that are not adapted to high-salt concentrations, indicating that adaptation to high salt evolved after their divergence from Nanohaloarchaeota. Furthermore, our analyses identified genomic hallmarks of salt adaptation in another recently discovered halophilic DPANN lineage within Aenigmatarchaeota, the Haloaenigmatarchaeaceae. We found that the Nanohaloarchaeota and Haloaenigmatarchaeaceae have distinct sets of proteins that enable life at high salt concentrations but share a common mechanism of evolutionary adaptation, in which niche-relevant genes were acquired horizontally from their halophilic hosts. This work provides the first detailed investigation into the enigmatic Terrarchaeota, and new insights into the convergent evolution of high salt adaptation within symbiotic clades of Archaea.

Abstract Magnetotactic bacteria form a highly diverse group of microorganisms, yet early exploration of their diversity was largely centered on the Pseudomonadota. More recently, metagenomic studies have revealed that magnetotaxis, a form of chemotaxis guided by Earth's magnetic field, is widespread in other deep-branching phyla for which little to no ecological or biological information is available beyond that inferred from their genomes. For most of them, the morphology, ultrastructure and magnetosome chain characteristics responsible for the magnetic guidance remain unknown. While screening extreme environments for novel magnetotactic species, we observed magnetotactic Bdellovibrionota in the anoxic and ferruginous sediments of the Fontaine Goyon spring (France). We characterized their cell morphology and ultrastructure using magnetic enrichment, a single-cell sorting approach, and high-resolution electron microscopy. Cells display the morphology typical of the few predatory bacteria described in this phylum, and biomineralize, on average, five irregularly faceted, bullet-shaped magnetite magnetosomes along the concave side of the cell. Metagenomic analysis of approximately one hundred cells revealed a potentially predatory and heterotrophic lifestyle adapted to low-O2 conditions. It also suggests a flexible respiratory metabolism under varying redox conditions, using iron as an alternative terminal electron acceptor. Exploring the diversity of Bdellovibrionota in public databases, we found 21 metagenome-assembled-genomes containing magnetosome genes. None of them harbor the canonical mamK actin-like gene implicated in aligning magnetosomes in described magnetotactic models. Affiliated to an undescribed class, we propose a classification scheme for the magnetotactic Bdellovibrionota species representing the class Bdellonasia class nov., for which no species had been formally described.

A hydrogenotrophic methanogen, designated strain ACI-7 T , was isolated from an alkaline, saline wetland located in eastern Nebraska, USA. Cells were identified as non-motile rods (0.9–4.2 µm in length and 0.2–0.4 µm in diameter) that occurred singly, in chains, or as twisted chains and aggregates. Strain ACI-7 T utilized H 2 +CO 2 or formate as methanogenic substrates and required growth factors present in yeast extract for continuous cultivation. The strain grew at 20–45 °C (optimum, 40 °C), at pH 6.5–8.5 (optimum, pH 7.3) and with 0–2.5% NaCl (optimum, 0–1%). The genomic G+C content of ACI-7 T was 31.79 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated strain ACI-7 T was affiliated with the genus Methanobacterium , most closely related to Methanobacterium oryzae FPi T (97.1% sequence similarity) and Methanobacterium veterum MK4 T (96.9% sequence similarity). Overall genome relatedness indices for strain ACI-7 T compared to other Methanobacterium species ranged from 68.45 to 78.17% for average nucleotide identity and 18.9 to 25.7% for digital DNA–DNA hybridization. Morphological, physiological and genomic characteristics indicate that strain ACI-7 T represents a novel species, for which the name Methanobacterium nebraskense sp. nov. is proposed. The type strain is ACI-7 T (=DSM 118696 T =ATCC TSD-487 T ).

ABSTRACT The oil refinery (OR) consortium is a model methanogenic enrichment culture used to study anaerobic benzene degradation. Over half of the culture’s bacterial community consists of two closely related Desulfobacterota strains, designated ORM2a and ORM2b, whose mechanisms of benzene activation are unknown. Three proteomics data sets were integrated and analyzed using high-quality OR metagenomes and metagenome-assembled genomes (MAGs), including a complete circularized ORM2a MAG, to identify active metabolic pathways and proteins expressed during methanogenic benzene degradation. Among the proteins identified were Bam-like subunits of an ATP-independent benzoyl-CoA degradation pathway, as well as downstream β-oxidation proteins yielding acetate. The most abundant proteins identified mapped to two ORM2a gene clusters of unknown function. Homologous and syntenic gene clusters were identified in the MAGs of ORM2b and a sulfate-reducing Pelotomaculum that also degrades benzene, as well as in nine contigs assembled from hydrothermal vent metagenomes. Extensive homology and structural predictions suggest that the first cluster—termed the “Magic” gene cluster—encodes for enzymes catalyzing the chemically challenging activation of benzene and subsequent transformation steps yielding benzoyl-CoA. The second (“Nanopod”) gene cluster encodes a transmembrane complex that may facilitate benzene transport across the cell membrane. Phylogenomic analyses place ORM2a and ORM2b within a novel genus of strict anaerobes specialized for benzene degradation, which we propose naming “ Candidatus Anaerobenzenivorax.” IMPORTANCE Benzene is a widespread, persistent, and toxic pollutant that can accumulate in anoxic environments such as groundwater and sediments. Benzene can be metabolized in the absence of oxygen; however, despite decades of research, the biochemical mechanisms for benzene activation under anaerobic conditions remain unproven. This study provides strong genetic and proteomic evidence for a new suite of enzymes that initiate anaerobic benzene activation. These findings lay a foundation for future biochemical studies and expand our understanding of how microbes carry out difficult chemical reactions in the absence of oxygen.

‘Candidatus Phytoplasma rubi’ (elm yellows group, 16SrV-E phytoplasma subgroup) is the causal agent of rubus stunt disease, a disorder affecting economically important plants—raspberries and blackberries. Although this phytoplasma has been extensively studied in cultivated Rubus crops, its occurrence and molecular identity in wild Rubus species populations in North-Eastern Europe remain poorly documented. In this study, phytoplasmas associated with symptomatic wild Rubus idaeus L. and Rubus nessensis Hall plants were investigated in natural forest ecosystems of the Curonian Spit, Lithuania. A total of 65 symptomatic plants were surveyed, and phytoplasma infection was detected in 30 samples by nested PCR targeting the 16S rRNA gene. Positive samples were characterized using a multilocus molecular approach based on sequence analysis of the additional cpn60 and secA genes. All strains showed high nucleotide sequence similarity across the analysed loci and consistently clustered with reference strains of ‘Candidatus Phytoplasma rubi’. Virtual RFLP profiles derived from the 16S rRNA and cpn60 genes were nearly identical to those of established 16SrV-E phytoplasma subgroup reference strains and clearly distinct from other 16SrV phytoplasma subgroups. These results provide not only the first detailed multilocus molecular characterization of ‘Candidatus Phytoplasma rubi’-related strains infecting wild Rubus species in Lithuania but also represent the first report of this phytoplasma in naturally occurring R. idaeus and R. nessensis plants in the country, thereby extending the known geographical occurrence of this pathogen and documenting its presence in wild Rubus hosts from unmanaged forest habitats in the Eastern Baltic region of Northern Europe.

Ocean warming, disease, and pollution have contributed to global declines in coral abundances and diversity. In the Caribbean, corals previously dominated reefs, providing an architectural framework for diverse ecological habitats, but have significantly declined due to infectious disease and anthropogenic climate change. Key species like the coral Acropora cervicornis are critically endangered, prompting researchers to focus on scientific endeavors to identify factors influencing coral disease resistance and resilience. We previously showed that disease susceptibility, growth rates, and bleaching risk were all associated with the abundance of a single bacterial parasite, Candidatus Aquirickettsia rohweri which proliferates in vivo under nutrient enrichment. Yet how nutrients influence parasite physiology in vivo remains unknown. Here, we analyzed parasite gene expression from a disease-susceptible A. cervicornis genotype exposed to ambient or nutrient enrichment conditions. Electron microscopy showed that Ca. A. rohweri was abundant in coral tissue and densely packed in mucocytes prior to nutrient enrichment. Under ambient conditions, the parasite upregulated genes involved in translation, protein maintenance, and cell envelope integrity, consistent with a conserve-and-maintain strategy. Nutrient enrichment induced expression of genes associated with central metabolism, nutrient import, stress response, host interaction, and two-component systems. Together, these results indicate that nutrient enrichment activates a growth-and-exploitation strategy, likely exacerbating parasitic pressure on A. cervicornis .

A field investigation was conducted during the winter seasons of 2021–22 and 2022–23 at the Jawaharlal Nehru Krishi Vishwa Vidyalaya (JNKVV), Jabalpur, Central India, to identify and characterize the phytoplasma strains associated with niger (var. JNC6) and sesame (var. RT315) exhibiting severe phyllody symptoms. Disease incidence ranged from 0-3%, and 3 to 15% were recorded in niger and sesame crops during the study period. Nested PCR assays using universal phytoplasma primer pairs P1/P7 followed by R16F2n/R16R2 consistently amplified ~1.25 kb fragments of the 16S rRNA gene from both the symptomatic plants, confirming phytoplasma association. Additional amplification of phytoplasma-specific sec A (~600 bp) and sec Y (~1.7 kb) gene fragments further validated the phytoplasma infection in both crops. Two predominant leafhopper species, Amrasca biguttula and Orosius albicinctus , colonizing and feeding niger and sesame fields were identified. Notably, only Amrasca biguttula was tested positive for phytoplasma using the similar PCR assays mentioned above, implicating it as a putative vector. Sequence identity, phylogenetic analysis, and in silico RFLP profiling of 16S rRNA, sec A, and sec Y gene sequences from both the symptomatic plants and leafhoppers confirmed the presence of a ‘ Ca. P. asiaticum’ related strain (16SrII-C subgroup). This study represents the first global report of 16SrII-C subgroup phytoplasma infection in niger, highlighting a new host record. The detection of an identical phytoplasma strain in niger, sesame, and Amrasca biguttula suggests a possible epidemiological link and underscores the role of these natural hosts and the leafhopper in the dissemination of ‘ Ca. P. asiaticum’-related phytoplasma strains in Central India.

Candidatus Liberibacter asiaticus (CLas) is an unculturable, phloem-limited gram-negative bacterium associated with citrus Huanglongbing (HLB) disease in Florida and most other citrus-producing countries around the world. HLB is considered the most destructive disease of citrus and is severely affecting the fruit production worldwide. Natural transmission of CLas occurs by the Asian citrus psyllid (ACP) vector Diaphorina citri. Under greenhouse conditions, the experimental transmission of CLas to non-infected trees usually is accomplished using infected ACP or by grafting infected citrus tissue. CLas was confirmed infecting citrus in Florida in 2005 and many characteristics of the disease have been described. However, the transmission of CLas for controlled greenhouse studies is often prob lematic and poorly understood. Due the complexity in maintenance of ACP colonies for controlled vector transmission, the experimental infection of new plants is accomplished primarily by grafting, but several factors may influence the reliability of this method. This study evaluates the propagation and transmission efficiency of CLas using graft inoculation methods under greenhouse conditions. Three experiments were conducted for with graft inoculation: 1) seedlings of 12 cultivars; 2) ‘Tango’ mandarin on 13 rootstock cultivars; and 3) ‘Navel’ orange on 14 rootstock cultivars. Plants were inoculated with buds from infected greenhouse-grown ‘Valencia’ orange plants that were poly merase chain reaction (PCR) positive for CLas and symptomatic for HLB. Three bud pieces were used to inoculate each plant using the inverted-T method.