How ultraviolet disinfection increases mortality of mud crabs in recirculating aquaculture systems (RAS): Insights from microbial community analysis

Ultraviolet (UV) disinfection is widely used to enhance water quality in recirculating aquaculture systems (RAS); however, concerns about its potential adverse effects on aquatic animal survival have emerged. The mechanisms underlying UV-induced mortality are poorly understood, complicating the effective health management of aquatic species in RAS. Our study evaluated the impact of UV disinfection on juvenile mud crab survival in RAS, revealing a statistically significant decline from 76.4 % to 48.6 %. To identify contributing factors, we analyzed microbial composition in RAS water, as well as in the gut and carapace of the crabs and the biofilm of moving bed biofilm reactors (MBBRs). Our analysis indicated that UV disinfection compromised the complexity and stability of bacterial communities in the water, which subsequently affected the microbial composition in the gut, carapace, and biofilm. FEAST (fast expectation-maximization for microbial source tracking) analysis revealed an interaction between biofilm and water bacterial communities, with both contributing to the carapace and gut bacterial communities to varying degrees. Metagenomic analysis of the biofilm suggested that changes in microbial dynamics contributed to elevated nitrite levels in the rearing water. Functional analysis of gut and carapace microbial dynamics indicated increased biofilm formation on the carapace, along with strengthened antibiotic biosynthesis and protein export functions in the gut. Structural equation modeling revealed that variation in the carapace bacterial community is correlated with juvenile crab survival. Furthermore, UV disinfection also reduced the relative abundance of ARGs in RAS, thereby reducing the threat to environmental safety posed by aquaculture wastewater. Therefore, it is recommended to adjust the UV dose and disinfection duration, and to avoid prolonged disinfection before the rearing tanks.