Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Tamar L. Goulet, Kartick P. Shirur, Blake D. Ramsby, Roberto Iglesias-Prieto, Chaolun Allen Chen.
Global climate change not only leads to elevated seawater temperatures but also to episodic anomalously high or low temperatures lasting for several hours to days. Scleractinian corals are detrimentally affected by thermal fluctuations, which often lead to an uncoupling of their mutualism with Symbiodinium spp. (coral bleaching) and potentially coral death. Consequently, on many Caribbean reefs scleractinian coral cover has plummeted. Conversely, gorgonian corals persist, with their abundance even increasing. How gorgonians react to thermal anomalies has been investigated utilizing limited parameters of either the gorgonian, Symbiodinium or the combined symbiosis (holobiont). We employed a holistic approach to examine the effect of an experimental five-day elevated temperature episode on parameters of the host, symbiont, and the holobiont in Eunicea tourneforti, E. flexuosa and Pseudoplexaura porosa. These gorgonian corals reacted and coped with 32°C seawater temperatures. Neither Symbiodinium genotypes nor densities differed between the ambient 29.5°C and 32°C. Chlorophyll a and c2 per Symbiodinium cell, however, were lower at 32°C leading to a reduction in chlorophyll content in the branches and an associated reduction in estimated absorbance and increase in the chlorophyll a specific absorption coefficient. The adjustments in the photochemical parameters led to changes in photochemical efficiencies, although these too showed that the gorgonians were coping. For example, the maximum excitation pressure, Qm, was significantly lower at 32°C than at 29.5°C. In addition, although per dry weight the amount of protein and lipids were lower at 32°C, the overall energy content in the tissues did not differ between the temperatures. Antioxidant activity either remained the same or increased following exposure to 32°C further reiterating a response that dealt with the stressor. Taken together, the capability of Caribbean gorgonian corals to modify symbiont, host and consequently holobiont parameters may partially explain their persistence on reefs faced with climate change.
Global climate change affects many ecosystems, including coral reefs . One aspect of climate change is the rise of seawater temperatures that is anticipated to continue into the future [1, 2]. In addition, short-term fluctuations in prevailing temperatures over several hours or days are also projected to occur more frequently [3–5]. Exposure to seawater temperatures even 2°C above the mean summer maximum can adversely affect corals and their mutualistic endosymbiotic dinoflagellate algae, Symbiodinium spp. . Numerous studies have investigated the predominantly detrimental effects of elevated seawater temperatures on scleractinian coral—Symbiodinium symbioses (reviewed in [6, 7, 8]), but such data on other abundant coral reef cnidarians, such as octocorals, lag behind.
Thermal fluctuations that expose coral reefs to anomalously high or low seawater temperatures for several hours or days are projected to occur more frequently in the future [3–5]. In scleractinian corals subjected to experimental conditions simulating such events, a 50–80% reduction in Symbiodinium density often occurs [39, 43, 91–96]. Subsequently, scleractinian corals may recover from the bleaching event. Conversely, the loss of Symbiodinium, compounded with the other stress responses, may lead to the demise of the host [39, 43, 94–96]. While we mimicked a short term thermal event by exposing branches of three gorgonian species, Eunicea tourneforti, E. flexuosa, and Pseudoplexaura porosa, to an elevated 32°C seawater temperature, the Symbiodinium densities in these branches did not significantly differ from Symbiodinium densities in branches from the same colonies maintained at the ambient temperature of 29.5°C. Furthermore, in P. porosa, Symbiodinium densities at the elevated temperature were actually higher, not lower, compared to those at the ambient temperature, although not significantly so (Fig 2). The ability to continue hosting the same Symbiodinium density at elevated temperatures may be one reason why Caribbean gorgonians are maintaining or increasing their abundance on Caribbean coral reefs while scleractinian coral cover is declining [9, 10, 12–15].