Dissecting the heat stress altering immune responses and skin microbiota in fish in a recirculating aquaculture system in Singapore

Environmental factors, probiotics and feed additives affect microbiota diversity in fish. Water temperature disrupts the composition and diversity of microbiota, with temperature changes triggering stress and immune responses in fish. In Singapore, water heat treatment is used to control and prevent disease outbreaks in farmed fish. Although gut microbiota responses to heat stress have been reported, little is known about the effects of heat treatment on fish immune responses and fish skin microbiota dynamics. Over a 3-mo interval, we investigated microbiota dynamics and fish immune responses associated with a heat treatment practice in a commercial fish farm with a recirculating aquaculture system (RAS). Tank water temperature was raised to 37-39 {degrees}C for 1 hour, every 2 weeks. Tank water and fish microbial communities were analysed by 16S amplicon sequencing, and host molecular expressions were determined by qPCR. We inferred that heat treatment increased stress and immune responses that protected fish from opportunistic infections. However, overreaction to temperature stress can cause dysbiosis of the skin microbiota and death. We also suggested the value of the skin microbiota Pseudomonadota: Bacteroidota (P:B) ratio as a biomarker for aquaculture fish health. IMPORTANCEAquaculture is an emerging economic activity to supply high-quality animal protein and reduce reliance on wild-caught fish products. Recently, there has been emphasis on intensive aquaculture, using a Recirculating Aquaculture System (RAS). In RAS, management of pathogens/parasites prevalence is a major challenge. Developing practical solutions for producing healthy juveniles in nursery systems will make profound contributions to sustainable aquaculture. In this study, we used an unconventional strategy, exposing juveniles to the pathobiome in the environment, followed by non-lethal heat shock treatments to enhance immunity. Short-term stress induced heat shock proteins that protected fish from opportunistic infections. We concluded that manipulating environmental-microbial-host interactions, together with enhanced functional capacity of fish immune response, has potential for disease control in aquaculture.