Shifts in Vibrio abundance and community structure have been implicated as potential precursors to mass mortality events in oysters. This study investigates the role of Vibrios in these mortality events by comparing the genomes of 110 Vibrio strains isolated from the viscera of diploid and triploid Eastern oysters (Crassostrea virginica) in 2022 and 2023. Isolates were collected either pre-mortality or during intense mortality events across two consecutive years.
Phylogenomic analyses revealed the presence of distinct Vibrio clades that dominated pre-mortality (P clade), or during-mortality (D clades). P clade Vibrios were identified as Vibrio mediterranei, and D clade is comprised of three distinct Vibrio groups – V. astrianarerae, V. harveyi, and V. alginolyticus.
Our analysis revealed a striking absence of P clade Vibrios during mortality events, and co-isolation of P and D clade Vibrios occurred in only a single oyster. We applied pangenomic analyses to compare the metabolic capacities of these clades and identified 16 metabolic pathways enriched in either the P or D clades. Pathways enriched in P clades suggest symbiotic interactions, including the utilization of host-derived sugars and amino acids, detoxification, and waste removal. In contrast, D clade Vibrios are enriched in pathways indicative of pathogenicity and antagonistic interactions with the oyster host, including type III secretion systems and effectors.
Persistent diverticula pathology in oysters leading up to and during mortality events suggests that nutritional deprivation, compounded by microbial interactions, plays a critical role in oyster mortality. Based on our findings, we propose a mortality model where P clade Vibrios function symbiotically pre-mortality, contributing to oyster health through metabolic cooperation. However, shifts in the surrounding environment create anaerobic, nutrient-poor conditions within oyster tissue micro-niches that favor the invasion and persistence of D clade Vibrios. The displacement of P clade symbionts and opportunistic colonization by D clades likely triggers a positive feedback loop of environmental stress and microbial antagonism, culminating in oyster mortality.
Our findings emphasize the need for longitudinal studies that actively monitor microbial metabolic dynamics to determine (1) whether P clade Vibrios indeed provide essential functions to oyster health that are lost during dysbiosis and (2) how opportunistic D clade pathogens exploit these shifts to drive mortality. This work underscores the importance of microbiome interactions in oyster health and provides insights into the microbial contributions to aquaculture-related mortality events.