Corals are like greenhouses growing millions of symbiont algae inside their cells which provide the coral with more than 90% of its daily energy requirement. Coral reef ecosystems generate substantial social and economic benefits for more than 500 million people, but as global temperatures escalate, corals are experiencing thermal stress at increasing frequencies and intensities and are being eliminated at alarming rates.
Coral skeletons are highly scattering in the upper ~200 μm of the skeleton layer (measured as ‘microscopic’ reduced coefficient, µSʹ,m ) and effectively amplify and homogenize the ambient light-field of their endosymbiotic dinoflagellates. This effect enhances productivity of the symbiosis, but also contributes to the likelihood of catastrophic dissociation as densities of light-absorbing symbionts decrease and light levels for the remaining symbionts rapidly increase.
Understanding the factors involved in the failure of this symbiosis, which has repeatedly caused widespread mass mortality of ecosystem engineering coral species, has become increasingly critical because of the accelerating effects of global climate change.
The coral can be viewed as an intricate optical machine and its interaction with visible light is one of the key factors in the coral life cycle and bleaching episodes. The coral consists of two compartments: 1) the living tissue where the host photoprotective pigments and the symbiont algae with all its photosynthetic pigments are contained, and 2) the skeleton, a highly reflective limestone structure secreted by the coral polyp in a species-specific manner and affected by local environmental factors.
This design facilitates the incident light to travel through the coral tissues and be collected by the photosynthetic apparatus of the algae, and increases the amount of light available to the algae due to the light backscattered by the skeleton. We are studying the light transport properties of coral skeleton and tissue containing photoprotective pigments for coral species showing resistance and susceptibility to bleaching.
This work measures optical properties of coral skeletons using Low-coherence enhanced backscattering (LEBS) spectroscopy developed at the Backman laboratory for early cancer detection. Microscopic light-scattering was measured in over 150 coral skeletons from the 1893 World’s Fair in Chicago (Field Museum coral collection) and the Smithsonian Institution and we found that low- µSʹ,m corals are at higher risk of bleaching and dying. We also bleached 10 coral species that varied in their light-scattering properties by increasing thermal and/or light stress (in collaboration with the Shedd Aquarium) and demonstrated that low- µSʹ,m corals bleach at higher rate and severity than high- µSʹ,m corals.
Symbioses are often the most basal of interactions that provide the conditions necessary for the existence of an ecosystem and knowledge of their structure, function, and evolution can be the critical insight to mediate climate change related ecosystem collapse. As the first ecosystem that may be lost to climate change, coral reefs epitomize this point, where reef-building corals depend on endosymbiotic mutualisms with photosynthetic dinoflagellates for the majority of their carbon budgets and enable the construction of their calcium carbonate skeletons. Climate change induced thermal anomalies disrupt these associations (coral bleaching) and result in increases in mortality and reductions in resistance to disease, predation, and bioerosion, and reduced capacity for damage repair, competition, growth, and reproduction.
However, corals bleach differently, with some corals bleaching and dying while others hardly bleaching. We created a global index specific to each coral taxon (genus and species) to compare the bleaching response of about half of the world’s reef-building corals (374 taxa) exposed to bleaching episodes over the last 40 years. The index ranks the bleaching susceptibility of each species to thermal stress (scale of 1 to 100).
Corals may bleach differently because they are biologically different from each other, they are exposed to very distinct thermal stresses or their communities are exposed to distinct environmental conditions. It could also be due to inconsistencies in reporting among different studies since diverse communities exposed to various thermal stresses are being surveyed with a variety of protocols and criteria of bleaching severity. In order to test the effect of biological effects versus environmental effects we applied a model often used in the stock market to test how the value of a stock (a.k.a. a coral species) value reacts to fluctuations in the market (a.k.a the diverse coral communities where the coral is surveyed).
One of main prevailing hypothesis is that corals may resist thermal stress by associating themselves with symbiont algae that are more tolerant to heat (thermotolerant). However, there are over 400 genetic types of Symbiodinium that are known to associate with corals but only a few types have been assessed for their thermotolerance. We are working towards comparing known thermotolerance levels across different genetic types.