Biology subjects

Rappe, M. S.

Publications and source records attributed to Rappe, M. S..

12 recordsLinked to original sources

bioRxiv2026

Synteny-aware microbial pangenome graphs reveal blueprints of genomic variation

Pangenomics quantifies the conserved and variable gene repertoire among genomes, but popular implementations ignore gene synteny. Graph-based approaches incorporate both gene homology and synteny, but become difficult to interpret due to pervasive rearrangements. Here we present network-pruning and graph-layout algorithms that enable interactive, synteny-aware quantification and visualization of gene conservation and variability. Applied to 29 genomes of the marine genus Undatipelagibacter (formerly SAR11 subclade Ia.3.VI), we find that genomic variability forms not a few hypervariable islands against a static backbone but a structured continuum, whose variable regions differ in scale, topology, function, and evolutionary character. Genome variation spans from ancient, specialized regions of hundreds of genes whose propensity to vary is conserved across genera, to single hypervariable genes shaped by epistatic co-selection with partners dispersed genome-wide, and shows that chromosomal context carries evolutionary information synteny-unaware pangenomics cannot capture, and some evolutionary processes act on entire functional subsystems throughout a pangenome.

microbiology↗

bioRxiv2025

A high-resolution diel survey of surface ocean metagenomes, metatranscriptomes, and transfer RNA transcripts

The roles of marine microbes in ecosystem processes are inherently linked to their ability to sense, respond, and ultimately adapt to environmental change. Capturing the nuances of this perpetual dialogue and its long-term implications requires insight into the subtle drivers of microbial responses to environmental change that are most accessible at the shortest scales of time. Here, we present a multi-omics dataset comprising surface ocean metagenomes, metatranscriptomes, tRNA transcripts, and biogeochemical measurements, collected every 1.5 hours for 48 hours at two stations within coastal and adjacent offshore waters of the tropical Pacific Ocean. We expect that this integrated dataset of multiple sequence types and environmental parameters will facilitate novel insights into microbial ecology, microbial physiology, and ocean biogeochemistry and help investigate the different mechanisms of adaptation that drive microbial responses to environmental change.

microbiology↗

bioRxiv2025

Magnimaribacterales marine bacteria genetically partition across the nearshore to open-ocean in the tropical Pacific Ocean

The bacterial order Magnimaribacterales, previously known as the SAR86 lineage, is among the most abundant groups of planktonic bacteria inhabiting the global surface ocean. Despite their prevalence, our understanding of how this genetically diverse lineage partitions into units with coherent ecology and evolution remains limited. Here we surveyed multiple stations in the tropical Pacific Ocean using shotgun metagenomes and 16S rRNA gene amplicons to resolve distinct habitat preferences for Magnimaribacterales lineages across nearshore, offshore, and open-ocean environments. The comprehensive collection of genomes that captured a large fraction of the known evolutionary breadth of Magnimaribacterales, revealed patterns of ecotypic differentiation manifested primarily among genus-level clusters with specific clear preferences for distinct marine habitats. Enrichment analyses identified several functional genes associated with genomes from genera abundant in the nearshore environment, including those associated with sugar metabolism, peptide transport, and glycerophospholipid biosynthesis. Such metabolic adaptations likely facilitate the predominance of specific Magnimaribacterales genera in nearshore environments, promoting ecological partitioning across marine habitats. ImportanceUnderstanding the nature by which abundant, genetically-diverse planktonic marine bacteria organize into evolutionarily related and functionally coherent units remains an important question for scientists interested in the ecology of the global ocean. The bacterial order Magnimaribacterales (formerly SAR86) is one of the most prevalent clades in the global surface ocean, yet its ecological differentiation remains poorly resolved. By integrating metagenomic and amplicon analyses across a nearshore-to-open-ocean gradient in the tropical Pacific, this study reveals distinct habitat preferences within this lineage. The study also highlights how differences in distribution appear are primarily found between genus-level groupings. Our findings also highlight key functional traits, including sugar metabolism and glycerophospholipid biosynthesis, that were associated with the partitioning of nearshore and open-ocean Magnimaribacterales. This work enhances our understanding of the evolutionary processes shaping the diversity of one of the oceans most abundant bacterial clades.

microbiology↗

bioRxiv2024

Habitat-specificity in SAR11 is associated with a handful of genes under high selection

The order Pelagibacterales (SAR11) is the most abundant group of heterotrophic bacteria in the global surface ocean, where individual sublineages likely play distinct roles in oceanic biogeochemical cycles. Yet, understanding the determinants of niche partitioning within SAR11 has been a formidable challenge due to the high genetic diversity within individual SAR11 sublineages and the limited availability of high-quality genomes from both cultivation and metagenomic reconstruction. Here, we take advantage of 71 new SAR11 genomes from strains we isolated from the tropical Pacific Ocean to evaluate the distribution of metabolic traits across the Pelagibacteraceae, a recently classified family within the order Pelagibacterales encompassing subgroups Ia and Ib. Our analyses of metagenomes generated from stations where the strains were isolated reveals distinct habitat preferences across SAR11 genera for coastal or offshore environments, and subtle but systematic differences in metabolic potential that support these observations. We also observe higher levels of selective forces acting on habitat-specific metabolic genes linked to SAR11 fitness and polyphyletic distributions of habitat preferences and metabolic traits across SAR11 genera, suggesting that contrasting lifestyles have emerged across multiple lineages independently. Together, these insights reveal niche-partitioning within sympatric and parapatric populations of SAR11 and demonstrate that the immense genomic diversity of SAR11 bacteria naturally segregates into ecologically and genetically cohesive units, or ecotypes, that vary in spatial distributions in the tropical Pacific.

microbiology↗

bioRxiv2024

New isolate genomes and global marine metagenomes resolve ecologically relevant units of SAR11

The bacterial order Pelagibacterales (SAR11) is widely distributed across the global surface ocean, where its activities are integral to the marine carbon cycle. High-quality genomes from isolates that can be propagated and phenotyped are needed to unify perspectives on the ecology and evolution of this complex group. Here, we increase the number of complete SAR11 isolate genomes threefold by describing 81 new SAR11 strains from coastal and offshore surface seawater of the tropical Pacific Ocean. Our analyses of the genomes and their spatiotemporal distributions support the existence of 29 monophyletic, discrete Pelagibacterales ecotypes that we define as genera. The spatiotemporal distributions of genomes within genera were correlated at fine scales with variation in ecologically-relevant gene content, supporting generic assignments and providing indications of speciation. We provide a hierarchical system of classification for SAR11 populations that is meaningfully correlated with evolution and ecology, providing a valid and utilitarian systematic nomenclature for this clade.

microbiology↗

bioRxiv2024

Sharp transitions in phytoplankton communities across estuarine to open ocean waters of the tropical Pacific

Islands in the tropical Pacific supply elevated nutrients to nearshore waters that enhance phytoplankton biomass and create hotspots of productivity in otherwise nutrient-poor oceans. Despite the importance of these hotspots in supporting nearshore food webs, the spatial and temporal variability of phytoplankton enhancement and changes in the underlying phytoplankton communities across nearshore to open ocean systems remain poorly understood. In this study, a combination of flow cytometry, pigment analyses, 16S rRNA gene amplicons, and metagenomic sequencing provide a synoptic view of phytoplankton dynamics over a four-year, near-monthly time-series across coastal K[a]neohe Bay, Hawaii, spanning from an estuarine Indigenous aquaculture system to the adjacent offshore environment. Through comparisons with measurements taken at Station ALOHA located in the oligotrophic North Pacific Subtropical Gyre, we observed a sharp and persistent transition between picocyanobacterial communities, from Synechococcus clade II abundant in the nearshore to Prochlorococcus HLII proliferating in offshore and open ocean waters. In comparison to immediately adjacent offshore waters and the surrounding open ocean, phytoplankton biomass within K[a]neohe Bay was dramatically elevated. Members of the phytoplankton community revealed strong seasonal patterns, while nearshore phytoplankton biomass positively correlated with wind speed, rainfall, and wind direction, and not water temperatures. These findings elucidate the spatiotemporal dynamics underlying transitions in ocean biogeochemistry and phytoplankton dynamics across estuarine to open ocean waters in the tropical Pacific and provide a foundation for quantifying deviations from baseline conditions due to ongoing climate change.

microbiology↗

bioRxiv2024

Isolate-anchored comparisons reveal evolutionary and functional differentiation across SAR86 marine bacteria

SAR86 is one of the most abundant groups of bacteria in the global surface ocean. However, since its discovery over 30 years ago, it has remained recalcitrant to isolation and many details regarding this group are still unknown. Here we report the cellular characteristics from the first SAR86 isolate brought into culture, Candidatus Magnimaribacter mokuoloeensis strain HIMB1674, and use its closed genome in concert with over 700 environmental genomes to assess the phylogenomic and functional characteristics of this order-level lineage of marine Gammaproteobacteria. The Magnimaribacterales invest significant genomic resources into the capacity for {beta}-oxidation, which is present in most genomes in high gene copy numbers. This cyclical set of reactions is fed by components of cell membranes that includes lipids such as phosphatidylcholine, phosphatidylethanolamine, glycolipids, and sulfolipids. In addition to the widespread capacity to degrade the side chain of steroidal compounds via {beta}-oxidation, several SAR86 sublineages also appear able to fully degrade the steroid polycyclic ring structure as well as other aromatic, polycyclic, and heterocyclic molecules. Read recruitment from publicly available metagenomes reveals that the Magnimaribacterales compose up to 6% of the global surface ocean microbial community. Only a subset of genera drive these high relative abundances, with some more globally dominant and others restricted to specific oceanic regions. Candidatus Magnimaribacter mokuoloeensis provides an unprecedented foundation through which to understand this highly abundant yet poorly understood lineage of marine bacteria, and charts a path to bring more representatives of this order into laboratory culture.

microbiology↗

bioRxiv2022

Structure-informed microbial population genetics elucidate selective pressures that shape protein evolution

Comprehensive sampling of natural genetic diversity with metagenomics enables highly resolved insights into the interplay between ecology and evolution. However, intra-population genomic variation represents the outcome of both stochastic and selective forces, making it difficult to identify whether variants are maintained by adaptive, neutral, or purifying processes. This is partly due to the reliance on gene sequences to interpret variants, which disregards the physical properties of three-dimensional gene products that define the functional landscape on which selection acts. Here we describe an approach to analyze genetic variation in the context of predicted protein structures, and apply it to study a marine microbial population within the SAR11 subclade 1a.3.V, which dominates low-latitude surface oceans. Our analyses reveal a tight association between the patterns of nonsynonymous polymorphism, selective pressures, and structural properties of proteins such as per-site relative solvent accessibility and distance to ligands, which explain up to 59% of genetic variance in some genes. In glutamine synthetase, a central gene in nitrogen metabolism, we observe decreased occurrence of nonsynonymous variants from ligand binding sites as a function of nitrate concentrations in the environment, revealing genetic targets of distinct evolutionary pressures maintained by nutrient availability. Our data also reveals that rare codons are purified from ligand binding sites when genes are under high selection, demonstrating the utility of structure-aware analyses to study the variants that likely impact translational processes. Overall, our work yields insights into the governing principles of evolution that shape the genetic diversity landscape within a globally abundant population, and makes available a software framework for structure-aware investigations of microbial population genetics. SignificanceIncreasing availability of metagenomes offers new opportunities to study evolution, but the equal treatment of all variants limits insights into drivers of sequence diversity. By capitalizing on recent advances in protein structure prediction capabilities, our study examines subtle evolutionary dynamics of a microbial population that dominates surface oceans through the lens of structural biology. We demonstrate the utility of structure-informed metrics to understand the distribution of nonsynonymous polymorphism, establish insights into the impact of changing nutrient availability on protein evolution, and show that even synonymous variants are scrutinized strictly to maximize translational efficiency when selection is high. Overall, our work illustrates new opportunities for discovery at the intersection between metagenomics and structural bioinformatics, and offers an interactive and scalable software platform to visualize and analyze genetic variants in the context of predicted protein structures and ligand-binding sites.

microbiology↗

bioRxiv2021

Ecophysiology of the cosmopolitan OM252 bacterioplankton (Gammaproteobacteria)

Among the thousands of species that comprise marine bacterioplankton communities, most remain functionally obscure. One key cosmopolitan group in this understudied majority is the OM252 clade of Gammaproteobacteria. Although frequently found in sequence data and even previously cultured, the diversity, metabolic potential, physiology, and distribution of this clade has not been thoroughly investigated. Here we examined these features of OM252 bacterioplankton using a newly isolated strain and genomes from publicly available databases. We demonstrated that this group constitutes a globally distributed novel genus (Candidatus Halomarinus), sister to Litoricola, comprising two subclades and multiple distinct species. OM252 organisms have small genomes (median 2.21 Mbp) and are predicted obligate aerobes capable of alternating between chemoorganoheterotrophic and chemolithotrophic growth using reduced sulfur compounds as electron donors, with subclade I genomes encoding the Calvin-Benson-Bassham cycle for carbon fixation. One representative strain of subclade I, LSUCC0096, had extensive halotolerance but a mesophilic temperature range for growth, with a maximum of 0.36 doublings/hr at 35{degrees}C. Cells were curved rod/spirillum-shaped, ~1.5 x 0.2 m. Growth on thiosulfate as the sole electron donor under autotrophic conditions was roughly one third that of heterotrophic growth, even though calculations indicated similar Gibbs energies for both catabolisms. These phenotypic data show that some Ca. Halomarinus organisms can switch between serving as carbon sources or sinks and indicate the likely anabolic cost of lithoautotrophic growth. Our results thus provide new hypotheses about the roles of these organisms in global biogeochemical cycling of carbon and sulfur. ImportanceMarine microbial communities are teeming with understudied taxa due to the sheer numbers of species in any given sample of seawater. One group, the OM252 clade of Gammaproteobacteria, has been identified in gene surveys from myriad locations, and one isolated organism has even been genome sequenced (HIMB30). However, further study of these organisms has not occurred. Using another isolated representative (strain LSUCC0096) and publicly available genome sequences from metagenomic and single-cell genomic datasets, we examined the diversity within the OM252 clade, the distribution of these taxa in the worlds oceans, reconstructed the predicted metabolism of the group, and quantified growth dynamics in LSUCC0096. Our results generate new knowledge about the previously enigmatic OM252 clade and point towards the importance of facultative chemolithoautotrophy for supporting some clades of ostensibly "heterotrophic" taxa.

microbiology↗

bioRxiv2020

Isolation of SAR11 marine bacteria from cryopreserved seawater

In this study, we sought a means to increase current culture collections of SAR11 marine bacteria by testing the use of seawater cryopreserved with glycerol as an inoculum. In July 2017, raw seawater was collected outside of K[a]ne ohe Bay, Hawai i, in the tropical Pacific Ocean. A portion of this sample was diluted in seawater-based growth medium to create 576 x 2 mL dilution cultures containing 5 cells each and incubated for a high-throughput cultivation experiment, while another portion was cryopreserved in 10% glycerol. After ten months, a cryopreserved aliquot of seawater was thawed, diluted in seawater-based growth medium, and distributed to create a second high-throughput cultivation experiment of 480 x 2 mL dilution cultures containing 5 cells each and 94 cultures containing 105 cells each. The raw seawater cultivation experiment resulted in the successful isolation of 54 monocultures and 29 mixed-cultures, while cryopreserved seawater resulted in 59 monocultures and 29 mixed cultures. Combined, the cultures included 51 SAR11 isolates spanning 11 unique 16S rRNA gene amplicon sequence variants (ASVs) from raw seawater inoculum and 74 SAR11 isolates spanning 13 unique ASVs from cryopreserved seawater. A vast majority (115 of 125) of SAR11 isolates from the two HTC experiments were members of SAR11 subclade Ia, though isolates of subclades IIIa and Va were also recovered from cryopreserved seawater and subclade Ib was recovered from both. The four most abundant SAR11 subclade Ia ASVs found in the initial seawater sample used to create both culture experiments were isolated by both approaches. ImportanceHigh-throughput dilution culture has proved to be a successful approach to bring some difficult-to-isolate planktonic microorganisms into culture, including the highly abundant SAR11 lineage of marine bacteria. While the long-term preservation of bacterial isolates by freezing in the presence of cryoprotectants such as glycerol has been shown to be an effective method of storing viable cells over long time periods (i.e. years), to our knowledge it had not previously been tested for its efficacy in preserving raw seawater for later use as inoculum for high-throughput cultivation experiments. We found that SAR11 and other abundant marine bacteria could be isolated from seawater that was previously cryopreserved for nearly 10 months, at a rate of culturability similar to that of the same seawater used fresh, immediately after collection. Our findings expand the potential of high-throughput cultivation experiments to include opportunities where immediate isolation experiments are impractical, allow for targeted isolation experiments from specific samples based on analyses such as microbial community structure, and enable cultivation experiments across a wide range of other conditions that would benefit from having source inoculum available over extended periods of time.

microbiology↗

bioRxiv2017

The global biogeography of amino acid variants within a single SAR11 population is governed by natural selection

The diversity and geographical distribution of populations within major marine microbial lineages are largely governed by temperature and its co-variables. However, neither the mechanisms by which genomic heterogeneity emerges within a single population nor how it drives the partitioning of ecological niches are well understood. Here we took advantage of billions of metagenomic reads to study one of the most abundant and widespread microbial populations in the surface ocean. We characterized its substantial amount of genomic heterogeneity using single-amino acid variants (SAAVs), and identified systematic purifying selection and adaptive mechanisms governing non-synonymous variation within this population. Our Deep Learning analysis of SAAVs across metagenomes revealed two main ecological niches that reflect large-scale oceanic current temperatures, as well as six proteotypes demarcating finer-resolved niches. We identified significantly more protein variants in cold currents and an increased number of protein sweeps in warm currents, exposing a global pattern of alternating genomic diversity for this SAR11 population as it drifts along with surface ocean currents. Overall, the geographic partitioning of SAAVs suggests natural selection, rather than neutral evolution, is the main driver of the evolution of SAR11 in surface oceans.

microbiology↗

bioRxiv2017

Dramatic expansion of microbial groups that shape the global sulfur cycle

The biogeochemical cycle of sulfur on Earth is driven by microbial sulfate reduction, yet organisms from relatively few lineages have been implicated in this process. Recent studies using functional marker genes have detected abundant, novel dissimilatory sulfite reductases that confer the capacity for microbial sulfate reduction and could do not be affiliated with known organisms. Thus, the identity of a significant fraction of sulfate reducing microbes has remained elusive. Here we report the discovery of the capacity for sulfate reduction in the genomes of organisms from twelve bacterial and archaeal phyla, thereby doubling the number of microbial phyla associated with this process. Eight of the twelve newly identified groups are candidate phyla that lack isolated representatives, a finding only possible given genomes from metagenomes. Two candidate phyla, Candidatus Rokubacteria and Candidatus Hydrothermarchaeota contain the earliest evolved genes. The capacity for sulfate reduction has been laterally transferred in multiple events within some phyla, and a key gene potentially capable of switching sulfur oxidation to sulfate reduction in associated cells has been acquired by putatively symbiotic bacteria. We conclude that functional predictions based on phylogeny will significantly underestimate the extent of sulfate reduction across Earths ecosystems. Understanding the prevalence of this capacity is integral to interpreting the carbon cycle because sulfate reduction is often coupled to turnover of buried organic carbon. Our findings expand the diversity of microbial groups associated with sulfur transformations in the environment and motivate revision of biogeochemical process models based on microbial community composition.

microbiology↗