Abstract
Elevated seawater temperatures are associated with coral bleaching events and related mortality. Nevertheless, some coral species are able to survive bleaching and recover. The apoptotic responses associated to this ability were studied over 3 years in the coral Stylophora pistillata from the Gulf of Eilat subjected to long term thermal stress. These include caspase activity and the expression profiles of the S. pistillata caspase and Bcl-2 genes (StyCasp and StyBcl-2-like) cloned in this study. In corals exposed to thermal stress (32 or 34°C), caspase activity and the expression levels of the StyBcl-2-like gene increased over time (6–48 h) and declined to basal levels within 72 h of thermal stress. Distinct transcript levels were obtained for the StyCasp gene, with stimulated expression from 6 to 48 h of 34°C thermal stress, coinciding with the onset of bleaching. Increased cell death was detected in situ only between 6 to 48 h of stress and was limited to the gastroderm. The bleached corals survived up to one month at 32°C, and recovered back symbionts when placed at 24°C. These results point to a two-stage response in corals that withstand thermal stress: (i) the onset of apoptosis, accompanied by rapid activation of anti-oxidant/anti-apoptotic mediators that block the progression of apoptosis to other cells and (ii) acclimatization of the coral to the chronic thermal stress alongside the completion of symbiosis breakdown. Accordingly, the coral's ability to rapidly curb apoptosis appears to be the most important trait affecting the coral's thermotolerance and survival.
Highlights
At elevated seawater temperatures, scleractinian corals lose their endosymbiotic dinoflagellates (Symbiodinium spp; i.e., zooxanthellae), which leads to a bleached appearance and often to death
Thermal tolerance and bleaching In order to test thermotolerance and set the bleaching threshold of S. pistillata from the Gulf of Eilat, corals were subjected to long term thermal stress and temperatures of 32uC, 34uC and 35uC
A growing body of evidence indicates that apoptosis is one of the prominent host responses to thermal stress, and is dependent on the extent and duration of the stress [25,28,29,30]
Summary
Scleractinian corals lose their endosymbiotic dinoflagellates (Symbiodinium spp; i.e., zooxanthellae), which leads to a bleached appearance (pale or white color) and often to death. Some coral species are known to be able to survive and recover from bleaching [3]. This resistance has been attributed to coral morphology [4] or energy reserves and heterotrophic capability of the coral host [5], such resilience is poorly understood and the role of the host remains unclear [6]. One hypothesized mechanism of coral bleaching involves the increased production of reactive oxygen species (ROS) in the dinoflagellate symbionts, which would cause cellular damage and expulsion of symbionts [8]. As one of the signals for programmed cell death (PCD) [11,12], ROS could be involved in the initiation phase of apoptosis, resulting in coral death
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