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.
Highlights
Many invertebrates possess endosymbionts that support the metabolism and other physiological activities in the host and, often, the host provides nutrient resources to the endosymbionts
The coral host, on the other hand, provides carbon dioxide and nutrients in the form of waste products (N, P, and S) and urea to the zooxanthellae in hospite [3,4,5,6] - i.e., while they are still within the host, Zooxanthellae provide 65–100% [4,5,6] of the host coral’s metabolic energy requirements, other investigators have determined that the host corals receive a substantial portion of their metabolic requirements from plankton, organic, and inorganic matter in the water column [7,8,9,10,11]
Experimental Temperatures The monitored seawater temperatures in the tanks were very close to the target values assigned to the treatments
Summary
Many invertebrates possess endosymbionts that support the metabolism and other physiological activities in the host and, often, the host provides nutrient resources to the endosymbionts. The coral host, on the other hand, provides carbon dioxide and nutrients in the form of waste products (N, P, and S) and urea to the zooxanthellae in hospite [3,4,5,6] - i.e., while they are still within the host, Zooxanthellae provide 65–100% [4,5,6] of the host coral’s metabolic energy requirements, other investigators have determined that the host corals receive a substantial portion of their metabolic requirements from plankton, organic, and inorganic matter in the water column [7,8,9,10,11] This symbiotic relationship facilitates precipitation of the calcium carbonate skeleton and colony growth through skeletal extension [2,3,12,13,14]. These compounds can be precipitated as skeletal calcium carbonate through a calcification process, excreted as waste, or, through photosynthesis, used by zooxanthellae to continue the energy cycle
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