Effect of oxytetracycline treatment on the gastrointestinal microbiome of critically endangered white abalone (Haliotis sorenseni) treated for withering syndrome

White abalone (Haliotis sorenseni) are critically endangered marine gastropods that are native to kelp forests in the northeastern Pacific. White abalone are highly susceptible to withering syndrome, a fatal bacterial disease caused by Candidatus Xenohaliotis californiensis (CaXc), an intracellular, order Rickettsiales prokaryote that is endemic throughout the white abalone's range in California and Mexico. Oxytetracycline (OTC) baths at a dose of 500 mg/L are successful in clearing CaXc infections from the gastrointestinal tract of infected abalone. The impact of OTC treatment on the diversity and stability of the gut microbiome in white abalone is unknown. The objectives of this study were two-fold: (1) to characterize the gastrointestinal microbiome of clinically-normal white abalone and (2) to compare the gastrointestinal microbiomes of OTC-treated white abalone to those of control animals. Gastrointestinal tracts from five OTC-treated individuals and five untreated controls were sampled at each time point: day 0, one day after the 21-day OTC treatment (day 22), and at 203 days post-treatment. Gastrointestinal tract microbiomes were analyzed after amplification and sequence of the 16S rRNA. Gastrointestinal microbiomes of untreated animals were dominated by three core bacterial phyla: Proteobacteria, Fusobacteria, and Bacteroidetes. Reduced Shannon α-diversity and absence of various phyla in the microbiome of OTC-treated animals were observed in samples at day 22. Bacterial profiles were improved in terms of α-diversity at 203 days but some bacterial phyla, mainly Fusobacteria, remained absent. All animals remained clinically normal throughout the study period and there was no significant difference in a condition index between the two groups. OTC treatment for withering syndrome appears to be clinically safe in white abalone.