Depuration of a solitary ascidian depletes transient bacteria without altering microbiome alpha-diversity

Depuration, or the process of clearing impurities from the gut, is commonly applied to marine food products due to its efficacy in removing human pathogens from shellfish and edible ascidians. Recent studies also reported that depuration of filter-feeding animals helped reduce transient bacteria and identify resident symbionts in gut microbiome studies. Here, we examined the impact of depuration on bacteria in the branchial sac, gut, and hepatic gland of the solitary ascidian Pyura vittata. Replicates were kept in filtered seawater for 4 days prior to dissection (aquaria-depuration) and compared to samples that were immediately processed following collection (wild-no depuration) and replicates kept in unfiltered seawater for 4 days (aquaria-control). 16S rRNA gene sequence analysis revealed no significant differences among ascidian sources for microbial alpha-diversity but significant shifts in beta-diversity. Depuration reduced the number of core bacteria markedly (66%-84%) across all body regions, and bacteria that remained postdepuration consisted of genera associated with enhanced host health and resilience within other marine symbioses. Our results suggest that microbial profiles obtained following depuration do not substantially differ from those of nondepurated animals, but depuration can help differentiate transient from core and resident taxa in complex host-microbiome symbioses.