Sponges form symbiotic relationships with bacteria throughout their lifecycle, in a partnership that provides key services for the host. The transmission and acceptance of bacterial symbionts by a sponge host occurs via two distinct mechanisms – maternal vertical inheritance, predominantly during early embryogenesis via nurse cell mediated transfer, and horizontal acquisition of bacterial species during metamorphosis and adulthood to compliment the community structure. Here we use the demosponge Amphimedon queenslandica to investigate the role of the settlement substrate on the formation of the microbiome during metamorphosis and observe potential host-bacterial interactions during this integral life-cycle transition.
Using electron microscopy coupled with 16S amplicon sequencing, we investigated the horizontal acquisition of new bacterial diversity and stability of the microbiome of A. queenslandica throughout metamorphosis. To investigate these processes at physical level, transmission electron microscopy (TEM) was used to identify cell types involved in host cell-bacteria interactions and symbiont distribution within the sponge.
This work confirmed that a small specific core bacterial community of Proteobacteria are vertically inherited from the maternal sponge during embryogenesis, while horizontal inheritance of new and diverse species was reliant on the algal settlement site chosen for metamorphosis into the benthic adult. Additionally, electron microscopy data indicates these bacterial species initially access the post-larva through the settlement surface–sponge interface. Using TEM we demonstrated that A. queenslandica conducts cell-specific responses to symbiotic bacteria and new horizontally inherited morphologies – with epidermal cells primarily phagocytosing new morphologies, and endosomal cell-types either phagocytosing only the primary symbiont morphology or all present morphologies. These data shed light on the microbiome formation and cellular responses within A. queenslandica that allows for the enrichment of the microbiome through metamorphosis, and physical processes in place providing a stable host-symbiont relationship.