The Influence of Genes on the "Killer Plasmid" of Dinoroseobacter shibae on Its Symbiosis With the Dinoflagellate Prorocentrum minimum

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.

Bactericidal effectors of the Stenotrophomonas maltophilia type IV secretion system: functional definition of the nuclease TfdA and structural determination of TfcB

Stenotrophomonas maltophilia expresses a type IV protein secretion system (T4SS) that promotes contact-dependent killing of other bacteria and does so partly by secreting the effector TfcB. Here, we report the structure of TfcB, comprising an N-terminal domain similar to the catalytic domain of glycosyl hydrolase (GH-19) chitinases and a C-terminal domain for recognition and translocation by the T4SS. Utilizing a two-hybrid assay to measure effector interactions with the T4SS coupling protein VirD4, we documented the existence of five more T4SS substrates. One of these was protein 20845, an annotated nuclease. A S. maltophilia mutant lacking the gene for 20845 was impaired for killing Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Moreover, the cloned 20845 gene conferred robust toxicity, with the recombinant E. coli being rescued when 20845 was co-expressed with its cognate immunity protein. The 20845 effector was an 899 amino-acid protein, comprised of a GHH-nuclease domain in its N-terminus, a large central region of indeterminant function, and a C-terminus for secretion. Engineered variants of the 20845 gene that had mutations in the predicted catalytic site did not impede E. coli, indicating that the antibacterial effect of 20845 involves its nuclease activity. Using flow cytometry with DNA staining, we determined that 20845, but not its mutant variants, confers a loss in DNA content of target bacteria. Database searches revealed that uncharacterized homologs of 20845 occur within a range of bacteria. These data indicate that the S. maltophilia T4SS promotes interbacterial competition through the action of multiple toxic effectors, including a potent, novel DNase.IMPORTANCEStenotrophomonas maltophilia is a multi-drug-resistant, Gram-negative bacterium that is an emerging pathogen of humans. Patients with cystic fibrosis are particularly susceptible to S. maltophilia infection. In hospital water systems and various types of infections, S. maltophilia co-exists with other bacteria, including other pathogens such as Pseudomonas aeruginosa. We previously demonstrated that S. maltophilia has a functional VirB/D4 type VI protein secretion system (T4SS) that promotes contact-dependent killing of other bacteria. Since most work on antibacterial systems involves the type VI secretion system, this observation remains noteworthy. Moreover, S. maltophilia currently stands alone as a model for a human pathogen expressing an antibacterial T4SS. Using biochemical, genetic, and cell biological approaches, we now report both the discovery of a novel antibacterial nuclease (TfdA) and the first structural determination of a bactericidal T4SS effector (TfcB).

The microbiome of the dinoflagellate Prorocentrum cordatum in laboratory culture and its changes at higher temperatures

In the ocean, phytoplankton are dependent on communities of bacteria living in the phycosphere, a hot spot of metabolic and genetic exchange. Many types of interactions between phytoplankton and phycosphere bacteria have been shown, but it is unclear if the microbial communities associated with microalgae strains in culture collections are beneficial or harmful to the host strain. Here, we studied the microbial communities associated with four strains of the dinoflagellate Prorocentrum cordatum that had been isolated from distant geographical locations and maintained in culture collection for hundreds of generations. Community composition was determined by 16S rRNA gene amplicon sequencing. The dinoflagellate host strain was the strongest parameter separating communities, while growth phase, lifestyle (particle-attached versus free-living) and temperature had only a modulating effect. Although the strains had been isolated from distant locations in the Atlantic and Pacific Ocean, 14 ASVs were shared among all strains, the most abundant ones being Gilvibacter, Marivita, uncultivated Rhodobacteraceae, Marinobacter, Hyphomonadaceae, Cupriavidus, Variovorax, and Paucibacter. Adaptation to higher temperatures resulted in specific changes in each phycosphere microbiome, including increased abundance of rare community members. We then compared the growth of the four xenic cultures to that of the axenic P. cordatum CCMP1329. At 20°C, growth of the xenic cultures was similar or slower than that of CCMP1329. At 26°C, all four xenic cultures experienced a death phase, while the axenic culture stably remained in the stationary phase. At 30°C, only two of the xenic cultures were able to grow. A shift of dinoflagellate metabolism from autotrophy to mixotrophy and competition between dinoflagellate and bacteria for limiting nutrients, including essential vitamins, may contribute to these differences in growth patterns.

The Sixth Element: a 102-kb RepABC Plasmid of Xenologous Origin Modulates Chromosomal Gene Expression in Dinoroseobacter shibae

The model organism Dinoroseobacter shibae and many other marine Rhodobacterales (Roseobacteraceae, Alphaproteobacteria) are characterized by a multipartite genome organization. Here, we show that the original isolate (Dshi-6) contained six extrachromosomal replicons (ECRs), whereas the strain deposited at the DSMZ (Dshi-5) lacked a 102-kb plasmid. To determine the role of the sixth plasmid, we investigated the genomic and physiological differences between the two strains. Therefore, both genomes were (re)sequenced, and gene expression, growth, and substrate utilization were examined. For comparison, we included additional plasmid-cured strains in the analysis. In the Dshi-6 population, the conjugative 102-kb RepABC-9 plasmid was present in only about 50% of the cells, irrespective of its experimentally validated stability. In the presence of the sixth plasmid, copy number changes of other ECRs, in particular, a decrease of the 86-kb plasmid, were observed. The most conspicuous finding was the strong influence of plasmids on chromosomal gene expression, especially the repression of the CtrA regulon and the activation of the denitrification gene cluster. Expression is inversely controlled by either the presence of the 102-kb plasmid or the absence of the 86-kb plasmid. We identified regulatory genes on both plasmids, i.e., a sigma 70 factor and a quorum sensing synthase, that might be responsible for these major changes. The tremendous effects that were probably even underestimated challenge the current understanding of the relevance of volatile plasmids not only for the original host but also for new recipients after conjugation.

IMPORTANCE Plasmids are small DNA molecules that replicate independently of the bacterial chromosome. The common view of the role of plasmids is dominated by the accumulation of resistance genes, which is responsible for the antibiotic crisis in health care and livestock breeding. Beyond rapid adaptations to a changing environment, no general relevance for the host cell’s regulome was attributed to these volatile ECRs. The current study shows for the model organism D. shibae that its chromosomal gene expression is strongly influenced by two plasmids. We provide evidence that the gain or loss of plasmids not only results in minor alterations of the genetic repertoire but also can have tremendous effects on bacterial physiology. The central role of some plasmids in the regulatory network of the host could also explain their persistence despite fitness costs, which has been described as the “plasmid paradox.”

Identification of Volatiles of the Dinoflagellate Prorocentrum cordatum

The dinoflagellate Prorocentrum cordatum, often called P. minimum, is a potentially toxic alga found in algal blooms. Volatile compounds released by the alga might carry important information, e.g., on its physiological state, and may act as chemical messengers. We report here the identification of volatile organic compounds emitted by two strains, xenic P. cordatum CCMP 1529 and axenic P. cordatum CCMP 1329. The volatiles released during culture were identified despite their low production rates, using sensitive methods such as open-system-stripping analysis (OSSA) on Tenax TA desorption tubes, thermodesorption, cryofocusing and GC/MS-analysis. The analyses revealed 16 compounds released from the xenic strain and 52 compounds from the axenic strain. The majority of compounds were apocarotenoids, aromatic compounds and small oxylipins, but new natural products such as 3,7-dimethyl-4-octanolide were also identified and synthesized. The large difference of compound composition between xenic and axenic algae will be discussed.

Long-Term Stability of Bacterial Associations in a Microcosm of Ostreococcus tauri (Chlorophyta, Mamiellophyceae)

Phytoplankton-bacteria interactions rule over carbon fixation in the sunlit ocean, yet only a handful of phytoplanktonic-bacteria interactions have been experimentally characterized. In this study, we investigated the effect of three bacterial strains isolated from a long-term microcosm experiment with one Ostreococcus strain (Chlorophyta, Mamiellophyceae). We provided evidence that two Roseovarius strains (Alphaproteobacteria) had a beneficial effect on the long-term survival of the microalgae whereas one Winogradskyella strain (Flavobacteriia) led to the collapse of the microalga culture. Co-cultivation of the beneficial and the antagonistic strains also led to the loss of the microalga cells. Metagenomic analysis of the microcosm is consistent with vitamin B12 synthesis by the Roseovarius strains and unveiled two additional species affiliated to Balneola (Balneolia) and Muricauda (Flavobacteriia), which represent less than 4% of the reads, whereas Roseovarius and Winogradskyella recruit 57 and 39% of the reads, respectively. These results suggest that the low-frequency bacterial species may antagonize the algicidal effect of Winogradskyella in the microbiome of Ostreococcus tauri and thus stabilize the microalga persistence in the microcosm. Altogether, these results open novel perspectives into long-term stability of phytoplankton cultures.

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.