“Bringing the Heat”: Thermal stress responses of Pocillopora damicornis

As the rate of climate change accelerates due to anthropogenic global warming, coral reef ecosystems are experiencing greater threats. Elevated ocean temperatures and carbon dioxide concentration pose unprecedented risks to the highly biodiverse ecosystem. While the importance of the study area is on the rise, it is hard to quantify the social, ecological, and economic consequences. The study investigates the heat stress response of ​Pocillopora damicornis​, a highly resilient scleractinian coral with a wide geographic range. Specifically, the study observed heat stress effects on photosynthetic efficiency and color. Corals were given one day to acclimate and then split between the control system, which was maintained at 26°C, and the heat treatment system which rose 2°C daily until it reached the desired 32°C threshold. Coral color in the treatment tank was visually changing, but we found no significant difference in either photosynthetic efficiency or in the color analysis. The results of the study indicated no statistical significance in any heat-stressed colony, but ​P. damicornis​ photosynthetic efficiency and color were variable under extreme 32°C temperature. Ultimately, we concluded that ​P. damicornis​ was a highly resilient coral with the capability to endure consistent short-term heat stress. Introduction Coral reefs, which support thousands of different species, are one of the most biodiverse marine ecosystems on the planet (Roberts ​et al.​, 2002). While this highly complex system accounts for nearly one-third of the world’s marine species, their importance goes beyond significant species richness and productivity (Roberts ​et al.​, 2002; Cunning, ​et al​, 2018). Reefs provide social and economic value for coastal communities as they create jobs, sustain recreational uses, and promote a sense of livelihood (Stat ​et al.​, 2008). However, the rapid increase in human population size has, directly and indirectly, caused unprecedented degradation to coral reef ecosystems (Moberg ​et al.​, 1997). Recent work has established that rampant carbon dioxide emissions associated with fossil fuels are causing temperature increases in the ocean to be the most prominent factor (Hoegh-Guldberg ​et al.​, 2007). The diversity of corals in shallow oligotrophic waters can be largely attributed to their symbiotic relationships in the genus Symbiodiniaceae,where nutrients are exchanged between the two partners for survival (Stat ​et al.​, 2008; Pearse and Muscatine, 1971). Individual coral polyps host many algal cells in specialized structures called symbiosomes, and these algae photosynthesize and provide the corals with carbon sugars (Stat ​et al.​, 2008). Rising ocean temperatures disrupt this mutualistic relationship in a process called “coral bleaching” (Hoegh-Guldberg ​et al.​, 2007). During bleaching, corals expel their algae and begin to lose their characteristic coloring, and if the stressor the caused the bleaching does not subsist, the coral will die (Hoegh-Guldberg ​et al.​, 2007). Previous studies observing the heat responses of ​P. damicornis ​have different experimental setups limited by time and remote location of the field study (Wicks​ et al.​, 2010). Experimental limitations were also demonstrated by restricting each setup to one tank, posing the issue that any complication could negatively affect the entire experiment. A common procedure in similar studies is to immediately expose corals to heat stress, rather than allow for acclimation and progression of temperature increase in treatment systems (Hill ​et al.​, 2014). In this study we are testing how acute heat stress affects the symbiotic relationship of ​Pocillopora damicornis​, which is a common reef-building coral that exhibits fast growth rates (Cunning, ​et al.​, 2018). We selected ​P. damicornis​ as our study coral for a number of reasons. This coral has a wide geographic distribution across the western Indo-Pacific as well as the eastern tropical pacific, which means demonstrates it is exposed to a range of temperatures across this range. As a study subject of many previous heat stress studies, P. damicornis ​has shown to be a good indicator of responses of temperature change and subsequent bleaching. In the present study, ​P. damicornis ​nubbins will be subjected to a 16 day period of heat stress. The temperature in our treatment system will be increased by 2°C per day until the water temperature reaches 32°C, where the temperature of the water will remain for the remaining duration of the experiment. Our goal is to gain a better understanding of how increased water temperature can affect coral bleaching as global warming becomes a greater threat to all coral reef ecosystems.