Molecular and Cellular Mechanisms for Microbial Discrimination in Sponges

Sponges (Porifera) are basal metazoans that feed on  and establish symbiotic associations with microbes. How sponges discriminate between bacteria to digest as food, incorporate as symbionts, or eliminate as pathogens remains unknown. This thesis aimed to study mechanisms for microbial discrimination in sponges. Mechanisms likely to modulate sponge-microbe interactions are introduced (Chapter 1). The host transcriptomic response upon seawater and symbiont microbial consortia encounter was characterized by RNASeq in two sponges (Chapter 2). Aplysina aerophoba showed little differential gene expression and no participation of receptors, whereas Dysidea avara responded by regulating NLRs suggesting that microbial discrimination is driven by the repertoire of immune genes as well as to what degree they are induced. On the cellular level, an in-vivo phagocytosis assay was established in Halichondria panicea combining incubations with microalgae, bacteria, and latex beads with cell dissociation and fluorescence-activated cell sorting (FACS) to quantify particle incorporation into sponge cells (Chapter 3). After 30 min particles were predominantly incorporated into choanocyte-like cells and appeared to be translocated to archaeocyte-like cells after 60 min. Lastly, the established assay was combined with proteomic analysis to investigate H. panicea’s phagocytic response upon exposure to a “native” and a “foreign” Vibrio isolate (Chapter 4). Vibrio incorporation into sponge cells was indiscriminate, but the distribution of vibrios into different cell types differed between isolates. Phagocytic-related proteins were in a higher abundance in the foreign vs. the native treatment. These results indicate that bacterial discrimination in H. panicea occurs after internalization leading to differences in the processing of foreign vs native vibrio types.