Ecological Genomics of the Uncultivated Marine Roseobacter Lineage CHAB-I-5

How Big Is Big? The Effective Population Size of Marine Bacteria

Genome-reduced bacteria constitute most of the cells in surface-ocean bacterioplankton communities. Their extremely large census population sizes (N c) have been unfoundedly translated to huge effective population sizes (N e)-the size of an ideal population carrying as much neutral genetic diversity as the actual population. As N e scales inversely with the strength of genetic drift, constraining the magnitude of N e is key to evaluating whether natural selection can overcome the power of genetic drift to drive evolutionary events. Determining the N e of extant species requires measuring the genomic mutation rate, a challenging step for most genome-reduced bacterioplankton lineages. Results for genome-reduced Prochlorococcus and CHUG are surprising-their N e values are an order of magnitude lower than those of less abundant lineages carrying large genomes, such as Ruegeria and Vibrio. As bacterioplankton genome reduction commonly occurred in the distant past, appreciating their population genetic mechanisms requires constraining their ancient N e values by other methods.

Diversity and distribution of a prevalent Microviridae group across the global oceans

Small single-stranded DNA phages of the Microviridae family are diverse and prevalent in oceans. Our understanding of Microviridae phages that infect the ecologically important marine Roseobacter is currently limited, comprising few isolates. Here, we report six roseophages that infect Roseobacter RCA strains. Genomic and phylogenetic analyses revealed that they were new members of the previously identified subfamily Occultatumvirinae. Additionally, 232 marine uncultivated virus genomes (UViGs) affiliated to Occultatumvirinae were obtained from environmental genome datasets. Phylogenomic analysis revealed that marine Occultatumvirinae phages could be further grouped into 11 subgroups. Moreover, meta-omics based read-mapping analysis showed that Occultatumvirinae phages were globally distributed, with two low G + C subgroups showing the most prevalent distribution. Furthermore, one phage in subgroup 2 was found to be extremely ubiquitous. Overall, this study expands our understanding of the diversity and ecology of the Occultatumvirinae microviruses in the ocean and highlights their ecological impacts.

Novel phage infecting the Roseobacter CHUG lineage reveals a diverse and globally distributed phage family

Bacteriophages play an essential role in shaping the diversity and metabolism of bacterial communities. Marine Roseobacter group is an abundant heterotrophic bacterial group that is involved in many major element cycles, especially carbon and sulfur. Members of the Roseobacter CHUG (Clade Hidden and Underappreciated Globally) lineage are globally distributed and are activated in pelagic marine environments. In this study, we isolated and characterized a phage, CRP-810, that infects the CHUG strain FZCC0198. The genome of CRP-810 was dissimilar to those of other known phages. Additionally, 251 uncultured viral genomes (UViGs) closely related to CRP-810 were obtained from the uncultivated marine viral contig databases. Comparative genomic and phylogenetic analyses revealed that CRP-810 and these related UViGs exhibited conserved genome synteny, representing a new phage family with at least eight subgroups. Most of the CRP-810-type phages contain an integrase gene, and CRP-810 can be integrated into the host genome. Further analysis revealed that three CRP-810-type members were prophages found in the genomes of marine SAR11, Poseidonocella, and Sphingomonadaceae. Finally, viromic read-mapping analysis showed that CRP-810-type phages were globally distributed and displayed distinct biogeographic patterns related to temperature and latitude. Many members with a lower G + C content were mainly distributed in the trade station, whereas members with a higher G + C content were mainly distributed in polar and westerlies station, indicating that the niche differentiation of phages was subject to host adaptation. Collectively, these findings identify a novel phage family and expand our understanding of phylogenetic diversity, evolution, and biogeography of marine phages.

IMPORTANCE

The Roseobacter CHUG lineage, affiliated with the Pelagic Roseobacter Cluster (PRC), is widely distributed in the global oceans and is active in oligotrophic seawater. However, knowledge of the bacteriophages that infect CHUG members is limited. In this study, a CHUG phage, CRP-810, that infects the CHUG strain FZCC0198, was isolated and shown to have a novel genomic architecture. In addition, 251 uncultured viral genomes closely related to CRP-810 were recovered and included in the analyses. Phylogenomic analyses revealed that the CRP-810-type phages represent a new phage family containing at least eight genus-level subgroups. Members of this family were predicted to infect various marine bacteria. We also demonstrated that the CRP-810-type phages are widely distributed in global oceans and display distinct biogeographic patterns related to latitude. Collectively, this study provides important insights into the genomic organization, diversity, and ecology of a novel phage family that infect ecologically important bacteria in the global ocean.

New insights in bacterial organophosphorus cycling: From human pathogens to environmental bacteria

In terrestrial and aquatic ecosystems, phosphorus (P) availability controls primary production, with consequences for climate regulation and global food security. Understanding the microbial controls on the global P cycle is a prerequisite for minimising our reliance on non-renewable phosphate rock reserves and reducing pollution associated with excessive P fertiliser use. This recognised importance has reinvigorated research into microbial P cycling, which was pioneered over 75 years ago through the study of human pathogenic bacteria-host interactions. Immobilised organic P represents a significant fraction of the total P pool. Hence, microbes have evolved a plethora of mechanisms to transform this fraction into labile inorganic phosphate, the building block for numerous biological molecules. The 'genomics era' has revealed an extraordinary diversity of organic P cycling genes exist in the environment and studies going 'back to the lab' are determining how this diversity relates to function. Through this integrated approach, many hitherto unknown genes and proteins that are involved in microbial P cycling have been discovered. Not only do these fundamental discoveries push the frontier of our knowledge, but several examples also provide exciting opportunities for biotechnology and present possible solutions for improving the sustainability of how we grow our food, both locally and globally. In this review, we provide a comprehensive overview of bacterial organic P cycling, covering studies on human pathogens and how this knowledge is informing new discoveries in environmental microbiology.

Metagenome-assembled genomes reveal greatly expanded taxonomic and functional diversification of the abundant marine Roseobacter RCA cluster

BACKGROUND

The RCA (Roseobacter clade affiliated) cluster belongs to the family Roseobacteracea and represents a major Roseobacter lineage in temperate to polar oceans. Despite its prevalence and abundance, only a few genomes and one described species, Planktomarina temperata, exist. To gain more insights into our limited understanding of this cluster and its taxonomic and functional diversity and biogeography, we screened metagenomic datasets from the global oceans and reconstructed metagenome-assembled genomes (MAG) affiliated to this cluster.

RESULTS

The total of 82 MAGs, plus five genomes of isolates, reveal an unexpected diversity and novel insights into the genomic features, the functional diversity, and greatly refined biogeographic patterns of the RCA cluster. This cluster is subdivided into three genera: Planktomarina, Pseudoplanktomarina, and the most deeply branching Candidatus Paraplanktomarina. Six of the eight Planktomarina species have larger genome sizes (2.44-3.12 Mbp) and higher G + C contents (46.36-53.70%) than the four Pseudoplanktomarina species (2.26-2.72 Mbp, 42.22-43.72 G + C%). Cand. Paraplanktomarina is represented only by one species with a genome size of 2.40 Mbp and a G + C content of 45.85%. Three novel species of the genera Planktomarina and Pseudoplanktomarina are validly described according to the SeqCode nomenclature for prokaryotic genomes. Aerobic anoxygenic photosynthesis (AAP) is encoded in three Planktomarina species. Unexpectedly, proteorhodopsin (PR) is encoded in the other Planktomarina and all Pseudoplanktomarina species, suggesting that this light-driven proton pump is the most important mode of acquiring complementary energy of the RCA cluster. The Pseudoplanktomarina species exhibit differences in functional traits compared to Planktomarina species and adaptations to more resource-limited conditions. An assessment of the global biogeography of the different species greatly expands the range of occurrence and shows that the different species exhibit distinct biogeographic patterns. They partially reflect the genomic features of the species.

CONCLUSIONS

Our detailed MAG-based analyses shed new light on the diversification, environmental adaptation, and global biogeography of a major lineage of pelagic bacteria. The taxonomic delineation and validation by the SeqCode nomenclature of prominent genera and species of the RCA cluster may be a promising way for a refined taxonomic identification of major prokaryotic lineages and sublineages in marine and other prokaryotic communities assessed by metagenomics approaches. Video Abstract.

Developing a Base Editing System for Marine Roseobacter Clade Bacteria

The Roseobacter clade bacteria are of great significance in marine ecology and biogeochemical cycles, and they are potential microbial chassis for marine synthetic biology due to their versatile metabolic capabilities. Here, we adapted a CRISPR-Cas-based system, base editing, with the combination of nuclease-deactivated Cas9 and deaminase for Roseobacter clade bacteria. Taking the model roseobacter Roseovarius nubinhibens as an example, we achieved precise and efficient genome editing at single-nucleotide resolution without generating double-strand breaks or requesting donor DNAs. Since R. nubinhibens can metabolize aromatic compounds, we interrogated the key genes in the β-ketoadipate pathway with our base editing system via the introduction of premature STOP codons. The essentiality of these genes was demonstrated, and for the first time, we determined PcaQ as a transcription activator experimentally. This is the first report of CRISPR-Cas-based genome editing in the entire clade of Roseobacter bacteria. We believe that our work provides a paradigm for interrogating marine ecology and biogeochemistry with direct genotype-and-phenotype linkages and potentially opens a new avenue for the synthetic biology of marine Roseobacter bacteria.

Ecological divergence of syntopic marine bacterial species is shaped by gene content and expression

Identifying mechanisms by which bacterial species evolve and maintain genomic diversity is particularly challenging for the uncultured lineages that dominate the surface ocean. A longitudinal analysis of bacterial genes, genomes, and transcripts during a coastal phytoplankton bloom revealed two co-occurring, highly related Rhodobacteraceae species from the deeply branching and uncultured NAC11-7 lineage. These have identical 16S rRNA gene amplicon sequences, yet their genome contents assembled from metagenomes and single cells indicate species-level divergence. Moreover, shifts in relative dominance of the species during dynamic bloom conditions over 7 weeks confirmed the syntopic species' divergent responses to the same microenvironment at the same time. Genes unique to each species and genes shared but divergent in per-cell inventories of mRNAs accounted for 5% of the species' pangenome content. These analyses uncover physiological and ecological features that differentiate the species, including capacities for organic carbon utilization, attributes of the cell surface, metal requirements, and vitamin biosynthesis. Such insights into the coexistence of highly related and ecologically similar bacterial species in their shared natural habitat are rare.

Towards the outwelling hypothesis in a Patagonian estuary: First support from lipid markers and bacterial communities

Biogeochemical markers in combination with bacterial community composition were studied at two contrasting stations at the Río Negro (RN) estuary to assess the outwelling hypothesis in the Argentinian Patagonia. Inorganic nutrients and dissolved organic matter were exported clearly during the last hours of the ebb at the station Wetland. Moreover, a considerable outwelling of polyunsaturated fatty acids (PUFA), particulates and microalgae was inferred by this combined approach. The exported 22:6(n-3) and 20:5(n-3) contributed very likely to sustain higher trophic levels in the coasts of the Southwest Atlantic. The stable isotopes did not evidence clearly the outwelling; nevertheless, the combination of δ13C with fatty acid bacterial markers indicated organic matter degradation in the sediments. The dominance of Desulfobacterales and Desulfuromonadales suggested sulphate reduction in the sediments, a key mechanism for nutrient outwelling in salt marshes. Marivivens and other Rhodobacterales (Alphaproteobacteria) in the suspended particulate matter were clear indicators of the nutrient outwelling. The colonization of particles according to the island biogeography theory was a good hypothesis to explain the lower bacterial biodiversity at the wetland. The copiotrophic conditions of the RN estuary and particularly at the wetland were deduced also by the dynamic of some Actinobacteria, Bacteroidia and Gammaproteobacteria. This high-resolution snapshot combining isotopic, lipid and bacterial markers offers key pioneer insights into biogeochemical and ecological processes of the RN estuary.

Genomic analysis and characterization of phages infecting the marine Roseobacter CHAB-I-5 lineage reveal a globally distributed and abundant phage genus

Marine phages play an important role in marine biogeochemical cycles by regulating the death, physiological metabolism, and evolutionary trajectory of bacteria. The Roseobacter group is an abundant and important heterotrophic bacterial group in the ocean, and plays an important role in carbon, nitrogen, sulfur and phosphorus cycling. The CHAB-I-5 lineage is one of the most dominant Roseobacter lineages, but remains largely uncultured. Phages infecting CHAB-I-5 bacteria have not yet been investigated due to the lack of culturable CHAB-I-5 strains. In this study, we isolated and sequenced two new phages (CRP-901 and CRP-902) infecting the CHAB-I-5 strain FZCC0083. We applied metagenomic data mining, comparative genomics, phylogenetic analysis, and metagenomic read-mapping to investigate the diversity, evolution, taxonomy, and biogeography of the phage group represented by the two phages. The two phages are highly similar, with an average nucleotide identity of 89.17%, and sharing 77% of their open reading frames. We identified several genes involved in DNA replication and metabolism, virion structure, DNA packing, and host lysis from their genomes. Metagenomic mining identified 24 metagenomic viral genomes closely related to CRP-901 and CRP-902. Genomic comparison and phylogenetic analysis demonstrated that these phages are distinct from other known viruses, representing a novel genus-level phage group (CRP-901-type). The CRP-901-type phages do not contain DNA primase and DNA polymerase genes, but possess a novel bifunctional DNA primase-polymerase gene with both primase and polymerase activities. Read-mapping analysis showed that the CRP-901-type phages are widespread across the world's oceans and are most abundant in estuarine and polar waters. Their abundance is generally higher than other known roseophages and even higher than most pelagiphages in the polar region. In summary, this study has greatly expanded our understanding of the genetic diversity, evolution, and distribution of roseophages. Our analysis suggests that the CRP-901-type phage is an important and novel marine phage group that plays important roles in the physiology and ecology of roseobacters.

Marine viruses and climate change: Virioplankton, the carbon cycle, and our future ocean

Interactions between marine viruses and microbes are a critical part of the oceanic carbon cycle. The impacts of virus-host interactions range from short-term disruptions in the mobility of microbial biomass carbon to higher trophic levels through cell lysis (i.e., the viral shunt) to long-term reallocation of microbial biomass carbon to the deep sea through accelerating the biological pump (i.e., the viral shuttle). The biogeochemical backdrop of the ocean-the physical, chemical, and biological landscape-influences the likelihood of both virus-host interactions and particle formation, and the fate and flow of carbon. As climate change reshapes the oceanic landscape through large-scale shifts in temperature, circulation, stratification, and acidification, virus-mediated carbon flux is likely to shift in response. Dynamics in the directionality and magnitude of changes in how, where, and when viruses mediate the recycling or storage of microbial biomass carbon is largely unknown. Integrating viral infection dynamics data obtained from experimental models and field systems, with particle motion microphysics and global observations of oceanic biogeochemistry, into improved ecosystem models will enable viral oceanographers to better predict the role of viruses in marine carbon cycling in the future ocean.

Newly identified HMO-2011-type phages reveal genomic diversity and biogeographic distributions of this marine viral group

Viruses play critical roles in influencing biogeochemical cycles and adjusting host mortality, population structure, physiology, and evolution in the ocean. Marine viral communities are composed of numerous genetically distinct subfamily/genus-level viral groups. Among currently identified viral groups, the HMO-2011-type group is known to be dominant and broadly distributed. However, only four HMO-2011-type cultivated representatives that infect marine SAR116 and Roseobacter strains have been reported to date, and the genetic diversity, potential hosts, and ecology of this group remain poorly elucidated. Here, we present the genomes of seven HMO-2011-type phages that were isolated using four Roseobacter strains and one SAR11 strain, as well as additional 207 HMO-2011-type metagenomic viral genomes (MVGs) identified from various marine viromes. Phylogenomic and shared-gene analyses revealed that the HMO-2011-type group is a subfamily-level group comprising at least 10 discernible genus-level subgroups. Moreover, >2000 HMO-2011-type DNA polymerase sequences were identified, and the DNA polymerase phylogeny also revealed that the HMO-2011-type group contains diverse subgroups and is globally distributed. Metagenomic read-mapping results further showed that most HMO-2011-type phages are prevalent in global oceans and display distinct geographic distributions, with the distribution of most HMO-2011-type phages being associated with temperature. Lastly, we found that members in subgroup IX, represented by pelagiphage HTVC033P, were among the most abundant HMO-2011-type phages, which implies that SAR11 bacteria are crucial hosts for this viral group. In summary, our findings substantially expand current knowledge regarding the phylogenetic diversity, evolution, and distribution of HMO-2011-type phages, highlighting HMO-2011-type phages as major ecological agents that can infect certain key bacterial groups.

Genomic Characterization of Two Novel RCA Phages Reveals New Insights into the Diversity and Evolution of Marine Viruses

Viruses are the most abundant living entities in marine ecosystems, playing critical roles in altering the structure and function of microbial communities and driving ocean biogeochemistry. Phages that infect Roseobacter clade-affiliated (RCA) cluster strains are an important component of marine viral communities. Here, we characterize the genome sequences of two new RCA phages, CRP-9 and CRP-13, which infect RCA strain FZCC0023. Genomic analysis reveals that CRP-9 and CRP-13 represent a novel evolutionary lineage of marine phages. They both have a DNA replication module most similar to those in Cobavirus group phages. In contrast, their morphogenesis and packaging modules are distinct from those in cobaviruses but homologous to those in HMO-2011-type phages. The genomic architecture of CRP-9 and CRP-13 suggests a genomic recombination event between distinct phage groups. Metagenomic data sets were examined for metagenome-assembled viral genomes (MAVGs) with similar recombinant genome architectures. Fifteen CRP-9-type MAVGs were identified from marine viromes. Additionally, 158 MAVGs were identified containing HMO-2011-type morphogenesis and packaging modules with other types of DNA replication genes, providing more evidence that recombination between different phage groups is a major driver of phage evolution. Altogether, this study significantly expands the understanding of diversity and evolution of marine roseophages. Meanwhile, the analysis of these novel RCA phages and MAVGs highlights the critical role of recombination in shaping phage diversity. These phage sequences are valuable resources for inferring the evolutionary connection of distinct phage groups. IMPORTANCE Diversity and evolution of phages that infect the relatively slow-growing but dominant Roseobacter lineages are largely unknown. In this study, RCA phages CRP-9 and CRP-13 have been isolated on a Roseobacter RCA strain and shown to have a unique genomic architecture, which appears to be the result of a recombination event. CRP-9 and CRP-13 have a DNA replication module most similar to those in Cobavirus group phages and morphogenesis and packaging modules most similar to those in HMO-2011-type phages. HMO-2011-type morphogenesis and packaging modules are found in combination with distinct types of DNA replication genes, suggesting compatibility with various DNA replication modules. Altogether, this study contributes toward a better understanding of marine viral diversity and evolution.

Phylogenomics of SAR116 Clade Reveals Two Subclades with Different Evolutionary Trajectories and an Important Role in the Ocean Sulfur Cycle

The SAR116 clade within the class Alphaproteobacteria represents one of the most abundant groups of heterotrophic bacteria inhabiting the surface of the ocean. The small number of cultured representatives of SAR116 (only two to date) is a major bottleneck that has prevented an in-depth study at the genomic level to understand the relationship between genome diversity and its role in the marine environment. In this study, we use all publicly available genomes to provide a genomic overview of the phylogeny, metabolism, and biogeography within the SAR116 clade. This increased genomic diversity has led to the discovery of two subclades that, despite coexisting in the same environment, display different properties in their genomic makeup. One represents a novel subclade for which no pure cultures have been isolated and is composed mainly of single-amplified genomes (SAGs). Genomes within this subclade showed convergent evolutionary trajectories with more streamlined features, such as low GC content (ca. 30%), short intergenic spacers (<22 bp), and strong purifying selection (low ratio of nonsynonymous to synonymous polymorphisms [dN/dS]). Besides, they were more abundant in metagenomic databases recruiting at the deep chlorophyll maximum. Less abundant and restricted to the upper photic layers of the global ocean, the other subclade of SAR116, enriched in metagenome-assembled genomes (MAGs), included the only two pure cultures. Genomic analysis suggested that both clades have a significant role in the sulfur cycle with differences in the way both clades can metabolize dimethylsulfoniopropionate (DMSP). IMPORTANCE The SAR116 clade of Alphaproteobacteria is a ubiquitous group of heterotrophic bacteria inhabiting the surface of the ocean, but the information about their ecology and population genomic diversity is scarce due to the difficulty of getting pure culture isolates. The combination of single-cell genomics and metagenomics has become an alternative approach to study these kinds of microbes. Our results expand the understanding of the genomic diversity, distribution, and lifestyles within this clade and provide evidence of different evolutionary trajectories in the genomic makeup of the two subclades that could serve to illustrate how evolutionary pressure can drive different adaptations to the same environment. Therefore, the SAR116 clade represents an ideal model organism for the study of the evolutionary streamlining of genomes in microbes that have relatively close relatedness to each other.

Mechanisms driving genome reduction of a novel Roseobacter lineage

Members of the marine Roseobacter group are key players in the global carbon and sulfur cycles. While over 300 species have been described, only 2% possess reduced genomes (mostly 3-3.5 Mbp) compared to an average roseobacter (>4 Mbp). These taxonomic minorities are phylogenetically diverse but form a Pelagic Roseobacter Cluster (PRC) at the genome content level. Here, we cultivated eight isolates constituting a novel Roseobacter lineage which we named 'CHUG'. Metagenomic and metatranscriptomic read recruitment analyses showed that CHUG members are globally distributed and active in marine pelagic environments. CHUG members possess some of the smallest genomes (~2.6 Mb) among all known roseobacters, but they do not exhibit canonical features of typical bacterioplankton lineages theorized to have undergone genome streamlining processes, like higher coding density, fewer paralogues and rarer pseudogenes. While CHUG members form a genome content cluster with traditional PRC members, they show important differences. Unlike other PRC members, neither the relative abundances of CHUG members nor their relative gene expression levels are correlated with chlorophyll a concentration across the global samples. CHUG members cannot utilize most phytoplankton-derived metabolites or synthesize vitamin B₁₂, a key metabolite mediating the roseobacter-phytoplankton interactions. This combination of features is evidence for the hypothesis that CHUG members may have evolved a free-living lifestyle decoupled from phytoplankton. This ecological transition was accompanied by the loss of signature genes involved in roseobacter-phytoplankton symbiosis, suggesting that relaxation of purifying selection owing to lifestyle shift is likely an important driver of genome reduction in CHUG.

Six-state Amino Acid Recoding is not an Effective Strategy to Offset Compositional Heterogeneity and Saturation in Phylogenetic Analyses

Six-state amino acid recoding strategies are commonly applied to combat the effects of compositional heterogeneity and substitution saturation in phylogenetic analyses. While these methods have been endorsed from a theoretical perspective, their performance has never been extensively tested. Here, we test the effectiveness of six-state recoding approaches by comparing the performance of analyses on recoded and non-recoded data sets that have been simulated under gradients of compositional heterogeneity or saturation. In our simulation analyses, non-recoding approaches consistently outperform six-state recoding approaches. Our results suggest that six-state recoding strategies are not effective in the face of high saturation. Furthermore, while recoding strategies do buffer the effects of compositional heterogeneity, the loss of information that accompanies six-state recoding outweighs its benefits. In addition, we evaluate recoding schemes with 9, 12, 15, and 18 states and show that these consistently outperform six-state recoding. Our analyses of other recoding schemes suggest that under conditions of very high compositional heterogeneity, it may be advantageous to apply recoding using more than six states, but we caution that applying any recoding should include sufficient justification. Our results have important implications for the more than 90 published papers that have incorporated six-state recoding, many of which have significant bearing on relationships across the tree of life. [Compositional heterogeneity; Dayhoff 6-state recoding; S&R 6-state recoding; six-state amino acid recoding; substitution saturation.]

Lipidomic Analysis of Roseobacters of the Pelagic RCA Cluster and Their Response to Phosphorus Limitation

The marine roseobacter-clade affiliated cluster (RCA) represents one of the most abundant groups of bacterioplankton in the global oceans, particularly in temperate and sub-polar regions. They play a key role in the biogeochemical cycling of various elements and are important players in oceanic climate-active trace gas metabolism. In contrast to copiotrophic roseobacter counterparts such as Ruegeria pomeroyi DSS-3 and Phaeobacter sp. MED193, RCA bacteria are truly pelagic and have smaller genomes. We have previously shown that RCA bacteria do not appear to encode the PlcP-mediated lipid remodeling pathway, whereby marine heterotrophic bacteria remodel their membrane lipid composition in response to phosphorus (P) stress by substituting membrane glycerophospholipids with alternative glycolipids or betaine lipids. In this study, we report lipidomic analysis of six RCA isolates. In addition to the commonly found glycerophospholipids such as phosphatidylglycerol (PG) and phosphatidylethanolamine (PE), RCA bacteria synthesize a relatively uncommon phospholipid, acylphosphatidylglycerol, which is not found in copiotrophic roseobacters. Instead, like the abundant SAR11 clade, RCA bacteria upregulate ornithine lipid biosynthesis in response to P stress, suggesting a key role of this aminolipid in the adaptation of marine heterotrophs to oceanic nutrient limitation.

Microbial Niche Diversification in the Galápagos Archipelago and Its Response to El Niño

The Galápagos Archipelago is located at the intersection of several major oceanographic features that produce diverse environmental conditions around the islands, and thus has the potential to serve as a natural laboratory for discerning the underlying environmental factors that structure marine microbial communities. Here we used quantitative metagenomics to characterize microbial communities in relation to archipelago marine habitats, and how those populations shift due to substantial environmental changes brought on by El Niño. Environmental conditions such as temperature, salinity, inorganic dissolved nutrients, and dissolved organic carbon (DOC) concentrations varied throughout the archipelago, revealing a diversity of potential microbial niches arising from upwelling, oligotrophic to eutrophic gradients, physical isolation, and potential island mass effects. The volumetric abundances of microbial community members shifted with these environmental changes and revealed several taxonomic indicators of different water masses. This included a transition from a Synechococcus dominated system in the west to an even mix of Synechococcus and Prochlorococcus in the east, mirroring the archipelago’s mesotrophic to oligotrophic and productivity gradients. Several flavobacteria groups displayed characteristic habitat distributions, including enrichment of Polaribacter and Tenacibaculum clades in the relatively nutrient rich western waters, Leeuwenhoekiella spp. that were enriched in the more nutrient-deplete central and eastern sites, and the streamlined MS024-2A group found to be abundant across all sites. During the 2015/16 El Niño event, both environmental conditions and microbial community composition were substantially altered, primarily on the western side of the archipelago due to the reduction of upwelling from the Equatorial Undercurrent. When the upwelling resumed, concentrations of inorganic nutrients and DOC at the western surface sites were more typical of mesopelagic depths. Correspondingly, Synechococcus abundances decreased by an order of magnitude, while groups associated with deeper water masses were enriched, including streamlined roseobacters HTCC2255 and HIMB11, Thioglobacaceae, methylotrophs (Methylophilaceae), archaea (Nitrosopumilaceae), and distinct subpopulations of Pelagibaceriales (SAR11 clade). These results provide a quantitative framework to connect community-wide microbial volumetric abundances to their environmental drivers, and thus incorporation into biogeochemical and ecological models.

Complementary energy acquisition via aerobic anoxygenic photosynthesis and carbon monoxide oxidation by Planktomarina temperata of the Roseobacter group

In marine pelagic ecosystems energy is often the limiting factor for growth of heterotrophic bacteria. Aerobic anoxygenic photosynthesis (AAP) and oxidation of carbon monoxide (CO) are modes to acquire complementary energy, but their significance in abundant and characteristic pelagic marine bacteria has not been well studied. In long-term batch culture experiments we found that Planktomarina temperata RCA23, representing the largest and most prominent subcluster of the Roseobacter group, maintains 2-3-fold higher cell numbers in the stationary and declining phase when grown in a light-dark cycle relative to dark conditions. Light enables P. temperata to continue to replicate its DNA during the stationary phase relative to a dark control such that when reinoculated into fresh medium growth resumed two days earlier than in control cultures. In cultures grown in the dark and supplemented with CO, cell numbers in the stationary phase remained significantly higher than in an unsupplemented control. Furthermore, repeated spiking with CO until day 372 resulted in significant CO consumption relative to an unsupplemented control. P. temperata represents a prominent marine pelagic bacterium for which AAP and CO consumption, to acquire complementary energy, have been documented.

Diverse, Abundant, and Novel Viruses Infecting the Marine Roseobacter RCA Lineage

Many major marine bacterial lineages such as SAR11, Prochlorococcus, SAR116, and several Roseobacter lineages have members that are abundant, relatively slow-growing, and genome streamlined. The isolation of phages that infect SAR11 and SAR116 have demonstrated the dominance of these phages in the marine virosphere. However, no phages have been isolated from bacteria in the Roseobacter RCA lineage, another abundant group of marine bacteria. In this study, seven RCA phages that infect three different RCA strains were isolated and characterized. All seven RCA phages belong to the Podoviridae family and have genome sizes ranging from 39.6 to 58.1 kb. Interestingly, three RCA phages (CRP-1, CRP-2, and CRP-3) show similar genomic content and architecture as SAR116 phage HMO-2011, which represents one of the most abundant known viral groups in the ocean. The high degree of homology among CRP-1, CRP-2, CRP-3, and HMO-2011 resulted in the contribution of RCA phages to the dominance of the HMO-2011-type group. CRP-4 and CRP-5 are similar to the Cobavirus group roseophages in terms of gene content and organization. The remaining two RCA phages, CRP-6 and CRP-7, show limited genomic similarity with known phages and represent two new phage groups. Metagenomic fragment recruitment analyses reveal that these RCA phage groups are much more abundant in the ocean than most existing marine roseophage groups. The characterization of these RCA phages has greatly expanded our understanding of the genomic diversity and evolution of marine roseophages and suggests the critical need for isolating phages from the abundant but "unculturable" bacteria.IMPORTANCE The RCA lineage of the marine Roseobacter group represents one of the slow-growing but dominant components of marine microbial communities. Although dozens of roseophages have been characterized, no phages infecting RCA strains have been reported. In this study, we reported on the first RCA phage genomes and investigated their distribution pattern and relative abundance in comparison with other important marine phage groups. Two of the four RCA phage groups were found closely related to previously reported SAR116 phage HMO-2011 and Cobavirus group roseophages, respectively. The remaining two groups are novel in the genome contents. Our study also revealed that RCA phages are widely distributed and exhibit high abundance in marine viromic data sets. Altogether, our findings have greatly broadened our understanding of RCA phages and emphasize the ecological and evolutionary importance of RCA phages in the marine virosphere.

Marker genes as predictors of shared genomic function

Background

Although high-throughput marker gene studies provide valuable insight into the diversity and relative abundance of taxa in microbial communities, they do not provide direct measures of their functional capacity. Recently, scientists have shown a general desire to predict functional profiles of microbial communities based on phylogenetic identification inferred from marker genes, and recent tools have been developed to link the two. However, to date, no large-scale examination has quantified the correlation between the marker gene based taxonomic identity and protein coding gene conservation. Here we utilize 4872 representative prokaryotic genomes from NCBI to investigate the relationship between marker gene identity and shared protein coding gene content.

Results

Even at 99–100% marker gene identity, genomes share on average less than 75% of their protein coding gene content. This occurs regardless of the marker gene(s) used: V4 region of the 16S rRNA, complete 16S rRNA, or single copy orthologs through a multi-locus sequence analysis. An important aspect related to this observation is the intra-organism variation of 16S copies from a single genome. Although the majority of 16S copies were found to have high sequence similarity (> 99%), several genomes contained copies that were highly diverged (< 97% identity).

Conclusions

This is the largest comparison between marker gene similarity and shared protein coding gene content to date. The study highlights the limitations of inferring a microbial community’s functions based on marker gene phylogeny. The data presented expands upon the results of previous studies that examined one or few bacterial species and supports the hypothesis that 16S rRNA and other marker genes cannot be directly used to fully predict the functional potential of a bacterial community.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5641-1) contains supplementary material, which is available to authorized users.

Cobaviruses - a new globally distributed phage group infecting Rhodobacteraceae in marine ecosystems

Bacteriophages are widely considered to influence bacterial communities, however most phages are still unknown or not studied well enough to understand their ecological roles. We have isolated two phages infecting Lentibacter sp. SH36, affiliated with the marine Roseobacter group, and retrieved similar phage genomes from publicly available metagenomics databases. Phylogenetic analysis placed the new phages within the Cobavirus group, in the here newly proposed genus Siovirus and subfamily Riovirinae of the Podoviridae. Gene composition and presence of direct terminal repeats in cultivated cobaviruses point toward a genome replication and packaging strategy similar to the T7 phage. Investigation of the genomes suggests that viral lysis of the cell proceeds via the canonical holin-endolysin pathway. Cobaviral hosts include members of the genera Lentibacter, Sulfitobacter and Celeribacter of the Roseobacter group within the family Rhodobacteraceae (Alphaproteobacteria). Screening more than 5,000 marine metagenomes, we found cobaviruses worldwide from temperate to tropical waters, in the euphotic zone, mainly in bays and estuaries, but also in the open ocean. The presence of cobaviruses in protist metagenomes as well as the phylogenetic neighborhood of cobaviruses in glutaredoxin and ribonucleotide reductase trees suggest that cobaviruses could infect bacteria associated with phototrophic or grazing protists. With this study, we expand the understanding of the phylogeny, classification, genomic organization, biogeography and ecology of this phage group infecting marine Rhodobacteraceae.

Characterization of Pustular Mats and Related Rivularia-Rich Laminations in Oncoids From the Laguna Negra Lake (Argentina)

Stromatolites are organo-sedimentary structures that represent some of the oldest records of the early biosphere on Earth. Cyanobacteria are considered as a main component of the microbial mats that are supposed to produce stromatolite-like structures. Understanding the role of cyanobacteria and associated microorganisms on the mineralization processes is critical to better understand what can be preserved in the laminated structure of stromatolites. Laguna Negra (Catamarca, Argentina), a high-altitude hypersaline lake where stromatolites are currently formed, is considered as an analog environment of early Earth. This study aimed at characterizing carbonate precipitation within microbial mats and associated oncoids in Laguna Negra. In particular, we focused on carbonated black pustular mats. By combining Confocal Laser Scanning Microscopy, Scanning Electron Microscopy, Laser Microdissection and Whole Genome Amplification, Cloning and Sanger sequencing, and Focused Ion Beam milling for Transmission Electron Microscopy, we showed that carbonate precipitation did not directly initiate on the sheaths of cyanobacterial Rivularia, which dominate in the mat. It occurred via organo-mineralization processes within a large EPS matrix excreted by the diverse microbial consortium associated with Rivularia where diatoms and anoxygenic phototrophic bacteria were particularly abundant. By structuring a large microbial consortium, Rivularia should then favor the formation of organic-rich laminations of carbonates that can be preserved in stromatolites. By using Fourier Transform Infrared spectroscopy and Synchrotron-based deep UV fluorescence imaging, we compared laminations rich in structures resembling Rivularia to putatively chemically-precipitated laminations in oncoids associated with the mats. We showed that they presented a different mineralogy jointly with a higher content in organic remnants, hence providing some criteria of biogenicity to be searched for in the fossil record.

Genome sequence of Acuticoccus yangtzensis JL1095T (DSM 28604T) isolated from the Yangtze Estuary

Acuticoccus yangtzensis JL1095^T is a proteobacterium from a genus belonging to the family Rhodobacteraceae; it was isolated from surface waters of the Yangtze Estuary, China. This strain displays the capability to utilize aromatic and simple carbon compounds. Here, we present the genome sequence, annotations, and features of A. yangtzensis JL1095^T. This strain has a genome size of 5,043,263 bp with a G + C content of 68.63%. The genome contains 4286 protein-coding genes, 56 RNA genes, and 83 pseudo genes. Many of the protein-coding genes were predicted to encode proteins involved in carbon metabolism pathways, such as aromatic degradation and methane metabolism. Notably, a total of 31 genes were predicted to encode form II carbon monoxide dehydrogenases, suggesting potential for carbon monoxide oxidation. The genome analysis helps better understand the major carbon metabolic pathways of this strain and its role in carbon cycling in coastal marine ecosystems.

The Biogeographical Distribution of Benthic Roseobacter Group Members along a Pacific Transect Is Structured by Nutrient Availability within the Sediments and Primary Production in Different Oceanic Provinces

By now, only limited information on the Roseobacter group thriving at the seafloor is available. Hence, the current study was conducted to determine their abundance and diversity within Pacific sediments along the 180° meridian. We hypothesize a distinct biogeographical distribution of benthic members of the Roseobacter group linked to nutrient availability within the sediments and productivity of the water column. Lowest cell numbers were counted at the edge of the south Pacific gyre and within the north Pacific gyre followed by an increase to the north with maximum values in the highly productive Bering Sea. Specific quantification of the Roseobacter group revealed on average a relative abundance of 1.7 and 6.3% as determined by catalyzed reported deposition-fluorescence in situ hybridization (CARD-FISH) and quantitative PCR (qPCR), respectively. Corresponding Illumina tag sequencing of 16S rRNA genes and 16S rRNA transcripts showed different compositions containing on average 0.7 and 0.9% Roseobacter-affiliated OTUs of the DNA- and RNA-based communities. These OTUs were mainly assigned to uncultured members of the Roseobacter group. Among those with cultured representatives, Sedimentitalea and Sulfitobacter made up the largest proportions. The different oceanic provinces with low nutrient content such as both ocean gyres were characterized by specific communities of the Roseobacter group, distinct from those of the more productive Pacific subarctic region and the Bering Sea. However, linking the community structure to specific metabolic processes at the seafloor is hampered by the dominance of so-far uncultured members of the Roseobacter group, indicating a diversity that has yet to be explored.

Rokubacteria: Genomic Giants among the Uncultured Bacterial Phyla

Recent advances in single-cell genomic and metagenomic techniques have facilitated the discovery of numerous previously unknown, deep branches of the tree of life that lack cultured representatives. Many of these candidate phyla are composed of microorganisms with minimalistic, streamlined genomes lacking some core metabolic pathways, which may contribute to their resistance to growth in pure culture. Here we analyzed single-cell genomes and metagenome bins to show that the "Candidate phylum Rokubacteria," formerly known as SPAM, represents an interesting exception, by having large genomes (6-8 Mbps), high GC content (66-71%), and the potential for a versatile, mixotrophic metabolism. We also observed an unusually high genomic heterogeneity among individual Rokubacteria cells in the studied samples. These features may have contributed to the limited recovery of sequences of this candidate phylum in prior cultivation and metagenomic studies. Our analyses suggest that Rokubacteria are distributed globally in diverse terrestrial ecosystems, including soils, the rhizosphere, volcanic mud, oil wells, aquifers, and the deep subsurface, with no reports from marine environments to date.

Linking Compositional and Functional Predictions to Decipher the Biogeochemical Significance in DFAA Turnover of Abundant Bacterioplankton Lineages in the North Sea

Deciphering the ecological traits of abundant marine bacteria is a major challenge in marine microbial ecology. In the current study, we linked compositional and functional predictions to elucidate such traits for abundant bacterioplankton lineages in the North Sea. For this purpose, we investigated entire and active bacterioplankton composition along a transect ranging from the German Bight to the northern North Sea by pyrotag sequencing of bacterial 16S rRNA genes and transcripts. Functional profiles were inferred from 16S rRNA data using Tax4Fun. Bacterioplankton communities were dominated by well-known marine lineages including clusters/genera that are affiliated with the Roseobacter group and the Flavobacteria. Variations in community composition and function were significantly explained by measured environmental and microbial properties. Turnover of dissolved free amino acids (DFAA) showed the strongest correlation to community composition and function. We applied multinomial models, which enabled us to identify bacterial lineages involved in DFAA turnover. For instance, the genus Planktomarina was more abundant at higher DFAA turnover rates, suggesting its vital role in amino acid degradation. Functional predictions further indicated that Planktomarina is involved in leucine and isoleucine degradation. Overall, our results provide novel insights into the biogeochemical significance of abundant bacterioplankton lineages in the North Sea.

Evolution of Dimethylsulfoniopropionate Metabolism in Marine Phytoplankton and Bacteria

The elucidation of the pathways for dimethylsulfoniopropionate (DMSP) synthesis and metabolism and the ecological impact of DMSP have been studied for nearly 70 years. Much of this interest stems from the fact that DMSP metabolism produces the climatically active gas dimethyl sulfide (DMS), the primary natural source of sulfur to the atmosphere. DMSP plays many important roles for marine life, including use as an osmolyte, antioxidant, predator deterrent, and cryoprotectant for phytoplankton and as a reduced carbon and sulfur source for marine bacteria. DMSP is hypothesized to have become abundant in oceans approximately 250 million years ago with the diversification of the strong DMSP producers, the dinoflagellates. This event coincides with the first genome expansion of the Roseobacter clade, known DMSP degraders. Structural and mechanistic studies of the enzymes of the bacterial DMSP demethylation and cleavage pathways suggest that exposure to DMSP led to the recruitment of enzymes from preexisting metabolic pathways. In some cases, such as DmdA, DmdD, and DddP, these enzymes appear to have evolved to become more specific for DMSP metabolism. By contrast, many of the other enzymes, DmdB, DmdC, and the acrylate utilization hydratase AcuH, have maintained broad functionality and substrate specificities, allowing them to carry out a range of reactions within the cell. This review will cover the experimental evidence supporting the hypothesis that, as DMSP became more readily available in the marine environment, marine bacteria adapted enzymes already encoded in their genomes to utilize this new compound.

Spontaneous mutations of a model heterotrophic marine bacterium

Heterotrophic marine bacterioplankton populations display substantive genomic diversity that is commonly explained to be the result of selective forces imposed by resource limitation or interactions with phage and predators. Here we use a mutation-accumulation experiment followed by whole-genome sequencing of mutation lines to determine an unbiased rate and molecular spectrum of spontaneous mutations for a model heterotrophic marine bacterium in the globally important Roseobacter clade, Ruegeria pomeroyi DSS-3. We find evidence for mutational bias towards deletions over insertions, and this process alone could account for a sizable portion of genome size diversity among roseobacters and also implies that lateral gene transfer and/or selection must also play a role in maintaining roseobacters with large genome sizes. We also find evidence for a mutational bias in favor of changes from A/T to G/C nucleobases, which explains widespread occurrences of G/C-enriched Roseobacter genomes. Using the calculated mutation rate of 1.39 × 10^-10 per base per generation, we implement a 'mutation-rate clock' approach to date the evolution of roseobacters by assuming a constant mutation rate along their evolutionary history. This approach gives an estimated date of Roseobacter genome expansion in good agreement with an earlier fossil-based estimate of ~250 million years ago and is consistent with a hypothesis of a correlated evolutionary history between roseobacters and marine eukaryotic phytoplankton groups.

Identification of dimethylamine monooxygenase in marine bacteria reveals a metabolic bottleneck in the methylated amine degradation pathway

Methylated amines (MAs) are ubiquitous in the marine environment and their subsequent flux into the atmosphere can result in the formation of aerosols and ultimately cloud condensation nuclei. Therefore, these compounds have a potentially important role in climate regulation. Using Ruegeria pomeroyi as a model, we identified the genes encoding dimethylamine (DMA) monooxygenase (dmmABC) and demonstrate that this enzyme degrades DMA to monomethylamine (MMA). Although only dmmABC are required for enzyme activity in recombinant Escherichia coli, we found that an additional gene, dmmD, was required for the growth of R. pomeroyi on MAs. The dmmDABC genes are absent from the genomes of multiple marine bacteria, including all representatives of the cosmopolitan SAR11 clade. Consequently, the abundance of dmmDABC in marine metagenomes was substantially lower than the genes required for other metabolic steps of the MA degradation pathway. Thus, there is a genetic and potential metabolic bottleneck in the marine MA degradation pathway. Our data provide an explanation for the observation that DMA-derived secondary organic aerosols (SOAs) are among the most abundant SOAs detected in fine marine particles over the North and Tropical Atlantic Ocean.

Ecotype diversification of an abundant Roseobacter lineage

The Roseobacter DC5-80-3 cluster (also known as the RCA clade) is among the most abundant bacterial lineages in temperate and polar oceans. Previous studies revealed two phylotypes within this cluster that are distinctly distributed in the Antarctic and other ocean provinces. Here, we report a nearly complete genome co-assembly of three closely related single cells co-occurring in the Antarctic, and compare it to the available genomes of the other phylotype from ocean regions where iron is more accessible but phosphorus and nitrogen are less. The Antarctic phylotype exclusively contains an operon structure consisting of a dicitrate transporter fecBCDE and an upstream regulator likely for iron uptake, whereas the other phylotype consistently carry a high-affinity phosphate pst transporter and the phoB-phoR regulatory system, a high-affinity ammonium amtB transporter, urea and taurine utilization systems. Moreover, the Antarctic phylotype uses proteorhodopsin to acquire light, whereas the other uses bacteriochlorophyll-a and the sulfur-oxidizing sox cluster for energy acquisition. This is potentially an iron-saving strategy for the Antarctic phylotype because only the latter two pathways have iron-requiring cytochromes. Therefore, the two DC5-80-3 phylotypes, while diverging by only 1.1% in their 16S rRNA genes, have evolved systematic differences in metabolism to support their distinct ecologies.

Marine Non-Cyanobacterial Diazotrophs: Moving beyond Molecular Detection

The nitrogen input through biological N₂ fixation is essential for life in vast areas of the global ocean. The belief is that cyanobacteria are the only relevant N₂-fixing (diazotrophic) organisms. It has, however, now become evident that non-cyanobacterial diazotrophs, bacteria and archaea with ecologies fundamentally distinct from those of cyanobacteria, are widespread and occasionally fix N₂ at significant rates. The documentation of a globally relevant nitrogen input from these diazotrophs would constitute a new paradigm for research on oceanic nitrogen cycling. Here we highlight the need for combining rate measurements and molecular analyses of field samples with cultivation studies in order to clarify the ecology of non-cyanobacteria and their contribution to marine N₂ fixation on local and global scales.

Differential Incorporation of Carbon Substrates among Microbial Populations Identified by Field-Based, DNA Stable-Isotope Probing in South China Sea

To determine the adapted microbial populations to variant dissolved organic carbon (DOC) sources in the marine environment and improve the understanding of the interaction between microorganisms and marine DOC pool, field-based incubation experiments were carried out using supplemental 13C-labeled typical substrates D-glucose and D-glucosamine (D-Glc and D-GlcN, respectively), which are two important components in marine DOC pool in the South China Sea. 13C- and 12C-DNA were then fractionated by ultracentrifugation and the microbial community was analyzed by terminal-restriction fragment length polymorphism and 454 pyrosequencing of 16S rRNA gene. 12C-DNA-based communities showed relatively high similarities with their corresponding in situ communities, and their bacterial diversities were generally higher than 13C-DNA-based counterparts. Distinct differences in community composition were found between 13C- and 12C-DNA-based communities and between two substrate-supplemented 13C-DNA-based communities; these differences distinctly varied with depth and site. In most cases, there were more genera with relative abundances of >0.1% in D-Glc-incorporating communities than in D-GlcN-incorporating communities. The Roseobacter clade was one of the prominent actively substrate-incorporating bacterial populations in all 13C-DNA-based communities. Vibrio was another prominent actively D-GlcN-incorporating bacterial population in most incubations. However notably, different OTUs dominated this clade or genus in different treatments at different depths. Altogether, these results suggested that there were taxa-specific differences in DOC assimilations and, moreover, their differences varied among the typical water masses, which could have been caused by the variant compositions of original bacterial communities from different hydrological environments. This implies that ecologically, the levels of labile or recalcitrance of DOC can be maintained only in a specific environmental context with specific bacterial community composition.

Gene Flow Across Genus Barriers - Conjugation of Dinoroseobacter shibae's 191-kb Killer Plasmid into Phaeobacter inhibens and AHL-mediated Expression of Type IV Secretion Systems

Rhodobacteraceae harbor a conspicuous wealth of extrachromosomal replicons (ECRs) and therefore the exchange of genetic material via horizontal transfer has been supposed to be a major evolutionary driving force. Many plasmids in this group encode type IV secretion systems (T4SS) that are expected to mediate transfer of proteins and/or DNA into host cells, but no experimental evidence of either has yet been provided. Dinoroseobacter shibae, a species of the Roseobacter group within the Rhodobacteraceae family, contains five ECRs that are crucial for anaerobic growth, survival under starvation and the pathogenicity of this model organism. Here we tagged two syntenous but compatible RepABC-type plasmids of 191 and 126-kb size, each encoding a T4SS, with antibiotic resistance genes and demonstrated their conjugational transfer into a distantly related Roseobacter species, namely Phaeobacter inhibens. Pulsed field gel electrophoresis showed transfer of those replicons into the recipient both individually but also together documenting the efficiency of conjugation. We then studied the influence of externally added quorum sensing (QS) signals on the expression of the T4SS located on the sister plasmids. A QS deficient D. shibae null mutant (ΔluxI₁) lacking synthesis of N-acyl-homoserine lactones (AHLs) was cultivated with a wide spectrum of chemically diverse long-chain AHLs. All AHLs with lengths of the acid side-chain ≥14 reverted the ΔluxI₁ phenotype to wild-type. Expression of the T4SS was induced up to log2 ∼3fold above wild-type level. We hypothesize that conjugation in roseobacters is QS-controlled and that the QS system may detect a wide array of long-chain AHLs at the cell surface.