Roseobacter sinensis sp. nov., a marine bacterium capable to synthesize arachidonic acid.

Strain WL0113T was isolated from surface seawater of the coast of Lianyungang, Jiangsu province, PR China. Strain WL0113T shared highest 16S rRNA gene sequence similarity with Roseobacter insulae YSTF-M11T (98.8%), followed by R. cerasinus AI77T (98.8%), R. ponti MM-7 T (98.0%). Strain WL0113T was Gram-stain-negative, cream, aerobic, non-motile and coccoid- to oval-shaped, and able to grow at pH 6.5-9.0 (optimum, pH 7.0-8.0), at 10-37  °C (optimum, 28 °C) and in the presence of 1-5% (w/v; optimum, 2.5%) NaCl. Ubiquinone-10 was detected as dominant. The main fatty acids (> 5%) of the strain WL0113T were C16:0, iso-C17:0 3OH, C20:4ω6,9,12,15c (arachidonic acid), and summed feature 8 (C18:1ω7c and/or C18:1ω6c). The major polar lipids include phosphatidylglycerol, diphosphatidylglycerol, phosphatidylcholine, glycophospholipid, unknown aminolipid, unknown phospholipid, and two unknown polar lipids. The ANI and dDDH values between strain WL0113T and Roseobacter cerasinus were 80.4% and 23.0%, respectively. The genomic DNA G + C content of strain WL0113T was 63.1%. Based on these data, it is proposed that strain WL0113T represent novel species of the genus Roseobacter, for which the name Roseobacter sinensis sp. nov. is proposed. The type strain is WL0113T (= GDMCC 1.3082T = JCM 35567T).

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.

Roseobacter weihaiensis sp. nov., a cellulose-degrading bacterium isolated from intestinal content of Nipponacmea schrenckii collected from golden beach in Weihai, China.

A Gram-staining-negative, dark pink, rod-shaped, amastigote and cellulose-degrading strain, designated H9T, was isolated from intestinal contents of Nipponacmea schrenckii. The isolate was able to grow at 4-42 °C (optimum, 25 °C), at pH 6.5-9.0 (optimum, pH 7.0), and with 0.0-11.0% (w/v) NaCl (optimum, 3.0-5.0%). Phylogenetic analysis of the 16S rRNA gene sequence suggested that isolate H9T belongs to the genus Roseobacter, neighboring Roseobacter insulae YSTF-M11T, Roseobacter cerasinus AI77T and Roseobacter ponti MM-7 T, and the pairwise sequence showed the highest similarity of 99.1% to Roseobacter insulae YSTF-M11T. The major fatty acid was summed feature 8 (C18:1ω7c and/or C18:1ω6c; 81.08%). The predominant respiratory quinone was Q-10. The polar lipids consisted of phosphatidylcholine, phosphatidylglycerol, an unknown lipid, and a small amount of an unknown phospholipid. The genome of strain H9T was 5,351,685 bp in length, and the DNA G + C content was 59.8%. The average amino acid identity (AAI), average nucleotide identity (ANI), and digital DNA hybridization (dDDH) values between strain H9T and closely related strains were 63.4-76.8%, 74.7-78.8%, and 13.4-19.7%, respectively. On the basis of the phenotypic, chemical taxonomic, and phylogenetic data, it is suggested that strain H9T should represent a novel species in the genus Roseobacter, for which the name Roseobacter weihaiensis sp. nov. is proposed. The type strain is H9T (= KCTC 82507 T = MCCC 1K04354T).

Algal exudates promote conjugation in marine Roseobacters.

Horizontal gene transfer (HGT) is a pivotal mechanism driving bacterial evolution, conferring adaptability within dynamic marine ecosystems. Among HGT mechanisms, conjugation mediated by type IV secretion systems (T4SSs) plays a central role in the ecological success of marine bacteria. However, the conditions promoting conjugation events in the marine environment are not well-understood. Roseobacters, abundant marine bacteria commonly associated with algae, possess a multitude of T4SSs. Many Roseobacters are heterotrophic bacteria that rely on algal secreted compounds to support their growth. These compounds attract bacteria, facilitating colonization and attachment to algal cells. Algae and their metabolites bring bacteria into close proximity, potentially promoting bacterial HGT. Investigation across various Roseobacters revealed that algal exudates indeed enhance plasmid transfer through conjugation. While algal exudates do not influence the transcription of bacterial conjugative machinery genes, they promote bacterial attachment, potentially stabilizing proximity and facilitating HGT. Notably, under conditions where attachment is less advantageous, the impact of algal exudates on conjugation is reduced. These findings suggest that algae enhance bacterial conjugation primarily by fostering attachment and highlight the importance of studying bacterial HGT within the context of algal-bacterial interactions. IMPORTANCE: This study explores how algal-bacterial interactions influence horizontal gene transfer (HGT) among marine bacteria. HGT, a key driver of bacterial evolution, is facilitated by conjugation mediated by type IV secretion systems (T4SSs). Through investigating Roseobacters, abundant marine bacteria often found to be associated with algae, the study reveals that algal exudates enhance plasmid transfer via conjugation. This enhancement is attributed to the promotion of bacterial attachment by algal compounds, emphasizing the role of algal-bacterial interactions in shaping genetic exchange within dynamic marine ecosystems. Understanding these mechanisms is crucial for elucidating bacterial adaptability and evolution in the marine environment.

Roseobacter fucihabitans sp. nov., isolated from the brown alga Fucus spiralis.

A Gram-negative, aerobic, pink-pigmented, and bacteriochlorophyll a-containing bacterial strain, designated B14T, was isolated from the macroalga Fucus spiralis sampled from the southern North Sea, Germany. Based on 16S rRNA gene sequences, species of the genera Roseobacter and Sulfitobacter were most closely related to strain B14T with sequence identities ranging from 98.15 % (Roseobacter denitrificans Och 114T) to 99.11 % (Roseobacter litoralis Och 149T), whereas Sulfitobacter mediterraneus CH-B427T exhibited 98.52 % sequence identity. Digital DNA-DNA hybridization and average nucleotide identity values between the genome of the novel strain and that of closely related Roseobacter and Sulfitobacter type strains were <20 % and <77 %, respectively. The novel strain contained ubiquinone-10 as the only respiratory quinone and C18 : 1 ω7c, C16 : 0, C18 : 0, C12 : 1 ω7c, C18 : 2 ω7,13c, and C10 : 0 3-OH as the major cellular fatty acids. The predominant polar lipids of strain B14T were phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. The genome of strain B14T comprises a chromosome with a size of 4.5 Mbp, one chromid, and four plasmids. The genome contains the complete gene cluster for aerobic anoxygenic photosynthesis required for a photoheterotrophic lifestyle. The results of this study indicate that strain B14T (=DSM 116946T=LMG 33352T) represents a novel species of the genus Roseobacter for which the name Roseobacter fucihabitans sp. nov. is proposed.

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.

Genome sequences of the first Autographiviridae phages infecting marine Roseobacter.

The ubiquitous and abundant marine phages play critical roles in shaping the composition and function of bacterial communities, impacting biogeochemical cycling in marine ecosystems. Autographiviridae is among the most abundant and ubiquitous phage families in the ocean. However, studies on the diversity and ecology of Autographiviridae phages in marine environments are restricted to isolates that infect SAR11 bacteria and cyanobacteria. In this study, ten new roseophages that infect marine Roseobacter strains were isolated from coastal waters. These new roseophages have a genome size ranging from 38 917 to 42 634 bp and G+C content of 44.6-50 %. Comparative genomics showed that they are similar to known Autographiviridae phages regarding gene content and architecture, thus representing the first Autographiviridae roseophages. Phylogenomic analysis based on concatenated conserved genes showed that the ten roseophages form three distinct subgroups within the Autographiviridae, and sequence analysis revealed that they belong to eight new genera. Finally, viromic read-mapping showed that these new Autographiviridae phages are widely distributed in global oceans, mostly inhabiting polar and estuarine locations. This study has expanded the current understanding of the genomic diversity, evolution and ecology of Autographiviridae phages and roseophages. We suggest that Autographiviridae phages play important roles in the mortality and community structure of roseobacters, and have broad ecological applications.

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 ß-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.

Anaerobic thiosulfate oxidation by the Roseobacter group is prevalent in marine biofilms.

Thiosulfate oxidation by microbes has a major impact on global sulfur cycling. Here, we provide evidence that bacteria within various Roseobacter lineages are important for thiosulfate oxidation in marine biofilms. We isolate and sequence the genomes of 54 biofilm-associated Roseobacter strains, finding conserved sox gene clusters for thiosulfate oxidation and plasmids, pointing to a niche-specific lifestyle. Analysis of global ocean metagenomic data suggests that Roseobacter strains are abundant in biofilms and mats on various substrates, including stones, artificial surfaces, plant roots, and hydrothermal vent chimneys. Metatranscriptomic analysis indicates that the majority of active sox genes in biofilms belong to Roseobacter strains. Furthermore, we show that Roseobacter strains can grow and oxidize thiosulfate to sulfate under both aerobic and anaerobic conditions. Transcriptomic and membrane proteomic analyses of biofilms formed by a representative strain indicate that thiosulfate induces sox gene expression and alterations in cell membrane protein composition, and promotes biofilm formation and anaerobic respiration. We propose that bacteria of the Roseobacter group are major thiosulfate-oxidizers in marine biofilms, where anaerobic thiosulfate metabolism is preferred.

Roseobacter insulae sp. nov. and Loktanella gaetbuli sp. nov., isolated from tidal flats in the Yellow Sea in Korea.

Two bacterial strains (designated as YSTF-M11T and TSTF-M6T) were isolated from tidal flat sediments of the Yellow Sea, Republic of Korea, and taxonomically characterized. A neighbour-joining phylogenetic tree based on 16S rRNA gene sequences showed that strain YSTF-M11T clusters with the type strains of Roseobacter species and strain TSTF-M6T clusters with the type strains of Loktanella salsilacus, Loktanella fryxellensis and Loktanella atrilutea. Strains YSTF-M11T and TSTF-M6T exhibited 16S rRNA gene sequence similarity values of 97.5-98.9 % and 94.1-97.2 % to the type strains of four Roseobacter species and to the type strains of four Loktanella species, respectively. An UBCG tree based on genomic sequences and a tree based on AAI showed that strains YSTF-M11T and TSTF-M6T form a cluster with the type strains of Roseobacter species and with the type strains of L. salsilacus, L. fryxellensis and L. atrilutea, respectively. The ANI and dDDH values between genomic sequences of strain YSTF-M11T and the type strains of four Roseobacter species and between those of strain TSTF-M6T and the type strains of the three Loktanella species were in ranges of 74.0-75.9 and 18.2-19.7 % and 74.7-75.5 and 18.8-19.3 %, respectively. The DNA G+C contents of strains YSTF-M11T and TSTF-M6T were 60.3 and 61.9 % based on their genomic sequences. Both strains contained Q-10 as the predominant ubiquinone and C18 : 1 ω7c as the major fatty acid. Strains YSTF-M11T and TSTF-M6T were separated from recognized Roseobacter species and L. salsilacus, L. fryxellensis and L. atrilutea, respectively, by their phenotypic properties together with the phylogenetic and genetic distinctiveness. Based on data presented in this study, strains YSTF-M11T (=KACC 21642T =NBRC 115155T) and TSTF-M6T (=KACC 21643T =NBRC 115154T) are considered to represent novel species of the genera Roseobacter and Loktanella, respectively, for which the names Roseobacter insulae sp. nov. and Loktanella gaetbuli sp. nov. are proposed.

Complete genome sequence of marine Roseobacter lineage member Ruegeria sp. YS9 with five plasmids isolated from red algae.

Ruegeria sp. YS9, an aerobic and chemoheterotrophic bacterium belonging to marine Roseobacter lineage, was a putative new species isolated from red algae Eucheuma okamurai in the South China Sea (Beihai, Guangxi province). The complete genome sequence in strain YS9 comprised one circular chromosome with 3,244,635 bp and five circular plasmids ranging from 38,085 to 748,160 bp, with a total length of 4.30 Mb and average GC content of 58.39%. In total, 4129 CDSs, 52 tRNA genes and 9 rRNA genes were obtained. Genomic analysis of strain YS9 revealed that 85 CAZymes were organized in 147 PUL-associated CAZymes involved in polysaccharides metabolism, which were the highest among its two closely related Ruegeria strains. Numerous PULs related to degradation on the cell wall of algae, especially agar, indicated its major player role in the remineralization of algal-derived carbon. Further, the existence of multiple plasmids provided strain YS9 with distinct advantages to facilitate its rapid environmental adaptation, including polysaccharide metabolism, denitrification, resistance to heavy metal stresses such as copper and cobalt, type IV secretion systems and type IV toxin-antitoxin systems, which were obviously different from the two Ruegeria strains. This study provides evidence for polysaccharide metabolic capacity and functions of five plasmids in strain YS9, broadening our understanding of the ecological roles of bacteria in the environment around red algae and the function patterns of plasmids in marine Roseobacter lineage members for environmental adaptation.

Sulfoquinovose is a widespread organosulfur substrate for Roseobacter clade bacteria in the ocean.

Sulfoquinovose (SQ) is one of the most abundant organosulfur compounds in the biosphere, and its biosynthesis and degradation can represent an important contribution to the sulfur cycle. To data, in marine environments, the microorganisms capable of metabolising SQ have remained unidentified and the sources of SQ are still uncertain. Herein, the marine Roseobacter clade bacteria (RCB) Dinoroseobacter shibae DFL 12 and Roseobacter denitrificans OCh 114 were found to grow using SQ as the sole source of carbon and energy. In the presence of SQ, we identified a set of highly up-regulated proteins encoded by gene clusters in these two organisms, of which four homologues to proteins in the SQ monooxygenase pathway of Agrobacterium fabrum C58 may confer the ability to metabolise SQ to these marine bacteria. The sulfite released from SQ desulfonation by FMN-dependent SQ monooxygenase (SmoC) may provide bacteria with reduced sulfur for assimilation, while proteins associated with sulfite production via assimilatory sulfate reduction were significantly down-regulated. Such SQ catabolic genes are restricted to a limited number of phylogenetically diverse bacterial taxa with the predominate genera belonging to the Roseobacter clade (Roseobacteraceae). Moreover, transcript analysis of Tara Oceans project and coastal Bohai Sea samples provided additional evidence for SQ metabolism by RCB. SQ was found to be widely distributed in marine phytoplankton and cyanobacteria with variable intracellular concentrations ranging from micromolar to millimolar levels, and the amounts of SQ on particulate organic matter in field samples were, on average, lower than that of dimethylsulfoniopropionate (DMSP) by one order of magnitude. Together, the phototroph-derived SQ actively metabolised by RCB represents a previously unidentified link in the marine sulfur cycle.

Aminolipids elicit functional trade-offs between competitiveness and bacteriophage attachment in Ruegeria pomeroyi.

Lipids play a crucial role in maintaining cell integrity and homeostasis with the surrounding environment. Cosmopolitan marine roseobacter clade (MRC) and SAR11 clade bacteria are unique in that, in addition to glycerophospholipids, they also produce an array of amino acid-containing lipids that are conjugated with beta-hydroxy fatty acids through an amide bond. Two of these aminolipids, the ornithine aminolipid (OL) and the glutamine aminolipid (QL), are synthesized using the O-acetyltransferase OlsA. Here, we demonstrate that OL and QL are present in both the inner and outer membranes of the Gram-negative MRC bacterium Ruegeria pomeroyi DSS-3. In an olsA mutant, loss of these aminolipids is compensated by a concurrent increase in glycerophospholipids. The inability to produce aminolipids caused significant changes in the membrane proteome, with the membrane being less permeable and key nutrient transporters being downregulated while proteins involved in the membrane stress response were upregulated. Indeed, the import of 14C-labelled choline and dimethylsulfoniopropionate, as a proxy for the transport of key marine nutrients across membranes, was significantly impaired in the olsA mutant. Moreover, the olsA mutant was significantly less competitive than the wild type (WT) being unable to compete with the WT strain in co-culture. However, the olsA mutant unable to synthesize these aminolipids is less susceptible to phage attachment. Together, these data reveal a critical role for aminolipids in the ecophysiology of this important clade of marine bacteria and a trade-off between growth and avoidance of bacteriophage attachment.

Distribution of rare N4-like viruses in temperate estuaries unveiled by viromics.

The relative abundance of N4-like viruses in two temperate estuaries was assessed using four different methods, read mapping to known N4-like virus isolates, read mapping to native viral contigs, reciprocal blast search based on core genes, and read taxonomy classification using Kaiju. Overall, N4-like viruses were found to be of low abundance in the estuarine viromes. When mapping reads to only known N4-like virus genomes, high occurrences of N4-like viruses infecting Roseobacter were found, with their diversity consisting mostly of locally isolated Roseobacter N4-like virus species. Both contig-based methods and Kaiju classification showed similar seasonal patterns for N4-like viruses, and redundancy analysis revealed a negative correlation between N4-like viruses and temperature, suggesting that N4-like viruses may be more abundant in colder water. The discrepancy of relative abundance estimates using different methods indicates that N4-like viruses are best represented by native viral sequences. Our study indicates that N4-like viruses are rare in the marine environment and also provide insight into the importance of including local viral sequences in reference databases.

TCA cycle enhancement and uptake of monomeric substrates support growth of marine Roseobacter at low temperature.

Members of the marine Roseobacter group are ubiquitous in global oceans, but their cold-adaptive strategies have barely been studied. Here, as represented by Loktanella salsilacus strains enriched in polar regions, we firstly characterized the metabolic features of a cold-adapted Roseobacter by multi-omics, enzyme activities, and carbon utilization procedures. Unlike in most cold-adapted microorganisms, the TCA cycle is enhanced by accumulating more enzyme molecules, whereas genes for thiosulfate oxidation, sulfate reduction, nitrate reduction, and urea metabolism are all expressed at lower abundance when L. salsilacus was growing at 5 °C in comparison with higher temperatures. Moreover, a carbon-source competition experiment has evidenced the preferential use of glucose rather than sucrose at low temperature. This selective utilization is likely to be controlled by the carbon source uptake and transformation steps, which also reflects an economic calculation balancing energy production and functional plasticity. These findings provide a mechanistic understanding of how a Roseobacter member and possibly others as well counteract polar constraints.

Nanomolar Responsiveness of Marine Phaeobacter inhibens DSM 17395 toward Carbohydrates and Amino Acids.

Phaeobacter inhibens DSM 17395 is a heterotrophic member of the ubiquitous, marine Roseobacter group and specializes in the aerobic utilization of carbohydrates and amino acids via pathways widespread among roseobacters. The in vivo responsiveness of P. inhibens DSM 17395 was studied with nonadapted cells (succinate-grown), which were exposed to a single pulse (100-0.01 µM) each of N-acetylglucosamine, mannitol, xylose, leucine, phenylalanine, or tryptophan (effectors). Responsiveness was then determined by time-resolved transcript analyses (quantitative reverse transcription-PCR) of "degradation" and "uptake" genes selected based on previously reported substrate-specific proteome profiles. The transcriptional response thresholds were: 50-100 nM for nagK (N-acetylglucosamine kinase), paaA (ring 1,2-phenylacetyl-CoA epoxidase), and kynA (tryptophan 2,3-dioxygenase), 10-50 nM for xylA (xylose isomerase), and around 10 nM for mtlK (mannitol 2-dehydrogenase). A threshold for leucine could not be determined due to the elevated intrinsic presence of leucine in the exometabolome of succinate-grown cells (no effector addition). Notably, the response thresholds for presumptive carbohydrate-binding proteins of ABC-transporters were in the same range or even lower: 0.1-1 µM for c27930 (N-acetylglucosamine) and even below 10 nM for c13210 (mannitol) and xylF (xylose). These results shed new light on the sensory/regulatory sensitivity of a well-studied roseobacter for recognizing potential substrates at low ambient concentrations and on the concentration threshold below which these might escape biodegradation ("emergent recalcitrance" concept of dissolved organic matter persistence).

Fatal affairs - conjugational transfer of a dinoflagellate-killing plasmid between marine Rhodobacterales.

The roseobacter group of marine bacteria is characterized by a mosaic distribution of ecologically important phenotypes. These are often encoded on mobile extrachromosomal replicons. So far, conjugation had only been experimentally proven between the two model organisms Phaeobacter inhibens and Dinoroseobacter shibae. Here, we show that two large natural RepABC-type plasmids from D. shibae can be transferred into representatives of all known major Rhodobacterales lineages. Complete genome sequencing of the newly established Phaeobacter inhibens transconjugants confirmed their genomic integrity. The conjugated plasmids were stably maintained as single copy number replicons in the genuine as well as the new host. Co-cultivation of Phaeobacter inhibens and the transconjugants with the dinoflagellate Prorocentrum minimum demonstrated that Phaeobacter inhibens is a probiotic strain that improves the yield and stability of the dinoflagellate culture. The transconjugant carrying the 191 kb plasmid, but not the 126 kb sister plasmid, killed the dinoflagellate in co-culture.

Metabolism of chiral sulfonate compound 2,3-dihydroxypropane-1-sulfonate (DHPS) by Roseobacter bacteria in marine environment.

The sulfonate compound 2,3-dihydroxypropane-1-sulfonate (DHPS) is one of the most abundant organic sulfur compounds in the biosphere. DHPS derived from dietary intake could be transformed into sulfide by intestinal microbiota and thus impacts human health. However, little is known about its sulfur transformation and subsequent impacts in marine environment. In this study, laboratory-culturing was combined with targeted metabolomic, chemical fluorescence probing, and comparative proteomic methods to examine the bioavailability of chiral DHPS (R and S isomers) for bacteria belonging to the marine Roseobacter clade. The metabolic potential of DHPS in bacteria was further assessed based on genomic analysis. Roseobacter members Ruegeria pomeroyi DSS-3, Dinoroseobacter shibae DFL 12, and Roseobacter denitrificans OCh 114 could utilize chiral DHPS for growth, producing sulfite. They all contained a similar gene cluster for DHPS metabolism but differed in the genes encoding enzymes for desulfonation. There was no significant difference in the growth rate and DHPS consumption rate for R. pomeroyi DSS-3 between R- and S-DHPS cultures, with few proteins expressed differentially were found. Proteomic data suggested that a series of hydrogenases oxidized DHPS, after which desulfonation could proceed via three distinct enzymatic pathways. Strain R. pomeroyi DSS-3 completed the desulfonation via L-cysteate sulfo-lyase, while D. shibae DFL 12 and R. denitrificans OCh 114 primarily utilized sulfolactate sulfo-lyase, and sulfopyruvate decarboxylase followed by sulfoacetaldehyde acetyltransferase, respectively, to complete desulfonation releasing the sulfonate-moiety. The sulfite could be further oxidized or incorporated into sulfate assimilation, indicated by the proteomic data. Furthermore, DHPS metabolic pathways were found primarily in marine bacterial groups, including the majority of sequenced Roseobacter genomes. Our results suggest that chiral DHPS, as a vital reduced sulfur reservoir, could be metabolized by marine bacteria, providing a resource for bacterial growth, rather than acting as a source of toxic sulfide within the marine ecosystem.

Muriiphilus fusiformis gen. nov., sp. nov., a novel non-marine bacterium belonging to the Roseobacter group, and reclassification of Maritimibacter lacisalsi (Zhong et al. 2015) as Muriicola lacisalsi gen. nov., comb. nov.

An aerobic, Gram-stain-negative, non-sporulating, flagellated and spindle-like bacterium, designated HY14T, was isolated from a pickle-processing factory wastewater sample. The isolate chemoheterotrophically grew at 4-42 °C (optimum, 35 °C) and pH 5.5-9.0 (optimum, pH 6.0-6.5). Salt was required for growth (0.5-12 % NaCl, w/v). A deep brown and water-soluble uncharacterized pigment was produced when grown in certain media. The predominant fatty acids (>5 %) included C16 : 0, C18 : 1 ω7c, 11-methyl C18 : 1 ω7c and C19 : 0 cyclo ω8c. The polar lipid profile consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, two unidentified aminolipids, two unidentified phospholipids, two unidentified glycolipids and five unknown lipids. The major isoprenoid quinone was ubiquinone-10. Pairwise alignment based on 16S rRNA gene sequences indicated that strain HY14T had the highest sequence similarity to genera Maritimibacter (95.61-96.05 %) and Boseongicola (95.82 %). Phylogenetic analysis based on core genome illustrated that strain HY14T formed a monophyletic lineage with members of the genus Maritimibacter in the clade of the Roseobacter group in the family Rhodobacteraeceae. The core-gene average amino acid identity used to define bacterial genera by a threshold of 60-80 % was calculated to be 68.56-76.5 % between HY14T and closely related taxa. Several genomic characteristics, such as carrying two RuBisCO-mediated pathways and different osmoprotectant transport pathways, exhibited the genotypic discrepancies of strain HY14T. Based on the polyphasic taxonomic characterization, strain HY14T is considered to represent a novel species of a novel genus belonging to the family Rhodobacteraeceae, for which the name Muriiphilus fusiformis gen. nov., sp. nov. is proposed. The type strain is HY14T (=CGMCC 1.15973T=KCTC 52499T). Maritimibacter lacisalsi (Zhong et al. 2015) is considered to diverge from Maritimibacter alkaliphilus at the genus level, and should be reassigned as a novel genus, for which the name Muriicola lacisalsi gen. nov., comb. nov. is proposed.