Abstract
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.
Highlights
Global climate change affects many ecosystems, including coral reefs [1]
Chlorophylls a (Chl a) and c2 contents per Symbiodinium cell were significantly lower after five days at elevated temperature than their levels in branches held at ambient temperature
This significant difference in Chl a per cell was primarily driven by P. porosa since the Chl a content per Symbiodinium cell in branches of the Eunicea species exposed to the elevated temperature did not significantly differ from levels in branches maintained at ambient temperature (Table 2)
Summary
One aspect of climate change is the rise of seawater temperatures that is anticipated to continue into the future [1, 2]. 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. In the Caribbean, for example, over the past few decades, scleractinian coral cover has dramatically declined [9, 10] concurrent with a rise in seawater temperatures by 0.2–0.4 ̊C/decade between 1985 and 2006 [11]. In order to understand the future of Caribbean reefs, it is imperative to determine the effects of potential stressors, such as elevated seawater temperatures, on gorgonian corals
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
