Trade-off between photo-symbiosis and innate immunity influences cnidarian’s response to pathogenic bacteria

Mutualistic relationships with photosynthetic organisms are common in
cnidarians, which form an intracellular symbiosis with dinoflagellates in
the family Symbiodiniaceae. The establishment and maintenance of these
symbionts are associated with the suppression of key host immune factors.
Due to this, there are potential trade-offs between the nutrition
cnidarian hosts gain from their symbionts and their ability to
successfully defend themselves from pathogens. To investigate this
potential trade-off, we utilized the facultatively symbiotic polyps of the
upside-down jellyfish Cassiopea xamachana and exposed aposymbiotic and
symbiotic polyps to the pathogen Serratia marcescens. Symbiotic polyps had
a lower probability of survival following S. marcescens exposure.
Gene expression analyses 24 hours following pathogen exposure indicate
that symbiotic animals mounted a more damaging immune response, with
higher levels of inflammation and oxidative stress likely resulting in
more severe disruptions to cellular homeostasis. Underlying this more
damaging immune response may be differences in constitutive and
pathogen-induced expression of immune transcription factors between
aposymbiotic and symbiotic polyps rather than broadscale immune
suppression during symbiosis. Our findings indicate that in facultatively
symbiotic polyps, hosting symbionts limits C. xamachana’s ability to
survive pathogen exposure, indicating a trade-off between symbiosis and
immunity that has potential implications for coral disease research.