Abstract
ABSTRACTThe endosymbiosis between dinoflagellate algae of the genus Symbiodinium and stony corals provides the foundation of coral reef ecosystems. Coral bleaching, the expulsion of endosymbionts from the coral host tissue as a consequence of heat or light stress, poses a threat to reef ecosystem functioning on a global scale. Hence, a better understanding of the factors contributing to heat stress susceptibility and tolerance is needed. In this regard, some of the most thermotolerant corals live in particularly saline habitats, but possible effects of high salinity on thermotolerance in corals are anecdotal. Here we test the hypothesis that high salinity may lead to increased thermotolerance. We conducted a heat stress experiment at low, intermediate, and high salinities using a set of host-endosymbiont combinations of the coral model Aiptasia. As expected, all host-endosymbiont combinations showed reduced photosynthetic efficiency and endosymbiont loss during heat stress, but the severity of bleaching was significantly reduced with increasing salinities for one of the host-endosymbiont combinations. Our results show that higher salinities can convey increased thermotolerance in Aiptasia, although this effect seems to be dependent on the particular host strain and/or associated symbiont type. This finding may help explain the extraordinarily high thermotolerance of corals in high salinity environments, such as the Red Sea and the Persian/Arabian Gulf, and provides novel insight regarding factors that contribute to thermotolerance. Since our results are based on a salinity effect in symbiotic sea anemones, it remains to be determined whether this salinity effect can also be observed in stony corals.
Highlights
Coral reefs are declining globally due to the direct and indirect effects of climate change (Albright et al, 2016; Anthony et al, 2008; Hoegh-Guldberg, 1999; Hughes et al, 2017a)
Higher salinities reduce bleaching during heat stress in Aiptasia H2 associated with SSB01 To test a possible effect of salinity on thermotolerance and bleaching in the sea anemone Aiptasia, we conducted a long-term heat stress experiment using Aiptasia of the host-symbiont combinations H2-SSB01 and CC7-SSA01 (Figs 1 and 2; Fig. S1)
Using the coral model Aiptasia, in this study we show that higher salinities can lead to increased thermotolerance accompanied by less bleaching in a cnidarian-dinoflagellate symbiosis
Summary
Coral reefs are declining globally due to the direct and indirect effects of climate change (Albright et al, 2016; Anthony et al, 2008; Hoegh-Guldberg, 1999; Hughes et al, 2017a). Elevated sea surface temperatures (SSTs) have caused and continue to cause coral bleaching, that is the loss of Symbiodinium as evidenced by the visible whitening of the coral host, on global scales, resulting in the loss of coral cover and destruction of reef ecosystems Red Sea Research Center, Division of Biological and Environmental Science and Engineering (BESE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. While the mechanisms of coral bleaching are still not completely understood (Baird et al, 2009; Pogoreutz et al, 2017; Weis, 2008), the production and accumulation of reactive oxygen species (ROS) and associated oxidative stress is likely playing a major role in heat-induced coral bleaching (Dubinsky and Stambler, 2011; Lesser, 1997)
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