Symbioses from hydrothermal vents and cold seeps

Rimicaris shrimp

K. van der Heijden, C. Borowski

The aim of this study is to shed light on the evolutionary history of Bathymodiolus mussels at the Mid-Atlantic Ridge. Previous phylogenetic analyses of the mussels showed that the classical marker gene for phylogenetic assessment, the mitochondrial mtCOI, does not provide enough resolution to clarify the relationship within this group.
To investigate the geographic structure, gene genealogies, and the major directions and relative amounts of historical gene flow between the four mussel populations at the Mid-Atlantic Ridge, we are using a multi-locus approach for this population genetic study. The results of our analyses are used to reconstruct the evolutionary history of the mussel populations which also provides us with insight on the colonization history of the Mid-Atlantic Ridge.

M. Ücker

In the Bathymodiolus symbiosis, little is known about the influence of the symbionts on the evolution of the host despite many studies that have been performed on the symbionts‘ physiology. Our study aims to understand the population structure and dynamics of Bathymodiolus mussels and the role of symbioses in animal hybridisation. We analyse mussel samples from a vent field at the Mid-Atlantic Ridge where two mussel species, B. azoricus and B. puteoserpentis, have been described to form viable hybrids. We use diagnostic markers and metagenomic techniques to assess the hybrid status of each mussel individual and to investigate the phylogeny of host mitochondria and the symbionts. We hope to shed new light on the symbiosis in hybrid Bathymodiolus mussels and investigate questions of symbiont specificity and recognition. Understanding hybridisation in symbiotic animals allows us to study species formation and evolution in natural communities and the role of bacterial symbionts in these processes.

M. Franke, N. Leisch

Adult Bathymodiolus mussels harbour their primary symbionts in the gill tissue. In juvenile mussels however, the symbionts colonize not only the gills but also all other epithelial tissues within the mantle cavity. Only little is known about how and when symbionts first colonize the mussels. To understand the processes and preferences of symbiont colonization, we examine early developmental stages of Bathymodiolus mussels. Using a correlative imaging approach, which unites light- and electron microscopy with synchrotron radiation-based X-ray micro tomography, we are able to generate a detailed three-dimensional morphological dataset. This dataset can be combined with symbiont localization using 16S rRNA FISH and allows us to determine the time of symbiont colonization revealed by distinct morphological adaptations during the development of the host. Our work serves as a starting point for further functional analyses to elucidate how bacteria can influence and shape early eukaryotic development.

B. Geier, M. Franke, E. Sogin, D. Michellod, M. Á. Gónzalez-Porras, A. Gruhl, N. Leisch, M. Liebeke

Endosymbioses such as in Bathymodiolus mussels are characterized by intimate metabolic interactions among the symbiotic partners. Yet, we know very little about the nature of these interactions in symbioses, particularly in their environmental context. Therefore, we created a spatial metabolomics pipeline (metaFISH) that links symbiont genotypes with metabolic phenotypes to allow a spatial assignment of host and symbiont metabolites at the scale of individual host cells. We could observe how the spatial chemistry of host cells differs with the presence of intracellular symbionts, found variations in metabolic phenotypes in a single symbiont type and detected novel metabolites. To gain a broader understanding of metabolite distribution at the scale of whole animals, we also developed a correlative workflow that allows imaging the 3D anatomy and molecular composition of animal-microbe systems. The high spatial resolution gained be these imaging approaches allows us to tackle some of the fundamental question in symbiosis research such as how do the symbiotic partners interact.

M. Tietjen

To understand the cellular interactions between Bathymodiolus mussels and their intracellular symbionts, we make use of advanced next-generation sequencing techniques and investigate gene expression patterns of both partners. In particular, we are investigating which gene (groups) are responsible for the maintenance of the symbiosis in the light of changing environmental conditions. One example for such conditions is the limited access to hydrothermal fluids that carry essential energy sources for the symbionts such as sulfur or methane. Such varying conditions are expected to occur regularly in hydrothermal systems. We simulate such conditions with in situ and laboratory experiments, and compare the physiological responses of both host and symbionts on the transcriptome and proteome level.

M. Á. Gónzalez-Porras, N. Leisch

Ca. Endonucleobacter is a gammaproteobacterial parasite that infects the nuclei of Bathymodiolus mussels. After a single Ca. Endonucleobacter cell invades the host nucleus, it proliferates massively (up to 80,000 bacteria per nucleus) and the volume of the nucleus increases up to 50 fold. We investigate how Ca. Endonucleobacter thrives in the nucleus, how it affects host cells and how it prevents the shutdown of the eukaryotic cell (e.g. apoptosis). To do so, we have sequenced the genome of the parasite and coupled laser capture microdissection of infected nuclei with ultra-low-input RNA-sequencing. We are now investigating how the transcriptional profiles of parasite and host change along the infectious cycle. By targeting the transcriptomic profiles of specific life stages of an intranuclear parasite in situ, our pipeline closes the gap between the visual, molecular and temporal characterization of a parasite-host interaction.

R. Ansorge, M. Á. Gónzalez-Porras

Beyond species boundaries, small genomic changes can strongly impact the lifestyle of a bacterial strain. However, currently it is not well understood how strain-level diversity in symbioses emerges, persists and if or how it impacts mutualistic associations. We developed a metagenomics approach to tease apart strain-level differences in Bathymodiolus symbionts and discovered striking functional differences among co-occurring symbiont strains. This high resolution allows us to study symbiont population structure that helps us understand symbiont colonization dynamics. Using cultivation-independent methods our research provides a snapshot of the symbiosis in its natural context, allowing us to investigate how such strain diversity evolved, persists and how it compares to free-living relatives of the symbionts.

P. Bourceau

Over the last years, our omic approaches have given us insights into the intricate relationship and metabolic interaction between the Bathymodiolus hosts and their symbionts. To fully show that particular processes take place, we apply manipulative experiments with symbiont relatives such as Methyloprofundus sedimenti, as culturing Bathymodiolus symbionts remains challenging. By using controlled incubation experiments and mass-spectrometry based analyses of the metabolome, we can measure traits of the M. sedimenti genome that are shared with the methane-oxidising symbiont of Bathymodiolus mussels. Not only can we identify possible substrates but also excreted metabolites, which could potentially be available to the host. Our overall aim is to characterize metabolic markers linked to environmental conditions to conclude the recent history of a specimen from the deep-sea with metabolomics data.

Expedition Blog of Research Cruise SO253

One of the most abundant animals at hydrothermal vent systems of the Western Pacific is the snail Ifremeria nautilei. These hosts harbor at least 4 bacterial symbionts in their gills, sulfide- and methane-oxidizing gammaproteobacteria and at least 2 alphaproteobacterial phylotypes of unknown function. We are currently using comparative sequence analysis of phylogenetic and functional genes to gain a better understanding of these symbioses (Borowski et al. in prep.).