Dissecting the impact of heat stress on heat-shock response and skin microbiota in farmed fish in a recirculating aquaculture system in Singapore.

Periodic heat treatment is a method used by aquaculture to reduce fish-borne pathogens. Few studies have examined the effects of heat treatment on fish gene expression and changes to skin microbiota, despite their role in fish health. Thus, this study aimed to evaluate the fish heat-shock response and changes to the skin microbiota during heat treatment in a commercial fish farm with a recirculating aquaculture system. Differences in host gene expression were determined by quantitative PCR (qPCR), and we found that heat treatment increased heat-shock protein ( and ) and cytokine (interleukin-1-beta and ) transcript expression. The 16S ribosomal RNA gene amplicon analysis of fish skin microbiota and qPCR detection of scale drop disease virus (SDDV) from skin revealed a reduction in opportunistic bacteria and viruses of surviving fish after heat treatment. However, after heat treatment, moribund fish exhibited the highest levels of heat-shock responses and opportunistic pathogen abundance. We inferred that heat treatment also induced an overreaction to temperature stress causing dysbiosis of fish microbiota and even death. We also propose the value of the skin microbiota Pseudomonadota/Bacteroidota ratio as a biomarker for aquaculture fish health.IMPORTANCEThe application of disruptive technologies such as recirculating aquaculture systems (RAS) to aquaculture aims to increase productivity and maximize economic yield. However, significant challenges remain in pathogen and parasite prevention despite RAS. 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 water heat treatments to enhance fish responses. We found that short-term water temperature fluctuations have no impact on skin microbiota, whereas induced heat-shock responses reduced opportunistic bacteria and viruses in surviving fish. We inferred that manipulating microbial-host and environmental interactions, together with the enhanced functional capacity of fish stress/immune response, could have an impact on disease control in aquaculture. We also determined the use of skin microbiota in fish health monitoring.