Responses to High Seawater Temperatures in Zooxanthellate Octocorals

Abstract

Increases in Sea Surface Temperatures (SSTs) as a result of global warming have caused reef-building scleractinian corals to bleach worldwide, a result of the loss of obligate endosymbiotic zooxanthellae. Since the 1980’s, bleaching severity and frequency has increased, in some cases causing mass mortality of corals. Earlier experiments have demonstrated that zooxanthellae in scleractinian corals from three families from the Great Barrier Reef, Australia (Faviidae, Poritidae, and Acroporidae) are more sensitive to heat stress than their hosts, exhibiting differential symptoms of programmed cell death – apoptosis and necrosis. Most zooxanthellar phylotypes are dying during expulsion upon release from the host. The host corals appear to be adapted or exapted to the heat increases. We attempt to determine whether this adaptation/exaptation occurs in octocorals by examining the heat-sensitivities of zooxanthellae and their host octocoral alcyonacean soft corals – Sarcophyton ehrenbergi (Alcyoniidae), Sinularia lochmodes (Alcyoniidae), and Xenia elongata (Xeniidae), species from two different families. The soft coral holobionts were subjected to experimental seawater temperatures of 28, 30, 32, 34, and 36°C for 48 hrs. Host and zooxanthellar cells were examined for viability, apoptosis, and necrosis (in hospite and expelled) using transmission electron microscopy (TEM), fluorescent microscopy (FM), and flow cytometry (FC). As experimental temperatures increased, zooxanthellae generally exhibited apoptotic and necrotic symptoms at lower temperatures than host cells and were expelled. Responses varied species-specifically. Soft coral hosts were adapted/exapted to higher seawater temperatures than their zooxanthellae. As with the scleractinians, the zooxanthellae appear to be the limiting factor for survival of the holobiont in the groups tested, in this region. These limits have now been shown to operate in six species within five families and two orders of the Cnidaria in the western Pacific. We hypothesize that this relationship may have taxonomic implications for other obligate zooxanthellate cnidarians subject to bleaching.

The use of in vivo microscopy to image the cnidarian stress response in three octocoral species

“…Merely observing these symbionts produced an effect comparable to 30 min of thermal and light stress. These results are consistent with previous findings, suggesting that C‐type symbionts are more sensitive than D‐type (Sammarco & Strychar ). In these octocorals, initial symbiont sensitivity thus seems to be governed largely by the genotype of Symbiodinium , not that of the host.…”

Why Do Corals Bleach? Conflict and Conflict Mediation in a Host/Symbiont Community

“…Suffice it to say that while not all the data are concordant, some support for this hypothesis can nevertheless be found. For instance, some studies have found differences in productivity and bleaching in corals with Symbiodinium clade C (which are often highly productive) versus D (which are usually less productive, yet more resistant to bleaching) . Note that the concordance of clade C and D nomenclature with our cooperator and defector categories is entirely coincidental.…”

Evaluation and validation of reference genes for qPCR analysis to study climate change-induced stresses in Sinularia cf. cruciata (Octocorallia: Alcyonidae)

“…The genus Sinularia is a widely studied genus among octocorals with respect to population genetics (Bastidas et al, 2001), bleaching and symbiont loss (Sammarco and Strychar, 2013;Strychar et al, 2005), cell culture (Khalesi et al, 2008), calcification (Jeng et al, 2011), and various secondary metabolites (Lakshmi and Kumar, 2009). Additionally, there are studies employing mRNA-pool profiling techniques using an informatics-based analysis of kinetic profiles (Hoover et al, 2007;Hoover et al, 2008).…”