Roseobacter Group Research Articles

A novel class of sulfur-containing aminolipids widespread in marine roseobacters

Marine roseobacter group bacteria are numerically abundant and ecologically important players in ocean ecosystems. These bacteria are capable of modifying their membrane lipid composition in response to environmental change. Remarkably, a variety of lipids are produced in these bacteria, including phosphorus-containing glycerophospholipids and several amino acid-containing aminolipids such as ornithine lipids and glutamine lipids. Here, we present the identification and characterization of a novel sulfur-containing aminolipid (SAL) in roseobacters. Using high resolution accurate mass spectrometry, a SAL was found in the lipid extract of Ruegeria pomeroyi DSS-3 and Phaeobacter inhibens DSM 17395. Using comparative genomics, transposon mutagenesis and targeted gene knockout, we identified a gene encoding a putative lyso-lipid acyltransferase, designated salA, which is essential for the biosynthesis of this SAL. Multiple sequence analysis and structural modeling suggest that SalA is a novel member of the lysophosphatidic acid acyltransferase (LPAAT) family, the prototype of which is the PlsC acyltransferase responsible for the biosynthesis of the phospholipid phosphatidic acid. SAL appears to play a key role in biofilm formation in roseobacters. salA is widely distributed in Tara Oceans metagenomes and actively expressed in Tara Oceans metatranscriptomes. Our results raise the importance of sulfur-containing membrane aminolipids in marine bacteria.

Breakdown of 3-(allylsulfonio)propanoates in bacteria from the Roseobacter group yields garlic oil constituents.

Two analogues of 3-(dimethylsulfonio)propanoate (DMSP), 3-(diallylsulfonio)propanoate (DAllSP), and 3-(allylmethylsulfonio)propanoate (AllMSP), were synthesized and fed to marine bacteria from the Roseobacter clade. These bacteria are able to degrade DMSP into dimethyl sulfide and methanethiol. The DMSP analogues were also degraded, resulting in the release of allylated sulfur volatiles known from garlic. For unknown compounds, structural suggestions were made based on their mass spectrometric fragmentation pattern and confirmed by the synthesis of reference compounds. The results of the feeding experiments allowed to conclude on the substrate tolerance of DMSP degrading enzymes in marine bacteria.

Pseudooceanicola algae sp. nov., isolated from the marine macroalga Fucus spiralis, shows genomic and physiological adaptations for an algae-associated lifestyle

The genus Pseudooceanicola from the alphaproteobacterial Roseobacter group currently includes ten validated species. We herein describe strain Lw-13eT, the first Pseudooceanicola species from marine macroalgae, isolated from the brown alga Fucus spiralis abundant at European and North American coasts. Physiological and pangenome analyses of Lw-13eT showed corresponding adaptive features. Adaptations to the tidal environment include a broad salinity tolerance, degradation of macroalgae-derived substrates (mannitol, mannose, proline), and resistance to several antibiotics and heavy metals. Notably, Lw-13eT can degrade oligomeric alginate via PL15 alginate lyase encoded in a polysaccharide utilization locus (PUL), rarely described for roseobacters to date. Plasmid localization of the PUL strengthens the importance of mobile genetic elements for evolutionary adaptations within the Roseobacter group. PL15 homologs were primarily detected in marine plant-associated metagenomes from coastal environments but not in the open ocean, corroborating its adaptive role in algae-rich habitats. Exceptional is the tolerance of Lw-13eT against the broad-spectrum antibiotic tropodithietic acid, produced by Phaeobacter spp. co-occurring in coastal habitats. Furthermore, Lw-13eT exhibits features resembling terrestrial plant-bacteria associations, i.e. biosynthesis of siderophores, terpenes and volatiles, which may contribute to mutual bacteria-algae interactions. Closest described relative of Lw-13eT is Pseudopuniceibacterium sediminis CY03T with 98.4% 16S rRNA gene sequence similarity. However, protein sequence-based core genome phylogeny and average nucleotide identity indicate affiliation of Lw-13eT with the genus Pseudooceanicola. Based on phylogenetic, physiological and (chemo)taxonomic distinctions, we propose strain Lw-13eT (=DSM 29013T=LMG 30557T) as a novel species with the name Pseudooceanicola algae.

Gene loss through pseudogenization contributes to the ecological diversification of a generalist Roseobacter lineage

Ecologically relevant genes generally show patchy distributions among related bacterial genomes. This is commonly attributed to lateral gene transfer, whereas the opposite mechanism-gene loss-has rarely been explored. Pseudogenization is a major mechanism underlying gene loss, and pseudogenes are best characterized by comparing closely related genomes because of their short life spans. To explore the role of pseudogenization in microbial ecological diversification, we apply rigorous methods to characterize pseudogenes in the 279 newly sequenced Ruegeria isolates of the globally abundant Roseobacter group collected from two typical coastal habitats in Hong Kong, the coral Platygyra acuta and the macroalga Sargassum hemiphyllum. Pseudogenes contribute to ~16% of the accessory genomes of these strains. Ancestral state reconstruction reveals that many pseudogenization events are correlated with ancestral niche shifts. Specifically, genes related to resource scavenging and energy acquisition were often pseudogenized when roseobacters inhabiting carbon-limited and energy-poor coral skeleton switched to other resource-richer niches. For roseobacters inhabiting the macroalgal niches, genes for nitrogen regulation and carbohydrate utilization were important but became dispensable upon shift to coral skeleton where nitrate is abundant but carbohydrates are less available. Whereas low-energy-demanding secondary transporters are more favorable in coral skeleton, ATP-driven primary transporters are preferentially kept in the energy-replete macroalgal niches. Moreover, a large proportion of these families mediate organismal interactions, suggesting their rapid losses by pseudogenization as a potential response to host and niche shift. These findings illustrate an important role of pseudogenization in shaping genome content and driving ecological diversification of marine roseobacters.

Quorum sensing regulates 'swim-or-stick' lifestyle in the phycosphere.

SummaryInteractions between phytoplankton and bacteria play major roles in global biogeochemical cycles and oceanic nutrient fluxes. These interactions occur in the microenvironment surrounding phytoplankton cells, known as the phycosphere. Bacteria in the phycosphere use either chemotaxis or attachment to benefit from algal excretions. Both processes are regulated by quorum sensing (QS), a cell–cell signalling mechanism that uses small infochemicals to coordinate bacterial gene expression. However, the role of QS in regulating bacterial attachment in the phycosphere is not clear. Here, we isolated a Sulfitobacter pseudonitzschiae F5 and a Phaeobacter sp. F10 belonging to the marine Roseobacter group and an Alteromonas macleodii F12 belonging to Alteromonadaceae, from the microbial community of the ubiquitous diatom Asterionellopsis glacialis. We show that only the Roseobacter group isolates (diatom symbionts) can attach to diatom transparent exopolymeric particles. Despite all three bacteria possessing genes involved in motility, chemotaxis, and attachment, only S. pseudonitzschiae F5 and Phaeobacter sp. F10 possessed complete QS systems and could synthesize QS signals. Using UHPLC–MS/MS, we identified three QS molecules produced by both bacteria of which only 3‐oxo‐C16:1‐HSL strongly inhibited bacterial motility and stimulated attachment in the phycosphere. These findings suggest that QS signals enable colonization of the phycosphere by algal symbionts.

Cryptic speciation of a pelagic Roseobacter population varying at a few thousand nucleotide sites.

A drop of seawater contains numerous microspatial niches at the scale relevant to microbial activities. Examples are abiotic niches such as detrital particles that show different sizes and organic contents, and biotic niches resulting from bacteria-phage and bacteria-phytoplankton interactions. A common practice to investigate the impact of microenvironments on bacterial evolution is to separate the microenvironments physically and compare the bacterial inhabitants from each. It remains poorly understood, however, which microenvironment primarily drives bacterioplankton evolution in the pelagic ocean. By applying a dilution cultivation approach to an undisturbed coastal water sample, we isolate a bacterial population affiliated with the globally dominant Roseobacter group. Although varying at just a few thousand nucleotide sites across the whole genomes, members of this clonal population are diverging into two genetically separated subspecies. Genes underlying speciation are not unique to subspecies but instead clustered at the shared regions that represent ~6% of the genomic DNA. They are primarily involved in vitamin synthesis, motility, oxidative defense, carbohydrate, and amino acid utilization, consistent with the known strategies that roseobacters take to interact with phytoplankton and particles. Physiological assays corroborate that one subspecies outcompetes the other in these traits. Our results indicate that the microenvironments in the pelagic ocean represented by phytoplankton and organic particles are likely important niches that drive the cryptic speciation of the Roseobacter population, though microhabitats contributed by other less abundant pelagic hosts cannot be ruled out.

Genomic Characteristics and Potential Metabolic Adaptations of Hadal Trench Roseobacter and Alteromonas Bacteria Based on Single-Cell Genomics Analyses.

Heterotrophic bacteria such as those from the Roseobacter group and genus Alteromonas dominate the hadal zones of oceans; however, we know little about the genomic characteristics and potential metabolic adaptations of hadal trench-dwelling bacteria. Here, we report multiple single amplified genomes (SAGs) belonging to Roseobacter and Alteromonas, recovered from the hadal zone of the Mariana Trench. While phylogenetic analyses show that these hadal SAGs cluster with their surface relatives, an analysis of genomic recruitment indicates that they have higher relative abundances in the hadal zone of the Mariana Trench. Comparative genomic analyses between the hadal SAGs and reference genomes of closely related shallow-water relatives indicate that genes involved in the mobilome (prophages and transposons) are overrepresented among the unique genes of the hadal Roseobacter and Alteromonas SAGs; the functional proteins encoded by this category of genes also shows higher amino acid sequence variation than those encoded by other gene sets within the Roseobacter SAGs. We also found that genes involved in cell wall/membrane/envelope biogenesis, transcriptional regulation, and metal transport may be important for the adaptation of hadal Roseobacter and Alteromonas lineages. These results imply that the modification of cell surface-related proteins and transporters is the major direction of genomic evolution in Roseobacter and Alteromonas bacteria adapting to the hadal environment, and that prophages and transposons may be the key factors driving this process.

Complete Genome Sequences of Two Marine Biofilm Isolates, Leisingera sp. nov. Strains 201A and 204H, Novel Representatives of the Roseobacter Group.

Here, we report the complete-genome assemblies of biofilm isolates 201A and 204H. They possess six and seven plasmids, respectively, with a size ranging from 44 kb to 159 kb. Genomic comparisons place the two strains into one new species belonging to the genus Leisingera as novel representatives of the Roseobacter group.

Complete Genome of Roseobacter ponti DSM 106830T.

Members of the Roseobacter group are known for their different ecologically relevant metabolic traits and high abundance in many marine environments. This includes traits like carbon monoxide oxidation, sulfur oxidation, nitrogen oxidation, DMSP demethylation, denitrification, and production of bioactive compounds. Nevertheless, their role in the marine biogeochemical cycles remains to be elucidated. Roseobacter ponti DSM 106830T, also designated strain MM-7T (=KCTC 52469T =NBRC 112431T), is a novel type strain of the Roseobacter group, which was proposed as new Roseobacter species. It was isolated from seawater of the Yellow Sea in South Korea. We report the complete genome sequence of R. ponti DSM 106830T, which belongs to the family Rhodobacteraceae. The genome of R. ponti DSM 106830T comprises a single circular chromosome (3,861,689 bp) with a GC content of 60.52% and an additional circular plasmid (p1) of 100,942 bp with a GC content of 61.51%. The genome encodes 3,812 putative genes, including 3 rRNA, 42 tRNA, 1 tmRNA, and 3 ncRNA. The genome information was used to perform a phylogenetic analysis, which confirmed that the strain represents a new species. Moreover, the genome sequence enabled the investigation of the metabolic capabilities and versatility of R. ponti DSM 106830T. Finally, it provided insight into the high niche adaptation potential of Roseobacter group members.

Diversity of the epiphytic bacterial communities associated with commercially cultivated healthy and diseased Saccharina japonica during the harvest season

Disease outbreaks cause severe economic losses in commercially cultivated Saccharina japonica. Epiphytic bacteria are increasingly recognized to play an important role in S. japonica health and may also become opportunistic pathogens when the environment deteriorates. However, there is little knowledge about the epiphytic bacterial communities associated with commercially cultivated S. japonica and how these communities change over the harvest season. We analyzed the diversity, composition, and temporal dynamics of epiphytic bacterial communities associated with both healthy and diseased tissue of S. japonica over a harvest season (from April to June, 2015) at Ailian Bay, Rongcheng, China. Our results showed that Proteobacteria and Bacteroidetes were the main phyla and Halomonas spp. were the most dominant bacterial species. Halomonadaceae, Hyphomonadaceae, Rhodobacteraceae, and Phyllobacteriaceae families were abundant in both healthy and diseased S. japonica, but the relative abundance of these bacteria differed between healthy and diseased groups. Spatial (site 1, site 2, and site 3 in April) and temporal comparisons (site 3 from April to June) revealed that the bacterial community associated with cultivated S. japonica was influenced by sampling time and sampling site independent of the health status. Several OTUs from the Roseobacter group, including members of the genera Sulfitobacter and Loktanella, were enriched in diseased tissues suggesting these taxa could be candidate opportunistic pathogens. In contrast, OTUs belonging to the genera Halomonas were enriched in healthy tissues, indicating a potential beneficial role for group during the cultivation of S. japonica. Our results provide important baseline knowledge of the epiphytic bacterial communities and possibly pathogenic bacteria associated with commercially cultivated S. japonica.

Global Response of Phaeobacter inhibens DSM 17395 to Deletion of Its 262-kb Chromid Encoding Antibiotic Synthesis

The marine alphaproteobacterium Phaeobacter inhibens DSM 17395, a member of the Roseobacter group, was recently shown to markedly enhance growth upon deletion of its 262-kb chromid encoding biosynthesis of tropodithietic acid (TDA). To scrutinize the metabolic/regulatory adaptations that underlie enhanced growth of the Δ262 mutant, its transcriptome and proteome compared to the wild type were investigated in process-controlled bioreactors with Casamino Acids as growth substrate. Genome resequencing revealed only few additional genetic changes (a heterogenic insertion, prophage activation, and several point mutations) between wild type and Δ262 mutant, albeit with no conceivable effect on the studied growth physiology. The abundances of the vast majority of transcripts and proteins involved in the catabolic network for complete substrate oxidation to CO₂ were found to be unchanged, suggesting that the enhanced amino acid utilization of the Δ262 mutant did not require elevated synthesis of most enzymes of the catabolic network. Similarly, constituents of genetic information processing and cellular processes remained mostly unchanged. In contrast, 426 genes displayed differential expression, of which 410 were localized on the 3.2-Mb chromosome, 5 on the 65-kb chromid, and 11 on the 78-kb chromid. Notably, the branched-chain amino transferase IlvE acting on rapidly utilized Val, Ile, and Leu was upregulated. Moreover, the transportome was reconfigured, as evidenced from increased abundances of transcripts and proteins of several uptake systems for amino acids and inorganic nutrients (e.g., phosphate). Some components of the respiratory chain were also upregulated, which correlates with the higher respiration rates of the Δ262 mutant. Furthermore, chromosomally encoded transcripts and proteins that are peripherally related to TDA biosynthesis (e.g., the serine acyl transferase CysE) were strongly downregulated in the Δ262 mutant. Taken together, these observations reflect adaptations to enhanced growth as well as the functional interconnectivity of the replicons of P. inhibens DSM 17395.

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.

Description of Palleronia rufa sp. nov., a biofilm-forming and AHL-producing Rhodobacteraceae, reclassification of Hwanghaeicola aestuarii as Palleronia aestuarii comb. nov., Maribius pontilimi as Palleronia pontilimi comb. nov., Maribius salinus as Palleronia salina comb. nov., Maribius pelagius as Palleronia pelagia comb. nov. and emended description of the genus Palleronia

Strain MOLA 401T was isolated from marine waters in the southwest lagoon of New Caledonia and was shown previously to produce an unusual diversity of quorum sensing signaling molecules. This strain was Gram-negative, formed non-motile cocci and colonies were caramel. Optimum growth conditions were 30°C, pH 8 and 3% NaCl (w/v). Based on 16S rRNA gene sequence analysis, this strain was found to be closely related to Pseudomaribius aestuariivivens NBRC 113039T (96.9% of similarity), Maribius pontilimi DSM 104950T (96.4% of similarity) and Palleronia marisminoris LMG 22959T (96.3% of similarity), belonging to the Roseobacter group within the family Rhodobacteraceae. As its closest relatives, strain MOLA 401T is able to form a biofilm on polystyrene, supporting the view of Roseobacter group strains as prolific surface colonizers. An in-depth genomic study allowed us to affiliate strain MOLA 401T as a new species of genus Palleronia and to reaffiliate some of its closest relatives in this genus. Consequently, we describe strain MOLA 401T (DSM 106827T=CIP 111607T=BBCC 401T) for which we propose the name Palleronia rufa sp. nov. We also propose to emend the description of the genus Palleronia and to reclassify Maribius and Hwanghaeicola species as Palleronia species.

A marine plasmid hitchhiking vast phylogenetic and geographic distances

Horizontal gene transfer (HGT) plays an important role in bacterial evolution and serves as a driving force for bacterial diversity and versatility. HGT events often involve mobile genetic elements like plasmids, which can promote their own dissemination by associating with adaptive traits in the gene pool of the so-called mobilome. Novel traits that evolve through HGT can therefore lead to the exploitation of new ecological niches, prompting an adaptive radiation of bacterial species. In this study, we present phylogenetic, biogeographic, and functional analyses of a previously unrecognized RepL-type plasmid found in diverse members of the marine Roseobacter group across the globe. Noteworthy, 100% identical plasmids were detected in phylogenetically and geographically distant bacteria, revealing a so-far overlooked, but environmentally highly relevant vector for HGT. The genomic and functional characterization of this plasmid showed a completely conserved backbone dedicated to replication, stability, and mobilization as well as an interchangeable gene cassette with highly diverse, but recurring motifs. The majority of the latter appear to be involved in mechanisms coping with toxins and/or pollutants in the marine environment. Furthermore, we provide experimental evidence that the plasmid has the potential to be transmitted across bacterial orders, thereby increasing our understanding of evolution and microbial niche adaptation in the environment.

Early Colonization of Weathered Polyethylene by Distinct Bacteria in Marine Coastal Seawater

Plastic debris in aquatic environments is rapidly colonized by a diverse community of microorganisms, often referred to as the “Plastisphere.” Given that common plastics are derived from fossil fuels, one would expect that Plastispheres should be enriched with obligate hydrocarbon-degrading bacteria (OHCB). So far, though, different polymer types do not seem to exert a strong effect on determining the composition of the Plastisphere, and putative biodegrading bacteria are only found as rare taxa within these biofilms. Here, we show through 16S rRNA gene sequencing that the enrichment of a prominent OHCB member on weathered and non-weathered polyethylene only occurred at early stages of colonization (i.e., after 2 days of incubation in coastal marine water; 5.8% and 3.7% of relative abundance, respectively, vs. 0.6% on glass controls). As biofilms matured, these bacteria decreased in relative abundance on all materials (< 0.3% after 9 days). Apart from OHCB, weathered polyethylene strongly enriched for other distinct organisms during early stages of colonization, such as a specific member of the Roseobacter group and a member of the genus Aestuariibacter (median 26.9% and 1.8% of the community, respectively), possibly as a consequence of the availability of short-oxidized chains generated from weathering. Our results demonstrate that Plastispheres can vary in accordance with the weathering state of the material and that very early colonizing communities are enriched with taxa that can potentially degrade hydrocarbons. Given the lack of persistent enrichment and overall community convergence between materials over time, common non-hydrolysable polymers might not serve as an important source of carbon for mature Plastispheres once the labile substrates generated from weathering have been depleted.

Genome sequence of Epibacterium ulvae strain DSM 24752T, an indigoidine-producing, macroalga-associated member of the marine Roseobacter group

Strain U95T (= DSM 24752T = LMG 26464T) is the type strain of Epibacterium ulvae, which is the type species of the genus Epibacterium. This genus belongs to the marine Roseobacter group. E. ulvae Strain U95T was isolated from the macroalga Ulva australis, is Gram-negative, rod-shaped and motile. Here we describe the permanent draft genome sequence and annotation of E. ulvae U95T with a focus on secondary metabolite production and interaction with its host. The genome contains 4,092,893 bp, 3977 protein-coding genes and 60 RNA genes. The genome encodes a gene cluster for synthesis of the blue-pigmented secondary metabolite indigoidine and contains several genes for adhesion mechanisms, putative bacteriocin, siderophores, a type VI secretion system, and enzymes that confer oxidative stress resistance. Combined, these features may aid in the successful colonization and persistence of E. ulvae on host surfaces and in competition with the surrounding microbial consortium.

Distinct relationships between fluorescence in situ hybridization and 16S rRNA gene- and amplicon-based sequencing data of bacterioplankton lineages

Catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) and amplicon sequencing of the total (16S rRNA gene) and potentially active (16S rRNA transcripts), community are the major state of the art approaches for assessing the composition of bacterial communities in marine pelagic and other ecosystems. However, CARD-FISH and amplicon sequencing methods have not yet been directly compared to assess the composition of bacterioplankton communities. Therefore, these approaches were used to study the composition of bacterial communities in two North Sea seawater mesocosm experiments supplemented with diatom-derived organic matter (OM). All approaches revealed Proteobacteria and Bacteroidetes as major components of the bacterioplankton communities. The Roseobacter group and its RCA cluster, as well as Bacteroidetes and Gammaproteobacteria, responded most strongly to OM addition, whereas the SAR11 clade responded in only one of the two mesocosms. A correlation analysis showed that CARD-FISH and amplicon sequencing data of the SAR11 clade and the Roseobacter group, together with its RCA cluster, were highly significantly correlated, whereas Bacteroidetes did not yield any significant correlation and Gammaproteobacteria was only correlated with the potentially active fraction. However, subgroups of these phylogenetic groups, the SAR92 clade, the genera Pseudoalteromonas and Polaribacter, exhibited significant correlations in one of the two mesocosms. Correlations of CARD-FISH with amplicon sequencing data from the total and potentially active fractions of these lineages exhibited distinct differences. The study showed that CARD-FISH and amplicon sequencing data of distinct bacterioplankton groups and especially the phylogenetic lineages at a higher taxonomic level were correlated but reflected different aspects of their growth dynamics.

Tropodithietic acid induces oxidative stress response, cell envelope biogenesis and iron uptake in Vibrio vulnificus.

The Roseobacter group is a widespread marine bacterial group, of which some species produce the broad-spectrum antibiotic tropodithietic acid (TDA). A mode of action for TDA has previously been proposed in Escherichia coli, but little is known about its effect on non-producing marine bacteria at in situ concentrations. The purpose of this study was to investigate how a sub-lethal level of TDA affects Vibrio vulnificus at different time points (30 and 60 min) using a transcriptomic approach. Exposure to TDA for as little as 30 min resulted in the differential expression of genes associated with cell regeneration, including the up-regulation of those involved in biogenesis of the cell envelope. Defence mechanisms including oxidative stress defence proteins and iron uptake systems were also up-regulated in response to TDA, while motility-related genes were down-regulated. Gene expression data and scanning electron microscopy imaging revealed a switch to a biofilm phenotype in the presence of TDA. Our study shows that a low concentration of this antibiotic triggers a defence response to reactive oxygen species and iron depletion in V. vulnificus, which indicates that the mode of action of TDA is likely more complex in this bacterium than what is known for E. coli.

Diversity and Distribution of Bacteria Producing Known Secondary Metabolites.

There is an increasing interest in the utilisation of marine bioactive compounds as novel biopharmaceuticals and agrichemicals; however, little is known about the environmental distribution for many of these molecules. Here, we aimed to elucidate the environmental distribution and to detect the biosynthetic gene clusters in environmental samples of four bioactive compounds, namely violacein, tropodithietic acid (TDA), tambjamine and the antibacterial protein AlpP. Our database analyses revealed high bacterial diversity for AlpP and violacein producers, while TDA-producing bacteria were mostly associated with marine surfaces and all belonged to the roseobacter group. In contrast, the tambjamine cluster was only found in the genomes of two Pseudoalteromonas species and in one terrestrial species belonging to the Cupriavidus genus. Using a PCR-based screen of different marine samples, we detected TDA and violacein genes associated with the microbiome of Ulva and Protohyale niger and tambjamine genes associated with Nodilittorina unifasciata; however, alpP was not detected. These results highlight the variable distribution of the genes encoding these four bioactive compounds, including their detection from the surface of multiple marine eukaryotic hosts. Determining the natural distribution of these gene clusters will help to understand the ecological importance of these metabolites and the bacteria that produce them.

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.