Climate Change Coral Genetics

A new thermally tolerant species of algae is found!

Paper: Hume BCC, D’Angelo C, Smith EG, Stevens JR, Burt J, and Wiedenmann J. “Symbiodinium thermophilum sp. nov., a thermotolerant sumbiotic alga prevalent in corals of the world’s hottest sea, the Persian/Arabian Gulf” (2015) Scientific Reports. 5: 8562. DOI: 10.1038/srep08562

Resident Symbiodinium spp. remained stable within corals following a cold-water stress event off Baja Cali fornia, Mexico. Photo: T.C. LaJeunesse, M. D. Aschaffenburg, D.T. Pettay 2008

Coral reefs are in rapid decline due to several factors including climate change and human activities. Shallow water corals depend on the symbiotic (mutually beneficial) relationship they have with a certain type of algae, zooxanthellae, belonging in the genus Symbiodinium. Changes in water temperature can cause the loss of zooxanthellae, which often leads to coral bleaching. Large coral bleaching events have increased in the last couple decades due to the rising ocean temperatures. Several coral reefs in the Persian/Arabian Gulf have been seen to cope with large (~20°C) temperature fluctuations, indicating at least some algal species are able to survive the changes in temperature.

Corals can use this temperature-coping type of Symbiodinium algae to survive temperature fluctuations. By having the thermally tolerant Symbiodinium host with them, corals will be less likely to bleach out. Researchers from the UK set out to understand a little bit more about this symbiotic relationship and ended up finding a brand-new algae species, Symbiodinium thermophilum.

The researchers used molecular phylogenetic analysis to better understand the new algae. This involved breaking down and analyzing the DNA in the algae in order to better understand its relationship to other algae species in the genus Symbiodinium. By analyzing how this new algae’s genes fit together, researchers can determine if it is a completely new strain of algae, or if it is an evolved form of zooxanthelllae that “learned” to cope with temperature fluctuations. They also monitored the symbiotic partnership of the corals and the new algae over a span of twenty-two months to ensure that this association was stable through a range of thermal conditions. Six different species of corals were tagged and analyzed over the seasons in the Persian/Arabian Gulf (Fig. 1).

fig 1
Fig. 1 Coral- Symbiodinium sampling locations with the Persian/Arabian Gulf. (A-E) represents previous sampling locations from Baker et al. 2004. 1-3 represent sampling locations from this study; Dalma (1), Saadiyat (2), and Umm Al Quwain (3).



Results and Implications

fig 2
Fig. 2 Phenotypic plasticity can be defined as the ability of one genotype to produce more than one phenotype when exposed to different environments.

All six species of corals analyzed showed an association with the new Symbiodinium algae, and it is indeed prevalent all year round in the Persian/Arabian Gulf. The prevalence of this new alga in the extreme temperature conditions of the Persian/Arabian Gulf contradicts the current understanding of sensitive zooxanthellae. These new findings could imply the new algae species has a greater phenotypic plasticity (Fig. 2) than previously thought.

If this new algae starts to host in shallow water corals across the globe, it could potentially reduce coral bleaching. It is, however, unknown if this new species is indeed conferring thermal tolerance to the coral, or if it is simply a strain that prefers to inhabit corals that are already a little more hardy and tolerant of changing water temperatures.

It is promising that corals have a way to adjust to stressful environmental conditions and gives hope that there might be more thermally-tolerant host species out there for the corals. Temperature, however, is not the only condition troubling coral reefs. Pollution, excess nutrients, overfishing, and coastal development are all threats coral reefs face. Simply being resilient against rising sea temperatures will, unfortunately, not be enough to save all the reefs from bleaching and dying off.

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