Abstract
Responses of cnidarian-Symbiodiniaceae associations to warming are determined, in part, by high-frequency temperature variability. Yet, the role of such variability in determining specific maximum temperature thresholds of cnidarian holobionts (the ecological units comprised of cnidarian hosts and associated microorganisms, including Symbiodiniaceae) remains untested. Here we contrasted the thermal resilience (that is the ability to resist stress) of a model symbiotic cnidarian from the Red Sea (jellyfish of the genus Cassiopea) under stable and diel oscillating temperature conditions that provide night-time reprieves from daily maximum temperatures. Holobionts were subjected to two thermal trajectories; one that increased but plateaued at 2 °C below identified bleaching thresholds and another that increased incrementally until holobionts bleached. We used behavior, growth, photochemical efficiency, Symbiodiniaceae (symbiont) cell density, and total chlorophyll cell content to characterize thermal resilience and examined Symbiodiniaceae community composition responses at 1 and 13 days of exposure, and post-bleaching. Lower night-time temperatures, resulting in lower daily mean temperatures, allowed holobionts to withstand daily maximum temperatures close to their bleaching thresholds for two extra days than those under stable maximum temperature conditions. Lower night-time temperatures increased the bleaching threshold of the holobionts, whereby holobionts exposed to night-time thermal reprieves tolerated a more extreme daily mean temperature of 40.6 oC and reached a daily thermal maxima 4 oC higher than those under stable temperature conditions. However, post-bleaching observations indicate that night-time temperature reprieves did not prevent symbiont cell or pigment loss. Symbiodiniaceae communities were unaffected by lower night-time temperatures and no directional changes indicative of symbiont shuffling/ selection of thermally tolerant lineages were observed. We show that stable experimental treatments may fail to accurately identify maximum thermal thresholds of non-calcifying cnidarians and limit their relevance to in situ environments that are often characterized by high levels of temperature fluctuations.
Highlights
Ongoing anthropogenic climate change has heightened the need to understand potential drivers of resilience of marine biota
Mass mortality of Cassiopea sp. medusae occurred 5–7 days after bleaching of the holobionts was first observed on the 12th September 2017
Lower night-time temperatures that yielded lower daily mean temperatures extended the duration at which the holobionts could withstand daily thermal maxima close to their bleaching threshold by 2 days and increased the daily maximum and mean temperatures medusae could withstand before bleaching by 4 and 1.7◦C, respectively
Summary
Ongoing anthropogenic climate change has heightened the need to understand potential drivers of resilience (defined here as the ability to resist stress, sensu Palumbi et al, 2008) of marine biota This is especially urgent for the endosymbiotic relationship between cnidarian hosts and dinoflagellates of the family Symbiodiniaceae that fundamentally drives the productivity of tropical and temperate reefs worldwide (Muscatine and Porter, 1977). Many experimental assessments of cnidarian responses to thermal stress apply static, carefully controlled elevated temperature scenarios (typically within ±1◦C of mean treatment levels, e.g., Bahr et al, 2016; Horvath et al, 2016) to calcifying cnidarians. Cnidarian bleaching predominately occurs in response to elevated temperature (Glynn, 1993; Hughes et al, 2017), physiological responses to thermal stress depend on multiple factors (Fitt et al, 2001), including absolute temperature, the duration of exposure to elevated temperature, and the rate at which temperatures rise (Hoegh-Guldberg and Smith, 1989; Jokiel and Coles, 1990; Middlebrook et al, 2010)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
