RUI: Biochemical adaptation to temperature in the coral-dinoflagellate symbiosis

Franklin And Marshall College, Lancaster PA

Investigators

Abstract

Reef-building corals throughout the tropical oceans depend on a relationship with single-celled algae (Symbiodinium) that live within coral tissue and that provide the coral most of its energy. However, coral bleaching, a process in which corals expel these algal symbionts in response to environmental stress, has been occurring at increasing rates over the past few decades. Bleaching is especially widespread during times of high water temperature, and often leads to the death of the coral and damage to the reef. Indeed, there is concern that much of the coral reef habitat will be lost in the next fifty years due to the impacts of ocean warming and coral bleaching. This project aims to increase biologists' understanding of why bleaching occurs, by comparing the temperature sensitivity of important enzymes in corals to those of different types of Symbiodinium. The researchers hypothesize that one reason bleaching occurs is because corals are adapted to live at higher temperatures than the algae (which live not only within coral tissue but also free in the water), and furthermore that different types of algae may be more or less adapted to withstand high temperature. The research will provide opportunities for teaching and training undergraduates interested in careers in science, as well as in-depth summer opportunities for socioeconomically disadvantaged high school students. The results of the study will help researchers better predict the likely extent of future bleaching events, and the possibility that corals and algae may be able to adapt to withstand increasing water temperatures. This project will test two specific hypotheses: First, that there is a mismatch in adaptation temperature between the partners, leading to a loss of metabolic function in Symbiodinium at higher temperatures. The hypothesis is based on the observation that Symbiodinium occur in the coral host, but also are free-living in environments across a broad, and often cooler, temperature range. At the biochemical level, adaptation to a eurythermal environment requires enzymes to maintain catalytic rate and substrate affinity over a wide temperature range, but might concomitantly reduce structural stability. To test this hypothesis, metabolic enzymes occurring both in corals and Symbiodinium will be expressed recombinantly, and temperature sensitivity of kinetic parameters of enzyme function will be assessed. Thermal stability of each enzyme also will be measured through residual activity and circular dichroism spectrometry. A second hypothesis is that Symbiodinium types differing in thermal tolerance are biochemically adapted to different temperatures. It has been shown that different clades of Symbiodinium have varying thermal sensitivities, providing the holobiont different levels of resistance to heat-induced bleaching. The hypothesis will be tested by examining representative enzymes from different Symbiodinium types using the approaches described above. In addition, because Symbiodinium orthologs are structurally similar, 3D models of target enzymes will be examined to determine the extent, location, and types of amino acid substitutions that lead to changes in thermal sensitivity; the temperature-adaptive importance of these substitutions will be confirmed through site-directed mutagenesis followed by expression, enzyme kinetics and stability analyses.