Establishing the breadcrumb sponge Halichondria panicea as an experimental model for sponge symbioses
The early divergent metazoan phylum Porifera (sponges) holds potentially important emerging model systems to study microbiome stability, colonization and evolutionarily early mechanisms of animal-microbe interactions. However, experimental approaches that allow the manipulation of sponge microbiomes are currently lacking. To fill this gap, the overarching aim of my PhD thesis was to establish the breadcrumb sponge Halichondria panicea and its dominant bacterial symbiont Candidatus Halichondribacter symbioticus as an experimental model. First, I approached the sponge holobiont by summarizing current knowledge on multispecies interactions within the host. Then, I focused on the host-side by exposing sponges to bacterial LPS and characterized their immune repertoire as well as induced immune response by RNAseq. We suggest that H. paniceahas context-dependent strategies of immune gene expression (constitutive vs inducible; ubiquitous vs individual-specific), reflecting the diverse roles of innate immunity in sponges. Lastly, I approached the microbial-side of the sponge holobiont and developed an experimental set-up to culture sponges under sterile conditions and manipulate sponge-associated bacteria with antibiotics. Antibiotics induced repeatable shifts in bacterial community composition towards a dysbiotic state, while the total number of associated bacteria increased. We tested recolonization with the natural microbiome to recover dysbiotic microbiomes. This strategy was not successful, however single bacterial taxa were transferred by recolonization. Importantly, the combination of relative bacterial abundance data together with absolute data uncovered a high stability of Ca. H. symbioticus in spite of dysbiosis at the microbial community level. Overall, my findings contribute to an improved understanding of microbiome dynamics, host-microbe and microbe-microbe interactions in the H. panicea holobiont.