Are super-corals really a super solution ?
Written by: Richard Singhroy
Acknowledgements to Dr. Emma Camp
While reefs are adaptable ecosystems, they are struggling to adapt to the quick changes brought on by man-made climate change, habitat destruction and overfishing (Eddy et al., 2021). Global coral reefs’ degradation calls for a variety of mitigation, adaptation, and conservation measures to cope with local, regional and global pressures. Among these measures, the use of super-corals for conservation purposes has been explored through research, propagation, outplant, relocation or protection. Research on “super-corals” is still being explored and a unique definition of the term is not yet available, as we have discussed in our previous article. Nevertheless, some scientists define super-coral as corals with superior tolerance/survivorship when exposed to one or more prolonged conditions (press) or severe events (pulse) considered “stressful” for most coral species (Camp et al., 2018a).
Some corals that can help bolster reefs are naturally occurring. An example of this is in the Gulf of Aqaba in the Red Sea. Here, coral communities’ species have been found to tolerate extreme temperature conditions: above 6 degrees their average summertime maximum for around three to four weeks (Fine et al., 2013). This is significant because most corals can only tolerate temperatures above 1 to 2 degrees above the average summertime maximum, and can only do so for 2 to 4 weeks (e.g.Hoegh-Guldberg, 2011). The species Stylophora pistillata, Pocillopora damicornis, and Favia favus have been found as prime candidates to be species of super-corals in the context of high temperatures and bleaching events due to this thermal tolerance (Grottoli et al., 2017).
Such tolerance has been suggested to be based on the genetic traits of the coral populations, resulting from the natural selection process over the coral larvae settled in the area (Fine et al., 2013; Grottoli et al., 2017). Only larvae tolerant to these extreme conditions on the species’ tolerance range were to survive and form the coral reef.
Microalgae resistance to heat strain and bleaching events
Taking this a step further, the thermal tolerance of corals can also be increased by improving the tolerance of symbiotic microalgae strains. Coral bleaching is heavily influenced by the strain of microalgae within the polyps (Buerger et al., 2020). Therefore, more resistant symbiotic algae can also help reduce the chance of bleaching. These algae also evolve far more rapidly than the coral host itself (Chakravarti & van Oppen, 2018). This makes them easier to select for in laboratory conditions. Scientists expect that these heat and bleaching-resistant strains can then be introduced to an ecosystem where corals can host them. The results of this research have been promising with some experiments seeing the continued growth of the symbiotic algae during thermal stress (Chakravarti & van Oppen, 2018).
Application of super-corals
Evidence supports that identifying and introducing heat-tolerant corals to a reef can increase the overall tolerance of corals to heat stress, as well as the ability of the reef to recover from stressful events (Morikawa & Palumbi, 2019). The biodiversity of the ecosystem is an important asset for reefs to endure bleaching events, this is equally true for the diversity of coral species as it is for the microbiome (Gardner et al., 2019).
The timing of the introduction of super-corals or microalgae can also be important for recovering from thermal stress. Some studies have found that the introduction of resistant symbiotic microalgae is most effective right after a bleaching event so that bleached corals may recover by taking in the resistant algae (Silverstein et al., 2015). It was found that the corals that were bleached, and exposed to resistant microalgae after, were better able to endure high temperatures later (Silverstein et al., 2015).
In addition to introducing super-corals to maintain reef health, there are some locations where corals survive better than in the surrounding reefs, called refuges, refugia, and hope spots, amongst other terms (Camp, 2022). Here, conditions are maintained favourable for corals when lost elsewhere. Management and protection of such locations are critical in maintaining regional reef health, as corals in such areas are sheltered from stress present in surrounding areas and can therefore persist and eventually repopulate areas that have been decimated by changing environmental conditions (see Camp, 2022).
What are limitations to super-corals?
There are however precautions and considerations when using super-corals for conservation purposes. Amongst them, first, to acknowledge that corals with enhanced tolerance to one stressor, such as thermal stress, may not be resistant to other stressors, such as low pH (Camp et al, 2018a). In the same way, that pulse stress-resistant corals might not be resistant to press events (Camp et al, 2018a). So, when using super-corals for conservation, an important step would be to identify under what environmental conditions it can operate and how it can perform under future changing conditions.
Second, super-corals have reported energetic trade-offs (see review by Camp et al, 2018b): higher thermal tolerance can be accompanied by a reduction in growth rate. So it must be considered how may ecosystem services supply change with the introduction of super-coral to reefs?
Despite the advances in this field, coral reefs remain among the most threatened ecosystems. Super-corals can be a potential tool in the fight to preserve or restore coral ecosystems. They can contribute to our understanding of the mechanisms of coral survival under different stress, as well as a wide range of applications for conservation strategies (see Camp, 2022). But limitations of super-corals should not be overlooked, and their feasibility should be assessed before implementation. To save our coral ecosystems we need to use all the tools at our disposal at a regional and local level. And of course, the elephant in the room worldwide is climate change, and our urgency to reduce CO2 emissions. As with all conservation efforts, the use of super-corals should not be implemented in isolation and must be considered carefully, yet they remain a powerful tool to ensure the future of coral reefs.
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