Abstract
In response to high salinity, algal endosymbionts of corals produce floridoside, an osmolyte with antioxidant potential.
The endosymbiosis between
Symbiodinium
dinoflagellates and stony corals provides the foundation of coral reef ecosystems. The survival of these ecosystems is under threat at a global scale, and better knowledge is needed to conceive strategies for mitigating future reef loss. Environmental disturbance imposing temperature, salinity, and nutrient stress can lead to the loss of the
Symbiodinium
partner, causing so-called coral bleaching. Some of the most thermotolerant coral-
Symbiodinium
associations occur in the Persian/Arabian Gulf and the Red Sea, which also represent the most saline coral habitats. We studied whether
Symbiodinium
alter their metabolite content in response to high-salinity environments. We found that
Symbiodinium
cells exposed to high salinity produced high levels of the osmolyte 2-
O
-glycerol-α-
d
-galactopyranoside (floridoside), both in vitro and in their coral host animals, thereby increasing their capacity and, putatively, the capacity of the holobiont to cope with the effects of osmotic stress in extreme environments. Given that floridoside has been previously shown to also act as an antioxidant, this osmolyte may serve a dual function: first, to serve as a compatible organic osmolyte accumulated by
Symbiodinium
in response to elevated salinities and, second, to counter reactive oxygen species produced as a consequence of potential salinity and heat stress.