The goals of this project were to investigate in detail 1) the ecology and physiological performance of the symbiotic algae Symbiodinium trenchii (type D1a) relative to other Symbiodinium spp. and 2) the role of S. trenchii as a functional symbiont critical to the health and growth of reef-building corals from the Caribbean and the Pacific. This work investigated the potential for competition between different Symbiodinium, which is important to understand how coral-Symbiodinium populations respond to thermal stress. We examined the physiological trade-offs that may exist for the coral host when associating with symbiont species of different thermal tolerances. The experiments we conducted were designed to address questions about the costs and benefits of S. trenchii spreading to other coral communities around the world as oceans continue to warm and/or as thermal regimes de-stabilize.

Experimental thermal stress treatments indicated differences in uptake and translocated dietary products between species and within biological compartments (animal, algal, skeleton). Interestingly, under thermal stress (32 °C) symbiotic algae retained the ability to uptake inorganic carbon. However, downstream translocation of the metabolized carbon appeared to be significantly reduced in the skeletal compartments of corals that did not host S. trenchii (with the exception of Porites rus). These results indicate that many corals in Palau, which do not associate with S. trenchii, translocate lesser amounts of carbon to skeletal components under thermal stress. These results provide alarming biological evidence of future climatic scenarios as predicted by IPCC. Our research locations in Nikko Bay have been characterized to have the biogeochemical and thermal properties of what is generally predicted for “normal” conditions in the tropical Pacific in the year 2100. That is, decreased pH to approximately 7.8 and seawater temperatures at least 1 °C greater than the present offshore Palauan coral reefs. Therefore, these biological measurements and our physiological comparisons are of paramount importance for understanding how coral respond to climatic stressors.

Additionally, in Palau we examined natural abundance isotopic ¹⁵N and ¹³C values of coral host tissue, Symbiodinium, and CaCO₃ skeleton. The results indicate that 1) corals located in Nikko Bay are receiving less autotrophically fixed carbon than conspecifics from offshore reefs and 2) corals from Nikko Bay are functioning at a higher trophic level (i.e., heterotrophy) as indicated by δ¹⁵N and δ¹³C than conspecifics only 6 km away on “pristine” offshore reefs. This apparent flexibility in trophic function (photoautotrophy to heterotrophy) is somewhat novel and is likely partially facilitated by the host’s ability to associate with S. trenchii. Using isotopes, we have documented, described, and compared food-web dynamics of 8 different scleractinian corals from Nikko Bay and an adjacent coral reef (current conditions). Corals from Nikko Bay: 1) mostly associate with the thermally tolerant S. trenchii, 2) show no differences in the amount of inorganic carbon that is metabolized and incorporated in the coral skeleton at the increased stressful temperature 32 °C, and 3) obtain more carbon via holozoic processes (i.e., heterotrophy) than offshore counterparts. This is in direct comparison to the offshore conspecific corals that 1) associate with various types of clade “C” Symbiodinium spp., 2) show significant decreased amounts of metabolized photosynthetically-fixed inorganic carbon in CaCO₃ skeleton at the increased stressful temperature 32 °C, and 3) obtain majority of carbon via photoautotrophic symbiosis instead of holozoic processes.

Our research was well represented at the 13^th International Coral Reef Symposium where PI’s and students presented some of our findings to the coral research community. Furthermore, we have given dozens of professional seminars at national and international venues that have featured our research findings in Palau. There is great interest in our results from this project and we have participated in several events to communicate our research to the “non-scientific” public and grade school children. Additionally, we have added infrastructure to Palau International Coral Reef Center by donating our experimental tank system (pumps, tanks, and shade cloth) for future research use of the facility (Figure 1).

In conclusion, the inshore coral reef habitats (Nikko Bay) of Palau resemble temperature and pH conditions similar to those projected by 2100 AD. In contrast to many studies that focus on one or two taxa, we compared the trophic ecology of eight species and thermal physiology of six species of reef-building coral found from both inshore and offshore reefs. Coral from inshore habitats were found to associate with Symbiodinium trenchii. Corals from Nikko Bay relied on zooplankton for metabolism considerably more than conspecifics from offshore reefs. Offshore corals depended more on autotrophy and harbored different species of symbiotic algae. These findings indicate that corals rely more on a combination of energy sources to acclimatize to stressful environments. Our findings across genetically diverse taxa demonstrate that trophic flexibility and symbiotic association are essential to maintaining coral metabolism to cope with increased ocean warming.

Last Modified: 07/09/2017
Modified by: Dustin W Kemp