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Apparently, elephants are not the only ones with a good memory! Corals seem to use a surprising strategy called 'environmental memory' to survive bleaching. Let's discover it together!

The phenomenon of coral bleaching is becoming more frequent, partly due to the increasing number of heat waves affecting these organisms. However, some coral reefs not only recover from such events but often show greater resilience to subsequent temperature increases.

This process, known as ‘environmental memory’, occurs when an organism changes its response to past environmental changes. Researchers are currently trying to better understand the reasons and mechanisms behind this adaptive capacity in corals.

Bleached corals – Photo via Canva

IMPORTANCE OF ENVIRONMENTAL MEMORY

As described in the article Il microbiota dei coralli: un universo nascosto, the resilience and survival of coral holobionts depends on the delicate balance between symbiotic partners, which is vulnerable to environmental changes. Moreover, anchored to the substrate, corals are forced to cope with all kinds of stress without being able to leave their habitat.

In this situation, environmental memory becomes a valuable strategy for these individuals! This adaptive capacity allows them to store information about the environmental conditions they have faced in the past to respond more quickly and efficiently when similar or worse situations occur in the future.

Photo via Canva

EVIDENCE OF MEMORY IN CORALS

The first evidence of environmental memory in corals was documented during a bleaching event in Phuket, Thailand, in 1995. On that occasion, the western parts of colonies of Coelastrea aspera, a species of stony coral, experienced less bleaching than the eastern parts of the same colonies. According to experts, this higher heat resistance was developed in response to previous exposure to increased solar radiation. Moreover, this ability persisted for 10 years in the absence of the stimulus that triggered it.

Since then, further evidence of environmental memory in corals has emerged. For example, a study published in Science in 2016 found that over 30 years, 75% of the bleaching episodes affecting the Great Barrier Reef occurred after periods when temperatures remained below the so-called ‘bleaching threshold’ – the critical temperature above which corals begin to lose their symbiotic partners. This initial exposure to heat proved beneficial as it protected the corals, reducing mortality and symbiont loss by 50% compared to times when the temperature directly exceeded the threshold without any initial stimulus.

Coral fitness is represented by three stages: healthy (no stress), pale (intermediate stress) and bleached (high stress), through the “memory stages” from the priming exposure to the triggering exposure. Through environmental memory, corals experience less bleaching during the triggering exposure than during the priming exposure, even if the temperature is higher than previously experienced.

MECHANISMS OF ENVIRONMENTAL MEMORY

Therefore, environmental memory allows corals to better prepare for conditions of high thermal stress. But how does this preparation occur? According to the researchers, it is possible thanks to a series of mechanisms, including:

  1. Symbiont shift: some coral species can change their symbionts, switching to species of algae that are more resistant to heat.
  2. Transcriptional boost: genes involved in stress management and coral survival are more highly activated.
  3. Epigenetic changes: these changes do not directly alter the ‘text’ of the DNA, but affect how genes are read and used, helping to make corals more resilient.
  4. Prepared offspring: although the mechanism is still unclear, early evidence suggests that corals can pass on their environmental memory to their offspring.

Understanding these mechanisms is crucial to learning how corals adapt to survive environmental changes.

Picture by © CATHERINE DELPHIA, MODIFIED BY THE SCIENTIST TEAM

IMPLICATIONS FOR CORAL REEF CONSERVATION

According to interviews conducted by The Scientist, the ultimate goal would be to incorporate environmental memory into coral reef restoration efforts. This could be done by ‘stressing’ corals in a controlled environment before replanting to create false stress memories. Such a strategy would favour the selection of more resilient symbionts and/or species with better memory.

Although scientists are in a race against time, some of these efforts are already underway. A deeper analysis of environmental memory could therefore better predict the response of coral holobionts to future environmental changes, paving the way for innovative and sustainable solutions for their conservation!

Photo via Canva

BIBLIOGRAPHY 
Autrice: Eleonora Tinto

Eleonora graduated from the University of the Algarve, Portugal, with a degree in Marine Biology. Having grown up close to the sea, she has always felt a deep connection to this world, which has led her to dedicate herself to its protection. She has recently been involved in science outreach and is even more motivated to continue in this field. If she were a marine animal, she would be a manta ray, dancing freely and lightly in the ocean currents.

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