Increased disease resilience in aiptasia after acclimation to elevated salinity

The tropical sea anemone Exaiptasia diaphana (commonly referred to as ‘Aiptasia’), serves as a model system for the coral holobiont. Previous research has shown that exposure to elevated salinity conveyed increased thermotolerance in Aiptasia. Based on this observation, we aimed to determine if acclimation to elevated salinity could lead to improved pathogen resilience. Aiptasia were acclimated to seawater at either 35 ppt or 42 ppt over a ‘short-term’ period (14–16 days) or ‘long-term’ period (90 + days). After the acclimation period, Aiptasia were exposed to Serratia marcescens PDL100 (a known coral pathogen) at 108 CFU/mL, 107 CFU/mL, or remained un-exposed. The exposed anemones were monitored for 10 days, and their mortality was scored daily. Aiptasia acclimated to 42 ppt, over both short- and long-term periods, exhibited a higher survival than that of control anemones. We then tested whether pre-exposure to sub-lethal concentrations of S. marcescens could lead to increased survival rates in comparison to naive anemones at both salinities. This resulted in 100% survival of the anemones exposed to the pathogen, suggesting priming of the Aiptasia immune system in response to exposure to non-lethal concentrations of the pathogen. Evidence of immune priming was confirmed by differential gene expression of known stress-response genes by qPCR. Our results show, for the first time, increased pathogen resilience due to exposure to elevated salinity in Aiptasia. Since our results are based on a salinity effect in symbiotic sea anemones, it remains unclear whether this salinity effect might also occur in stony corals.