Microbial Genomics

Phytoplasmas (genus ‘Candidatus Phytoplasma’) encompass a group of uncultivated bacteria affecting numerous plant species and causing significant damage in agriculture worldwide. They have a dual parasitic cycle, including colonization of both plant phloem and insect cells. Their genomes are small, diverse, repetitive, prone to rearrangements and harbour transposon-like elements known as potential mobile units (PMUs). In the Euro-Mediterranean region, ‘Ca. P. solani’ is an important species due to its broad range of plant hosts and insect vectors. To provide insights into the genomic diversity of this species, particularly the repertoire of putative effectors and PMUs, this study conducted genome sequencing and analyses of two ‘Ca. P. solani’ strains originating from different plants and transmitted by different insects. Based on de novo assembly, we obtained 19 contigs totalling 656 141 bp for strain STOL and 28 contigs totalling 707 036 bp for strain ST19. The prevalence of repetitive sequences and PMUs contributed to the fragmentation of these draft assemblies. The annotation identified 28 and 26 genes that encode putative secreted proteins in these two strains, respectively, including several homologues of previously characterized phytoplasma effectors. Our comparative analyses further identified species- and strain-specific genes. Frequently, genes that encode putative secreted proteins and effectors were found within PMU-like regions in both genomes. Moreover, strain STOL showed characteristics of a more reduced genome, having fewer PMU-like repetitive elements and genome rearrangements, while strain ST19 exhibited a higher level of sequence divergence in its PMU genes. The high levels of genomic diversity among ‘Ca. P. solani’ strains suggested rapid evolution of this species, which may contribute to its wide host range and adaptability potential. This study provides novel data on the diversification of ‘Ca. P. solani’ genomes. These results provide a foundation for future functional studies of putative effectors and their interactions with host targets, which could facilitate deciphering the pathogenicity strategies of this successful and versatile pathogen.

Marine sediments are vast, underexplored habitats and represent one of the largest carbon deposits on our planet. Microbial communities drive nutrient cycling in these sediments, but the full extent of their taxonomic and metabolic diversity remains to be explored. Here, we analysed shallow coastal and deep subseafloor sediment cores from 0.01 to nearly 600 metres below the seafloor, in the Western Pacific Region. Applying metagenomics, we identified several taxonomic clusters across all samples, which mainly aligned with depth and sediment type. Inferring functional patterns provided insights into possible ecological roles of the main microbial taxa. These included Chloroflexota, the most abundant phylum across all samples, whereby the classes Dehalococcoida and Anaerolineae dominated deep-subsurface and most shallow coastal sediments, respectively. Thermoproteota and Asgardarchaeota were the most abundant phyla among Archaea, contributing to high relative abundances of Archaea reaching over 50% in some samples. We recovered high-quality metagenome-assembled genomes for all main prokaryotic lineages and proposed names for three phyla, i.e. Tangaroaeota phyl. nov. (former RBG-13-66-14), Ryujiniota phyl. nov. (former UBA6262) and Spongiamicota phyl. nov. (former UBA8248). Metabolic capabilities across all samples ranged from aerobic respiration and photosynthesis in the shallowest sediment layers to heterotrophic carbon utilization, sulphate reduction and methanogenesis in deeper anoxic sediments. We also identified taxa with the potential to be involved in nitrogen and sulphur cycling and heterotrophic carbon utilization. In summary, this study contributes to our understanding of the taxonomic and functional diversity in benthic prokaryotic communities across marine sediments in the Western Pacific Region.

Dehalobacter is a genus of organohalide-respiring bacteria that is recognized for its fastidious growth using reductive dehalogenases (RDases). In the SC05 culture, however, a Dehalobacter population also mineralizes dichloromethane (DCM) produced by chloroform dechlorination using the mec cassette, just downstream of its active RDase. A closed genome of this DCM-mineralizing lineage has previously evaded assembly. Here, we present the genomes of two novel Dehalobacter strains, each of which was assembled from the metagenome of a distinct subculture from SC05. A pangenomic analysis of the Dehalobacter genus, including RDase synteny and phylogenomics, reveals at least five species of Dehalobacter based on average nucleotide identity, RDase and core gene synteny, as well as differential functional genes. An integration hotspot is also pinpointed in the Dehalobacter genome, in which many recombinase islands have accumulated. This nested recombinase island encodes the active RDase and mec cassette in both SC05 Dehalobacter genomes, indicating the transfer of key functional genes between species of Dehalobacter. Horizontal gene transfer between these two novel Dehalobacter strains has implications for the evolutionary history within the SC05 subcultures and of the Dehalobacter genus as a whole, especially regarding adaptation to anthropogenic chemicals.

This study presents the assembly and comparative genomic analysis of luminous Photobacterium strains isolated from the light organs of 12 fish species using Oxford Nanopore Technologies (ONT) sequencing. The majority of assemblies achieved chromosome-level continuity, consisting of one large (>3 Mbp) and one small (~1.5 Mbp) contig, with near complete BUSCO scores along with varying plasmid sequences. Leveraging this dataset, this study significantly expanded the available genomes for P. leiognathi and its subspecies P. ‘mandapamensis’, enabling a comparative genomic analysis between the two lineages. An analysis of the large and small chromosomes unveiled distinct patterns of core and accessory genes, with a larger fraction of the core genes residing on the large chromosome, supporting the hypothesis of secondary chromosome evolution from megaplasmids in Vibrionaceae. In addition, we discovered a proposed new species, Photobacterium acropomis sp. nov., isolated from an acropomatid host, with an average nucleotide identify (ANI) of 93 % compared to the P. leiognathi and P. ‘mandapamensis’ strains. A comparison of the P. leiognathi and P. ‘mandapamensis’ lineages revealed minimal differences in gene content, yet highlighted the former’s larger genome size and potential for horizontal gene transfer. An investigation of the lux-rib operon, responsible for light production, indicated congruence between the presence of luxF and host family, challenging its role in differentiating P. ‘mandapamensis’ from P. leiognathi . Further insights were derived from the identification of metabolic differences, such as the presence of the NADH:quinone oxidoreductase respiratory complex I in P. leiognathi as well as variations in the type II secretion system (T2S) genes between the lineages, potentially impacting protein secretion and symbiosis. In summary, this study advances our understanding of Photobacterium genome evolution, highlighting subtle differences between closely related lineages, specifically P. leiognathi and P. ‘mandapamensis’. These findings highlight the benefit of long read sequencing for bacterial genome assembly and pangenome analysis and provide a foundation for exploring early bacterial speciation processes of these facultative light organ symbionts.

Symbiotic microbes from the genus 'Candidatus Megaira' ( Rickettsiales ) are known to be common associates of algae and ciliates. However, genomic resources for these bacteria are scarce, limiting our understanding of their diversity and biology. We therefore utilize Sequence Read Archive and metagenomic assemblies to explore the diversity of this genus. We successfully extract four draft 'Ca. Megaira' genomes including one complete scaffold for a 'Ca. Megaira' and identify an additional 14 draft genomes from uncategorized environmental metagenome-assembled genomes. We use this information to resolve the phylogeny for the hyper-diverse 'Ca. Megaira', with hosts broadly spanning ciliates, and micro- and macro-algae, and find that the current single genus designation 'Ca. Megaira' significantly underestimates their diversity. We also evaluate the metabolic potential and diversity of ''Ca. Megaira' from this new genomic data and find no clear evidence of nutritional symbiosis. In contrast, we hypothesize a potential for defensive symbiosis in 'Ca. Megaira'. Intriguingly, one symbiont genome revealed a proliferation of ORFs with ankyrin, tetratricopeptide and leucine-rich repeats such as those observed in the genus Wolbachia where they are considered important for host–symbiont protein–protein interactions. Onward research should investigate the phenotypic interactions between 'Ca. Megaira' and their various potential hosts, including the economically important Nemacystus decipiens, and target acquisition of genomic information to reflect the diversity of this massively variable group.

Comparing obligate endosymbionts with their free-living relatives is a powerful approach to investigate the evolution of symbioses, and it has led to the identification of several genomic traits consistently associated with the establishment of symbiosis. ‘Candidatus Nebulobacter yamunensis’ is an obligate bacterial endosymbiont of the ciliate Euplotes that seemingly depends on its host for survival. A subsequently characterized bacterial strain with an identical 16S rRNA gene sequence, named Fastidiosibacter lacustris , can instead be maintained in pure culture. We analysed the genomes of ‘Candidatus Nebulobacter’ and Fastidiosibacter seeking to identify key differences between their functional traits and genomic structure that might shed light on a recent transition to obligate endosymbiosis. Surprisingly, we found almost no such differences: the two genomes share a high level of sequence identity, the same overall structure, and largely overlapping sets of genes. The similarities between the genomes of the two strains are at odds with their different ecological niches, confirmed here with a parallel growth experiment. Although other pairs of closely related symbiotic/free-living bacteria have been compared in the past, ‘Candidatus Nebulobacter’ and Fastidiosibacter represent an extreme example proving that a small number of (unknown) factors might play a pivotal role in the earliest stages of obligate endosymbiosis establishment.