Climate change represents a threat to the development of the aquaculture sector. As a consequence, heatwaves are becoming more frequent and have been implicated in the increased occurrence of jellyfish blooms. In particular, Aurelia aurita, a common jellyfish found in Norwegian waters, is sensitive to temperature changes. Blooms have been linked with high mortality events, and this is likely related to their capability of causing gill and skin damage, thereby compromising barrier functions. Fish gill mucosal microbiome interacts with the environment, being the primary barrier of defense against external agents, making them useful for monitoring mucosal health. In addition, fish microbiota plays critical roles in many host functions such as nutrient metabolism, stress response, immunity, and protection against pathogens. To date, a limited number of studies are exploring the different interactions between fish mucosal microbial communities and scyphozoans within a climate change context. Thus, this study aimed to explore how multiple climate change-related environmental stressors, such as increasing temperature and limited oxygen availability, interact with salmon gill mucosal microbiota and gill gene expression after jellyfish (Aurelia aurita) exposure.
Smolts were reared under three different conditions for three months: Control (C) group was maintained at 12°C, Heatwave (HW) group was maintained at 12°C, gradually increasing up to 17°C and then progressively lowered again to 12°C. Heatwave limited oxygen (HWDO) group was maintained as the HW group, lowering the percentage of dissolved oxygen to 70% at 17°C. Next, fish were tagged and exposed to macerated jellyfish in a common-garden trial for 72 hours. Gill samples were collected, and the microbiota and host transcriptome were sequenced.
Gill microbiota showed a significant difference between the C group towards the HW and HWDO groups, regardless of the jellyfish exposure (Fig.1), highlighting the impact of the temperature alone over the oxygen concentration. At the transcriptomic level, increased temperature was the factor inducing more changes in non-exposed fish, masking the effects of jellyfish exposure. All tested stimuli significantly impacted pathways related to ion homeostasis and extracellular matrix organization. Further studies of stress and immune-related genes will provide details on the specific impact of each stressor and their combined effect.
Increased temperature seems to have a particularly strong effect on fish health, which may aggravate secondary problems like decreased oxygen availability or jellyfish blooms; therefore, understanding the dynamics of how gill microbiota and gene expression are modulated by climate change stressors will be crucial for developing mitigation strategies.