Could there yet be a brighter future for coral reefs? Scientists have recently discovered something that could bring more hope: there are genes found in symbiotic algae that could provide protection against coral bleaching.

Climate change has many devastating effects on many different habitats, something we have heard a lot about in recent years. One of the habitats climate change disturbs most noticeably is coral reefs. These beautiful natural ecosystems house about 25% of all marine species despite only occupying 0.1% of the ocean’s surface. Many threats face them, meaning any change in the ecosystem can have dramatic effects on all the creatures living there. Threats such as ocean acidification, invasive species and overfishing all have an effect, but the most serious is the global rise of ocean temperatures and subsequent coral bleaching.

What is coral bleaching?

Corals gain their wonderful, bright colours from a symbiotic relationship with algae, specifically the Symbiodinium species. The algae live inside the corals to gain shelter and protection. In return, they give the corals food made from photosynthesis. In fact, the algae can provide the coral with about 90% of their food intake. This makes this relationship essential for the coral’s survival. However, when the water temperature rises, this puts the algae under stress and triggers the algae to release toxins called reactive oxygen species (ROS). These toxins, which include substances like peroxide, cause damage to both the algae and the coral. In order to protect itself, the coral expels the algae but in doing so it leaves itself vulnerable due to decreased food intake. If the water temperature does not decrease and the coral isn’t recolonised by new algae, the coral will die.

A staghorn coral (Acropora cervicornis) has been bleached in the middle because of the loss of its symbiotic algae (Gavin Bain/Flickr)

A staghorn coral (Acropora cervicornis) has been bleached in the middle because of the loss of its symbiotic algae (Gavin Bain/Flickr)

The loss of the algae is what causes the bleached look, as all colour is lost from the coral. This stress can severely damage coral populations and so a solution is needed to protect them. Recent research from the University of New South Wales in Australia provides insight into how genetics might be able to help us take a step towards an answer.

Protective genes

The research looked at two different types of algae living in the Great Barrier Reef, one that was temperature-sensitive and another that lived in a slightly warmer climate and so was more temperature resistant. Under heat stress, the warmer climate algae starting releasing ROS at higher temperatures than the colder climate algae, showing their greater resistance to higher temperatures. But it was also found that these more resistant algae also produced proteins as well that neutralised the ROS and so negated any damage that would usually occur, preventing bleaching from occurring.

All these algae did was switch on a few genes and it allows them to stop the coral from expelling them and so survive at higher ocean temperatures. The researchers also found that both types of algae switch from asexual reproduction (when there is only one parent) to sexual reproduction under heat stress. Sexual reproduction can help speed up evolution due to the mixing of two parent’s genes which gives genetic variation to the offspring. Genetic variation allows for a higher likelihood of advantageous genes to be passed on and so gives better adaptability. So the switch these algae make to sexual reproduction could help them to adapt to the rising ocean temperatures.

The Great Barrier Reef. The largest coral reef system in the world and the largest structure made by living organisms is under threat. (Kyle Taylor/Flickr)

The Great Barrier Reef. The largest coral reef system in the world and the largest structure made by living organisms is under threat. (Kyle Taylor/Flickr)

This understanding of how these algae species react under stress is valuable, and could possibly be utilised in aid of vulnerable corals, for example, it could be used to monitor coral’s risk of bleaching. By using these adaptive genes as a marker, looking at which algae possess these genes could allow an assessment of the risk of bleaching each area of coral has. By identifying the most susceptible areas, we can direct our efforts to help accordingly. With 93% of the Great Barrier Reef undergoing bleaching just from earlier this year, leading to a loss of 1/4 of corals, it is hoped that we will soon find a way to reduce the effect of warming oceans, and that this research is a step in the right direction.

references

Levin, R. A., Beltran, V. H., Hill, R., Kjelleberg, S., McDougald, D., Steinberg, P. D., & van Oppen, M. J. (2016). Sex, Scavengers, and Chaperones: Transcriptome Secrets of Divergent Symbiodinium Thermal Tolerances.Molecular Biology and Evolution, DOI:10.1093/molbev/msw119

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