The effects of elevated seawater temperatures on Caribbean gorgonian corals and their algal symbionts, <i>Symbiodinium</i> spp.

<div><p>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 <i>Symbiodinium</i> 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, <i>Symbiodinium</i> 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 <i>Eunicea tourneforti</i>, <i>E</i>. <i>flexuosa</i> and <i>Pseudoplexaura porosa</i>. These gorgonian corals reacted and coped with 32°C seawater temperatures. Neither <i>Symbiodinium</i> genotypes nor densities differed between the ambient 29.5°C and 32°C. Chlorophyll <i>a</i> and <i>c</i>_<i>2</i> per <i>Symbiodinium</i> 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 <i>a</i> 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, <i>Q</i>_m, 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.</p>