The cell-surface protein composition of a coral symbiont, Breviolum psygmophilum , reveals a mechanism for host specificity and displays dynamic regulation during temperature stress

The symbiosis between corals and dinoflagellates in the family Symbiodiniaceae is threatened by warming trends that induce coral bleaching, or symbiosis breakdown. Current models of symbiosis breakdown involve an immune response to an elevation in reactive oxygen species that ultimately results in the loss of the symbiont. However, the intimate nature of the symbiosis implies an important role for the symbiont surface as a point of interaction between partners. The response of symbiont cell surface proteins to experimental temperature stress was, therefore, investigated using a cell surface biotin probe. Cell-surface protein composition was found to be dynamically regulated in response to heat stress, particularly after 24 h of exposure to heat treatment. This pattern was primarily driven by an increased abundance in heat shock proteins, demonstrating that stress experienced by the symbiont can manifest at the cell surface. Elements known to activate host immunity were also increased in response to temperature stress, further demonstrating an avenue by which the symbiont can elicit a host immune response independent of reactive oxygen species. This work documents the cell surface protein composition of a Symbiodiniaceae species for the first time and highlights host–symbiont interaction mechanisms that may be important during symbiosis breakdown.