The role of diversity in mediating microbiota structural and functional responses in two sympatric species of abalone under stressed withering syndrome conditions

Background: The withering syndrome (WS) is an infectious disease initially affecting the gastro-intestinal tract (GI) of wild abalone populations of the coasts of Baja California. In spite of its high incidence, structural and functional changes in abalone GI microbiotas under WS-stressed conditions remain poorly investigated. Moreover, it is equally uncertain if interspecific microbiota features, such as the presence or absence of certain bacterial species, their abundances, and their functional capabilities, may prevent or at least lead to different microbiota responses. Healthy Haliotis fulgens and Haliotis corrugata from Baja California Sur (Mexico) harbor species-specific structural and functional microbiota profiles; hence, we hypothesize a distinctive microbiota response under WS-stressed conditions. Here, we compared both the structural arrangements and functional capabilities of healthy and dysbiotic microbiotas using 454 pyrosequencing high throughput sequencing technologies and PICRUSt v.2 outputs, respectively.Results: Our findings suggest that the extent to which WS may involve structural and functional changes in GI microbiotas is contingent on the microbiota diversity itself. Indeed, we report significant structual alterations in the less complex microbiotas of H. fulgens, which in turn led to a significant downregulation of several metabolic activities conducted by GI bacteria. Conversely, the effects of WS were marginal in more complex bacterial communities, as in H. corrugata, in which no significant structural and functional changes were detected. Conclusions: Our results provide new insights concerning the role of microbiome diversity in abalone health and the etiology of WS. Notably, complex bacterial communities appear to be less affected by WS than less complex microbiotas. Moreover, our insights suggest that structural changes observed under WS-stressed conditions may be considered stochastic, as predicted by the Anna Karenina principle, and result in the downregulation of several ecological functions conducted by GI bacteria. Overall, our results support the hypothesis that the occurrence of WS may be associated with shifts in GI microbiotas. Moreover, we propose that the susceptibility to WS that has been reported among abalone species may reflect the natural degree of complexity of the GI microbiomes harbored by each species.

Microbiota of the Digestive Gland of Red Abalone (Haliotis rufescens) Is Affected by Withering Syndrome

Withering syndrome (WS), an infectious disease caused by intracellular bacteria Candidatus Xenohaliotis californiensis, has provoked significant economic losses in abalone aquaculture. The pathogen infects gastroenteric epithelia, including digestive gland, disrupting the digestive system and causing a progressive wilting in abalone. Nonetheless, our knowledge about WS implications in digestive gland microbiota, and its role in diseases progress remains largely unknown. This study aims to determine whether digestive gland-associated microbiota differs between healthy red abalone (Haliotis rufescens) and red abalone affected with WS. Using high-throughput sequencing of the V4 region of the 16S rRNA gene, our results revealed differences in microbiota between groups. Bacterial genera, including Mycoplasma, Lactobacillus, Cocleimonas and Tateyamaria were significantly more abundant in healthy abalones, whilst Candidatus Xenohaliotis californiensis and Marinomonas were more abundant in WS-affected abalones. Whilst Mycoplasma was the dominant genus in the healthy group, Candidatus Xenohaliotis californiensis was dominant in the WS group. However, Candidatus Xenohaliotis californiensis was present in two healthy specimens, and thus the Mycoplasma/Candidatus Xenohaliotis californiensis ratio appears to be more determinant in specimens affected with WS. Further research to elucidate the role of digestive gland microbiota ecology in WS pathogenesis is required.