Tolerance of Cassiopea andromeda and its symbionts Symbiodinium microadriaticum to mild and long-lasting heat stress through proteomic adaptations in both partners

The symbiotic partnership between cnidarians and Symbiodiniaceae displays a complex energetic metabolism, involving respiration from both partners and photosynthesis from the dinoflagellates. Despite the major importance of these two processes, their inter play and regulations remain poorly studied. Abiotic factors can unsettle the symbiotic balance, leading to the collapse of the association and threatening the survival of entire ecosystems. Among them, the rise in sea water temperature is getting more and more concerning as global warming takes place. We explored this topic by examining the emerging model organism Cassiopea, which was found to be easily culturable and offers an alternative to stony corals for studying symbiosis.
Our approach involved examining the impact of a sustained rise in water temperature (four weeks at 32°C with growth temperature at 26 °C) on the energetic physiology and total proteome of Cassiopea andromeda medusae and their symbionts, Symbiodinium microadriaticum .
The jellyfish from both control and heat stressed groups exhibit similar growth patterns, alongside analogous symbiont and pigment density, as well as comparable photosynthetic yields and respiration rates. To understand the factors that ensure this stability at the cellular scale, bottom up proteomics was conducted on total protein extracts. Out of the approximately 4000 proteins identified in separated animal tissues or isolated symbionts , ca. 100 Cassiopea proteins and ca. 300 Symbiodinium proteins exhibit differential expression patterns between heat stressed and control jellyfish. A deeper analysis of the role of these proteins will allow us to unveil processes involved in the tolerance of this symbiosis. Despite this stability at 32°C, the heat stressed jellyfish showed an increased sensitivity to a subsequent increase in water temperature up to 34°C.
Overall, these results suggest that Cassiopea andromeda and its symbionts cope with a long lasting mild heat stress and that this tolerance is mediated by a significant change in proteome expression.