Biology subjects

Jaffe, A. L.

Publications and source records attributed to Jaffe, A. L..

At least 19 recordsLinked to original sources

bioRxiv2026

Deeply-sequenced metagenomes and over 1000 draft genomes from the epipelagic to bathypelagic Northeast Pacific Ocean

The deep ocean water column (>200 meters depth) hosts diverse and active microbial communities that play critical roles in Earths biogeochemical cycles. However, this habitat has been massively undersampled compared to shallower depths where sunlight penetrates, as well as benthic features like hydrothermal vents and seeps. In particular, few existing deep-sea datasets provide adequate spatial resolution to study the impact of physical and geochemical gradients on marine microbial communities over both horizontal and vertical spatial scales. Here, we present a deeply-sequenced, spatially-resolved set of 28 water column metagenomes from a [~]300 km horizontal transect off the coast of central California, spanning 50 to 4000 meters depth. We estimate that the deepest portion of the resulting [~]1.5 terabases of sequencing data comprises nearly 9% of all shotgun metagenomic sequencing currently available from 1000 meters depth or below. Additionally, we generate over one thousand draft genomes describing the bacteria and archaea present in our bulk metagenomic data. These genomes frequently account for the majority of reads sequenced, and show discernable patterns of abundance with depth, underscoring the presence of distinct microbial cohorts inhabiting different ocean compartments. Additionally, the depth of sequencing performed (mean of [~]51.9 gigabases per sample) enables resolution of rare community members that are, on average, more phylogenetically novel than their abundant counterparts, forming an important contribution to genomic catalogues for this habitat. We anticipate that the combined datasets will enable diverse research questions concerning the adaptation, functional potential, and distribution of deep sea microbes as well as their genes and proteins.

microbiology↗

bioRxiv2026

Microbial autotrophy is widespread across soils and most prevalent in deep and saturated environments

Genes for CO2 fixation occur in soil microorganisms, but little is known about the pathways that are most common across ecosystem types, the organisms with these genes, where different CO2 fixation pathways are most prevalent, and the energy sources that support autotrophy across ecosystems. Here, we investigated microbial capacity for autotrophy in soils using 853 metagenomes and 201 metatranscriptomes from a wide range of terrestrial ecosystems (agricultural soils, wetlands, weathering rock). Autotrophy-associated RuBisCO (Form I and II) is widely encoded across all soils and occurs in bacteria from numerous lineages (38 phyla). RuBisCO Form IE is consistently more phylogenetically diverse in soils than in marine ecosystems, suggesting that it may have evolved to function in soil-like environments. A newly discovered deeply branching Form I RuBisCO, Form I, supports the hypothesis that Form I RuBisCO originated in anaerobic environments. Further, saturated soils harbor more, and more distinct, autotrophic microbes, many of which may use the Calvin-Benson-Bassham cycle or Wood-Ljungdahl pathway for CO2 fixation. Overall, the results indicate that autotrophy is a particularly important metabolism in deep, saturated soils and weathering rock.

microbiology↗

bioRxiv2026

Substantial genetic potential for deep-sea chemoautotrophy extends beyond nitrifiers

I.Microbial chemoautotrophy in the deep sea has the potential to sustain ecosystems at depth, contribute to global carbon sequestration, and ameliorate the currently unbalanced deep-sea carbon budget. However, an understanding of its mechanisms and feasibility is still emerging, particularly beyond that fueled by nitrification. Here, we conduct both a gene-based and genome-resolved analysis of 28 metagenomes in the Northeast Pacific Ocean to investigate the prevalence, distribution, and phylogenetic and metabolic diversity of deep-sea chemoautotrophs. We find that organisms encoding marker genes for dissolved inorganic carbon (DIC) fixation are abundant and widespread at our study site, comprising 11-26% of microbial communities from 150-4000 m water depth. Marker genes for the Calvin cycle and 3-Hydroxypropionate (3HP) bi-cycles are more prevalent than those for the 3-Hydroxypropionate/4-Hydroxybutyrate (3HP/4HB) and reverse Tricarboxylic Acid (rTCA) cycles, the latter two of which are encoded by organisms conducting nitrification. We construct and identify 128 putatively chemoautotrophic metagenome-assembled genomes, spanning 14 phyla including the Proteobacteria, Actinobacteria, SAR324, and the Thermoproteota. They contained genes for the oxidation of carbon monoxide (77.3%), sulfur (68.8%), ammonia (5.5%), and/or methane (3.1%), suggesting diverse catabolisms fuel deep-sea DIC fixation and an underappreciated potential role for aerobic carbon monoxide oxidation. Fifty percent of these genomes encoded multiple inorganic catabolic pathways and 99% included genes for organic matter transport, suggesting catabolic flexibility and potentially facultative autotrophy, respectively. We create an inclusive inventory and map of potential chemoautotrophs at our study site, expanding their known phylogenetic breadth, metabolic repertoires, and potential to impact the carbon cycle. ImportanceThe deep-sea carbon cycle plays a central role regulating marine ecosystem productivity and the global climate. Chemoautotrophy, the microbial conversion of inorganic carbon (e.g., CO2 or HCO-3) into cellular biomass, is an understudied process with the potential to significantly shift models of the marine carbon budget. While some deep-sea chemoautotrophs are known, a wholistic analysis of organisms with the genetic potential for chemoautotrophy in the dark water column is lacking. Here, we find that microorganisms with the genetic potential for chemoautotrophy are widespread and abundant, and that they are more phylogenetically and metabolically diverse than previously appreciated. In particular, the prevalence of genes involved in CO oxidation suggests a currently unrecognized role in fueling deep-sea chemoautotrophy. By identifying the potential microbial mediators and coupled energy sources, our results allow for more accurate predictions of when, where, and how deep-sea chemoautotrophy occurs, and therefore its potential role in carbon cycling.

microbiology↗

bioRxiv2025

Fifteen-year microbiome survey of endangered killer whales (Orcinus orca) reveals declining diversity and population differences

The endangered Southern Resident killer whale (Orcinus orca) (SRKW) population is burdened by multiple anthropogenic stressors with limited non-invasive approaches for health surveillance. The gut microbiome is interconnected with host physiology and can be characterized using remotely collected fecal samples, creating unique opportunities to integrate environmental and individualized host-associated features of health. In this study, we used fecal samples collected over a 15-year period (2005-2019) from 77% of the living, wild SRKW population (56 individuals, 1-17 time points per individual), as well as fecal samples from the conspecific Northern and Alaska Resident killer whale populations, to characterize distal gut microbiota structure and genomic composition. SRKW microbiotas were individualized and distinct from those of the nearby Northern and Alaska Resident populations, both of which exhibit consistently higher fecundity and survivorship. During the study period, SRKW fecal microbiota species richness declined, despite stability over periods of time less than or equal to 1 year. Several potential bacterial pathogens, such as Fusobacterium spp., achieved dominance in the fecal microbiotas of SRKW individuals sampled within 6 months of death. These findings demonstrate the feasibility and value of harnessing non-invasively collected fecal samples and microbiome profiles for longitudinal killer whale health surveillance.

microbiology↗

bioRxiv2025

Diversity-driven biochemical survey reveals dimeric structural origin of rubisco

Rubisco is the entry point of nearly all organic carbon into the biosphere and is present in all domains of life. Despite its global importance, biochemical studies of this enzyme superfamily have been limited to a relatively narrow set of subclades. Recent advances in metagenomics have dramatically reshaped our understanding of both microbial and rubisco diversity; however, biochemical characterization of these sequences has not kept pace with the exponential growth in sequence data. To better survey the functional and structural diversity of rubisco, we systematically sampled and synthesized a library of diverse rubisco sequences with an emphasis on clades that have previously not been characterized. Our updated phylogenetic analysis reveals that many deep-branching rubiscos assemble as dimers, supporting a dimeric origin for the superfamily -- in contrast to the ecologically dominant hexadecameric form I. Additionally, we discover and structurally characterize the largest rubisco described to date, originating from a cryptic, early-branching subclade with novel structural folds that have previously not been observed in the rubisco superfamily. By integrating biochemical data with an updated phylogenetic framework, we propose a revised nomenclature for the rubisco protein family that reflects current insights and will better accommodate future discoveries.

biochemistry↗

bioRxiv2024

Cyanobacteria from marine oxygen deficient zones encode both form I and form II rubiscos

Cyanobacteria are highly abundant in the marine photic zone and primary drivers of the conversion of inorganic carbon to biomass. To date, all studied Cyanobacterial lineages encode carbon fixation machinery hinged upon form I rubisco enzymes within a CO2-concentrating carboxysome. Here, we report that the AMZ IB lineage of Prochlorococcus from global oxygen deficient zones (ODZs) harbor both form I and form II rubisco enzymes, the latter of which are typically non-carboxysomal and possess biochemical properties tuned towards low oxygen environments. Our analyses reveal that these cyanobacterial form II enzymes are functional in vitro and were likely acquired via lateral gene transfer from proteobacteria. Global metagenomic read recruitment demonstrates that Prochlorococcus with form II rubisco are essentially restricted to ODZs in the Eastern Tropical Pacific, suggesting that acquisition may confer an advantage specifically under low-O2 conditions. Populations of AMZ IB Prochlorococcus express both forms of rubisco in situ, with the highest form II rubisco expression at depths where both oxygen and light are particularly low, possibly as a mechanism to increase the efficiency of photoautotrophy under energy limitation. Our findings expand the diversity of carbon fixation configurations in the microbial world and may have implications for the overall capacity of ODZs to sequester carbon.

microbiology↗

bioRxiv2024

Prevalence of the Calvin-Benson-Bassham cycle in chemolithoautotrophic psychrophiles and the potential for cold-adapted Rubisco

The act of fixing inorganic carbon into the biosphere is largely facilitated by one enzyme, Rubisco. Beyond well-studied plants and cyanobacteria, many bacteria use Rubisco for chemolithoautotrophy in extreme environments on Earth. Here, we characterized the diversity of chemolithoautotrophic Rubiscos in subzero environments. First, we surveyed subzero environments and found that the Calvin-Benson-Bassham cycle was the most prevalent chemolithoautotrophic pathway. Second, we uncovered potential for chemolithoautotrophy in metagenomes from two distinct subzero, hypersaline Arctic environments: 40-kyr relic marine brines encased within permafrost (cryopeg brines) and first-year sea ice. Again, the Calvin-Benson-Bassham cycle was the dominant chemolithoautotrophic pathway in both environments, though with different Rubisco forms. From cryopeg brine, we reconstructed four metagenome-assembled genomes with the potential for chemolithoautotrophy, of which the sulfur-oxidizing genus Thiomicrorhabdus was most abundant. A broader survey of Thiomicrorhabdus genomes from diverse environments identified a core complement of three Rubisco forms (II, IAc, IAq) with distinct patterns of gain and loss. We developed a model framework and compared these different Rubisco forms across [CO2], [O2], and temperature. We found that form II outcompetes form I at low O2, but cold temperatures minimize this advantage. However, further inspection of form II from cold environments uncovered signals of thermal adaptation of key amino acids which resulted in a more exposed active site. These modifications suggest that these form II Rubisco proteins may have unique kinetics or thermal stability. This work can help address the limits of autotrophic functionality in extreme environments on Earth and other planetary bodies. ImportanceAutotrophy, or the fixation of inorganic carbon to biomass, is a key factor in lifes ability to thrive on Earth. Research on autotrophy has focused on plants and algae, but many bacteria are also autotrophic and can survive and thrive under more extreme conditions. These bacteria are a window to past autotrophy on Earth, as well as potential autotrophy in extreme environments elsewhere in the Universe. Our study focused on dark, cold, saline environments, which are likely to be found on Enceladus and Europa, as well as in the Martian subsurface. We found compelling evidence of cold adaptation in a key autotrophic enzyme, Rubisco, which could expand the known boundaries of autotrophy in rapidly disappearing icy environments on Earth. We also present a novel model framework that can be used to probe the limits of autotrophy not only on Earth but also on key astrobiological targets like Enceladus and Europa.

microbiology↗

bioRxiv2024

Urea assimilation and oxidation supports the activity of a phylogenetically diverse microbial community in the dark ocean

Urea is hypothesized to be an important source of nitrogen and chemical energy to microorganisms in the deep sea; however, direct evidence for urea use below the epipelagic ocean is lacking. Here, we explore urea utilization from 50 to 4000 meters depth in the northeastern Pacific Ocean using metagenomics, nitrification rates, and single-cell stable-isotope-uptake measurements with nanoscale secondary ion mass spectrometry (nanoSIMS). We find that the majority (>60%) of active cells across all samples assimilated urea-derived N, and that cell-specific nitrogen-incorporation rates from urea were higher than that from ammonium. Both urea concentrations and assimilation rates relative to ammonium generally increased below the euphotic zone. We detected ammonia- and urea-based nitrification at all depths at one of two sites analyzed, demonstrating their potential to support chemoautotrophy in the mesopelagic and bathypelagic regions. Using newly generated metagenomes we find that the ureC gene, encoding the catalytic subunit of urease, is found within 39% of deep-sea cells in this region, including the Nitrosophaerota (likely for nitrification) as well as thirteen other phyla such as Proteobacteria, Verrucomicrobia, Plantomycetota, Nitrospinota, and Chloroflexota (likely for assimilation). Analysis of public metagenomes revealed ureC within 10-46% of deep-sea cells around the world, with higher prevalance below the photic zone, suggesting urea is widely available to the deep-sea microbiome globally. Our results demonstrate that urea is a nitrogen source to abundant and diverse microorganisms in the dark ocean, as well as a significant contributor to deep-sea nitrification and therefore fuel for chemoautotrophy.

microbiology↗

bioRxiv2024

Abundant and metabolically flexible lineages within the SAR324 and gammaproteobacteria dominate the potential for rubisco-mediated carbon fixation in the dark ocean

BackgroundRubisco is among the most abundant enzymes on Earth and is a critical conduit for inorganic carbon into the biosphere. Despite this, the full extent of rubisco diversity and the biology of organisms that employ it for carbon fixation are still emerging, particularly in unlit ecosystems like the deep sea. ResultsHere, we generate fifteen deeply-sequenced metagenomes along a spatially-resolved transect off the California coast, and combine them with globally-distributed public data to examine the diversity, distribution, and metabolic features of rubisco-encoding organisms from the dark water column. Organisms with rubisco were detected in the vast majority of all samples, spanning over 1,000 species groups and together comprising up to [~]20% of the total microbial community. At 150 meters and below, potential for carbon fixation via rubisco was dominated by just two orders of gammaproteobacteria and SAR324, encoding either the form I or II rubisco. Many of these organisms also possessed genes for the oxidation of reduced sulfur compounds, which may energetically support carbon fixation. Transcriptomic profiling in the epi- and mesopelagic suggested that all major forms of rubisco can be highly expressed in the deep water column, but are not done so constitutively, consistent with metabolic flexibility. ConclusionOur results demonstrate that the genetic potential to fix carbon via rubisco is significant and spatially widespread in the dark ocean. We identify several rubisco-encoding species groups that are particularly abundant and cosmopolitan, highlighting the key role they may play in deep-sea chemoautotrophy and the global marine carbon cycle.

microbiology↗

bioRxiv2024

Characterization of sediment and granite hosted deep underground research laboratories reveals diverse microbiome functions, limited temporal variation and substantial genomic conservation

Underground research laboratories (URLs) provide a window on the deep biosphere and enable investigation of potential microbial impacts on nuclear waste, CO2 and H2 stored in the subsurface. We carried out the first multi-year study of groundwater microbiomes sampled from defined intervals between 140 and 400 m below the surface of the Horonobe and Mizunami URLs, Japan. We reconstructed draft genomes for >90% of all organisms detected over a four year period. The Horonobe and Mizunami microbiomes are dissimilar, likely because the Mizunami URL is hosted in granitic rock and the Horonobe URL in sedimentary rock. Despite this, hydrogen metabolism, rubisco-based CO2 fixation, reduction of nitrogen compounds and sulfate reduction are well represented functions in microbiomes from both URLs, although methane metabolism is more prevalent at the organic- and CO2-rich Horonobe URL. High fluid flow zones and proximity to subsurface tunnels select for candidate phyla radiation bacteria in the Mizunami URL. We detected near-identical genotypes for approximately one third of all genomically defined organisms at multiple depths within the Horonobe URL. This cannot be explained by inactivity, as in situ growth was detected for some bacteria, albeit at slow rates. Given the current low hydraulic conductivity and groundwater compositional heterogeneity, ongoing inter-site strain dispersal seems unlikely. Alternatively, the Horonobe URL microbiome homogeneity may be explained by higher groundwater mobility during the last glacial period. Genotypically-defined species closely related to those detected in the URLs were identified in three other subsurface environments in the USA. Thus, dispersal rates between widely separated underground sites may be fast enough relative to mutation rates to have precluded substantial divergence in species composition. Species overlaps between subsurface locations on different continents constrain expectations regarding the scale of global subsurface biodiversity. Overall, microbiome and geochemical stability over the study period has important implications for underground storage applications.

microbiology↗

bioRxiv2022

Host translation machinery is not a barrier to phages that infect both CPR and non-CPR bacteria

Within human microbiomes, Gracilibacteria, Absconditabacteria, and Saccharibacteria, members of Candidate Phyla Radiation (CPR), are increasingly correlated with human oral health and disease. We profiled the diversity of CRISPR-Cas systems in the genomes of these bacteria and sought phages that are capable of infecting them by comparing their spacer inventories to large phage sequence databases. Gracilibacteria and Absconditabacteria recode the typical TGA stop codon to glycine and are infected by phages that share their hosts alternate genetic code. Unexpectedly, however, other predicted phages of Gracilibacteria and Absconditabacteria do not use an alternative genetic code. Some of these phages are predicted to infect both alternatively coded CPR bacteria and standard coded bacteria. These phages rely on other stop codons besides TGA, and thus should be capable of producing viable gene products in either bacterial host type. Interestingly, we predict that phages of Saccharibacteria can replicate in Actinobacteria, which have been shown to act as episymbiotic hosts for Saccharibacteria. Overall, the broad host range of some CPR phages may be advantageous for the production of these phages for microscopic characterization or use as therapy agents, given the current difficulty of CPR cultivation. Absconditabacteria phages and Gracilibacteria phages may have avoided acquisition of in-frame stop codons to increase the diversity of bacteria in which they can replicate.

microbiology↗

bioRxiv2022

Autotrophic biofilms sustained by deeply-sourced groundwater host diverse CPR bacteria implicated in sulfur and hydrogen metabolism

BackgroundCandidate Phyla Radiation (CPR) bacteria are commonly detected yet enigmatic members of diverse microbial communities. Their host associations, metabolic capabilities, and potential roles in biogeochemical cycles remain under-explored. We studied chemoautotrophically-based biofilms that host diverse CPR bacteria and grow in sulfide-rich springs using bulk geochemical analysis, genome-resolved metagenomics and scanning transmission x-ray microscopy (STXM) at room temperature and 87{degrees} K. ResultsCPR-affiliated Gracilibacteria, Absconditabacteria, Saccharibacteria, Peregrinibacteria, Berkelbacteria, Microgenomates, and Parcubacteria are members of two biofilm communities dominated by chemolithotrophic sulfur-oxidizing bacteria including Thiothrix or Beggiatoa. STXM imaging revealed ultra-small cells along the surfaces of filamentous bacteria that we interpret are CPR bacterial episymbionts. STXM and NEXAFS spectroscopy at carbon K and sulfur L2,3 edges show protein-encapsulated elemental sulfur spherical granules associated with filamentous bacteria, indicating that they are sulfur-oxidizers, likely Thiothrix. Berkelbacteria and Moranbacteria in the same biofilm sample are predicted to have a novel electron bifurcating group 3b [NiFe]-hydrogenase, putatively a sulfhydrogenase, potentially linked to sulfur metabolism via redox cofactors. This complex could potentially underpin a symbiosis involving Berkelbacteria and/or Moranbacteria and filamentous sulfur-oxidizing bacteria such as Thiothrix that is based on cryptic sulfur cycling. One Doudnabacteria genome encodes adjacent sulfur dioxygenase and rhodanese genes that may convert thiosulfate to sulfite. We find similar conserved genomic architecture associated with CPR bacteria from other sulfur-rich subsurface ecosystems. ConclusionsOur combined metagenomic, geochemical, spectromicroscopic and structural bioinformatics analyses link some CPR bacteria to sulfur-oxidizing Proteobacteria, likely Thiothrix, and indicate roles for CPR bacteria in sulfur and hydrogen cycling.

microbiology↗

bioRxiv2022

Variable impact of geochemical gradients on the functional potential of bacteria, archaea, and phages from the permanently stratified Lac Pavin

Permanently stratified lakes contain diverse microbial communities that vary with depth, and so serve as useful models for studying the relationships between microbial community structure and geochemistry. Recent work has shown that these lakes can also harbor numerous bacteria and archaea from novel lineages, including those from the Candidate Phyla Radiation (CPR). However, the extent to which geochemical stratification differentially impacts carbon metabolism and overall genetic potential in CPR bacteria compared to other organisms is not well defined. Here, we determine the distribution of microbial lineages along an oxygen gradient in Lac Pavin, a deep, stratified lake in central France, and examine the influence of this gradient on their metabolism. Genome-based analyses revealed an enrichment of distinct C1 and CO2 fixation pathways in the oxic lake interface and anoxic zone/sediments, suggesting that oxygen likely plays a role in structuring metabolic strategies in non-CPR bacteria and archaea. Notably, we find that oxidation of methane and its byproducts is largely spatially separated from methane production, which is mediated by diverse communities of sediment methanogens that vary on the centimeter scale. In contrast, we detected evidence for RuBisCO throughout the water column and sediments, including form II/III and form III-related enzymes encoded by CPR bacteria in the water column and DPANN archaea in the sediments. Overall, CPR bacteria and phages did not show strong signals of gene content differentiation by depth, despite the fact that distinct species groups populate different lake and sediment compartments. Thus, environmental gradients in Lac Pavin probably select for the capacities of CPR bacteria and phages to a lesser extent than other bacteria and archaea, possibly because selection on the former groups is indirect and depends on host characteristics.

microbiology↗

bioRxiv2022

Saccharibacteria harness light energy using Type-1 rhodopsins that may rely on retinal sourced from microbial hosts

Microbial rhodopsins are a family of photoreceptive membrane proteins with a wide distribution across the Tree of Life. Within the Candidate Phyla Radiation (CPR), a diverse group of putatively episymbiotic bacteria, the genetic potential to produce rhodopsins appears to be confined to a small clade of organisms from sunlit environments. Here, we characterize the metabolic context and biophysical features of Saccharibacteria Type-1 rhodopsin sequences derived from metagenomic surveys and show that these proteins function as outward proton pumps. This provides one of the only known mechanisms by which CPR can generate a proton gradient for ATP synthesis. Intriguingly, Saccharibacteria do not encode the genetic machinery to produce all-trans-retinal, the chromophore essential for rhodopsin function, but their rhodopsins are able to rapidly uptake this cofactor when provided in experimental assays. We found consistent evidence for the capacity to produce retinal from {beta}-carotene in organisms co-occurring with Saccharibacteria, and this genetic potential was dominated by Actinobacteria, which are known hosts of Saccharibacteria in other habitats. If Actinobacteria serve as hosts for Saccharibacteria in freshwater environments, exchange of retinal for use by rhodopsin may be a feature of their associations.

microbiology↗

bioRxiv2021

Phage-encoded ribosomal protein S21 expression is linked to late stage phage replication

The ribosomal protein S21 (bS21) gene has been detected in diverse viruses with a large range of genome sizes, yet its in situ expression and potential significance have not been investigated. Here, we report five closely related clades of bacteriophages (phages) represented by 47 genomes (8 curated to completion and up to 331 kbp in length) that encode a bS21 gene. The bS21 gene is on the reverse strand within a conserved region that encodes the large terminase, major capsid protein, prohead protease, portal vertex proteins and some hypothetical proteins. These phages are predicted to infect Bacteroidetes species that inhabit a range of depths in freshwater lakes. Transcriptionally active bS21-encoding phages were sampled in the late-stage of replication, when core structural genes, bS21 and a neighboring gene of unknown function were highly expressed. Thus, our analyses suggest that bS21, which is involved in translation initiation, substitutes into the Bacteroidetes ribosomes and selects for phage transcripts during the late-stage replication when large-scale phage protein production is required for assembly of phage particles.

microbiology↗

bioRxiv2021

Long-term incubation of lake water enables genomic sampling of consortia involving Planctomycetes and Candidate Phyla Radiation bacteria

Microbial communities in lakes can profoundly impact biogeochemical processes through their individual activities and collective interactions. However, the complexity of these communities poses challenges, particularly for studying rare members. Laboratory enrichments can select for subsystems of interacting organisms and enable genome recovery for enriched populations. Here, a reactor inoculated with water from Lake Fargette, France, and maintained under dark conditions at 4{degrees}C for 31 months enriched for diverse Planctomycetes and Candidate Phyla Radiation (CPR) bacteria. We reconstructed draft genomes and predicted metabolic traits for 12 diverse Planctomycetes and 9 CPR bacteria, some of which are likely representatives of undescribed families or genera. One CPR genome representing the little-studied lineage Peribacter (1.239 Mbp) was curated to completion, and unexpectedly, encodes the full gluconeogenesis pathway. Metatranscriptomic data indicate that some Planctomycetes and CPR bacteria were active under the culture conditions. We also reconstructed genomes and obtained transmission electron microscope images for numerous phages, including one with a >300 kbp genome and several predicted to infect Planctomycetes. Together, our analyses suggest that freshwater Planctomycetes may act as hubs for interaction networks that include symbiotic CPR bacteria and phages.

microbiology↗

bioRxiv2021

Stop codon recoding is widespread in diverse phage lineages and has the potential to regulate translation of late stage and lytic genes

The genetic code is a highly conserved feature of life. However, some "alternative" genetic codes use reassigned stop codons to code for amino acids. Here, we survey stop codon recoding across bacteriophages (phages) in human and animal gut microbiomes. We find that stop codon recoding has evolved in diverse clades of phages predicted to infect hosts that use the standard code. We provide evidence for an evolutionary path towards recoding involving reduction in the frequency of TGA and TAG stop codons due to low GC content, followed by acquisition of suppressor tRNAs and the emergence of recoded stop codons in structural and lysis genes. In analyses of two distinct lineages of recoded virulent phages, we find that lysis-related genes are uniquely biased towards use of recoded stop codons. This convergence supports the inference that stop codon recoding is a strategy to regulate the expression of late stage genes and control lysis timing. Interestingly, we identified prophages with recoded stop codons integrated into genomes of bacteria that use standard code, and hypothesize that recoding may control the lytic-lysogenic switch. Alternative coding has evolved many times, often in closely related lineages, indicating that genetic code is plastic in bacteriophages and adaptive recoding can occur over very short evolutionary timescales.

microbiology↗

bioRxiv2021

Patterns of gene content and co-occurrence constrain the evolutionary path toward animal association in CPR bacteria

Candidate Phyla Radiation (CPR) bacteria are small, likely episymbiotic organisms found across Earths ecosystems. Despite their prevalence, the distribution of CPR lineages across habitats and the genomic signatures of transitions amongst these habitats remain unclear. Here, we expand the genome inventory for Absconditabacteria (SR1), Gracilibacteria, and Saccharibacteria (TM7), CPR bacteria known to occur in both animal-associated and environmental microbiomes, and investigate variation in gene content with habitat of origin. By overlaying phylogeny with habitat information, we show that bacteria from these three lineages have undergone multiple transitions from environmental habitats into animal microbiomes. Based on co-occurrence analyses of hundreds of metagenomes, we extend the prior suggestion that certain Saccharibacteria have broad bacterial host ranges and constrain possible host relationships for Absconditabacteria and Gracilibacteria. Full-proteome analyses show that animal-associated Saccharibacteria have smaller gene repertoires than their environmental counterparts and are enriched in numerous protein families, including those likely functioning in amino acid metabolism, phage defense, and detoxification of peroxide. In contrast, some freshwater Saccharibacteria encode a putative rhodopsin. For protein families exhibiting the clearest patterns of differential habitat distribution, we compared protein and species phylogenies to estimate the incidence of lateral gene transfer and genomic loss occurring over the species tree. These analyses suggest that habitat transitions were likely not accompanied by large transfer or loss events, but rather were associated with continuous proteome remodeling. Thus, we speculate that CPR habitat transitions were driven largely by availability of suitable host taxa, and were reinforced by acquisition and loss of some capacities. IMPORTANCEStudying the genetic differences between related microorganisms from different environment types can indicate factors associated with their movement among habitats. This is particularly interesting for bacteria from the Candidate Phyla Radiation because their minimal metabolic capabilities require symbiotic associations with microbial hosts. We found that shifts of Absconditabacteria, Gracilibacteria, and Saccharibacteria between environmental ecosystems and mammalian mouths/guts probably did not involve major episodes of gene gain and loss; rather, gradual genomic change likely followed habitat migration. The results inform our understanding of how little-known microorganisms establish in the human microbiota where they may ultimately impact health.

microbiology↗