Coral reefs are among the most diverse and productive ecosystems on Earth, providing habitat and food for millions of marine species and supporting the livelihoods of billions of people.
However, coral reefs are also highly vulnerable to global warming and other environmental changes, which can cause coral bleaching and mortality.
Coral bleaching occurs when environmental conditions, such as rising ocean temperatures, cause the relationship between the coral animals and their symbiotic algae to break down, resulting in a white, or bleached, colony.
The symbiotic algae, from the family Symbiodiniaceae, live within the coral tissues and provide them with energy from photosynthesis. Without these algae, corals can starve and die.
But not all corals and their symbionts are equally sensitive to environmental stress. Scientists have found that corals in the Indo-Pacific region, which covers the Pacific and Indian Oceans, are more resilient to climate change than those in the Atlantic Ocean.
The reason for this difference may lie in the diversity and flexibility of the symbiotic relationships between corals and algae in the Indo-Pacific.
Diversity of symbionts in the Indo-Pacific
An international team of researchers led by Penn State has described several species of symbiotic algae that are associated with corals across the Indo-Pacific region, as per Phys.org.
These species belong to different genera within the family Symbiodiniaceae, such as Cladocopium, Durusdinium, Symbiodinium and Breviolum.
The researchers found that some of these symbiont species are specialists, meaning they can only associate with one or a few species of coral hosts, whereas others are generalists, meaning they can associate with many species of coral hosts.
For example, Cladocopium goreaui is a specialist that only occurs in a single species of coral, Pocillopora damicornis, while Durusdinium trenchii is a generalist that can be found in over 100 species of corals.
The researchers also found that some of these symbiont species are more thermally tolerant than others, meaning they can survive higher ocean temperatures without causing bleaching in their coral hosts.
For example, Durusdinium trenchii is known to be one of the most heat-resistant symbionts, while Cladocopium goreaui is more sensitive to thermal stress.
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The flexibility of symbioses in the Indo-Pacific
The diversity of symbiont species in the Indo-Pacific region may provide corals with more options and flexibility to cope with environmental changes, as per Penn State University.
The researchers found that some corals can switch or shuffle their symbionts depending on the conditions they face.
For example, some corals can replace their original symbionts with more heat-tolerant ones when exposed to warmer waters, while others can adjust the relative abundance of different symbionts within their tissues to optimize their performance.
The researchers also found that some corals can acquire new symbionts from the environment or from other corals through horizontal transmission.
This may allow them to adapt to new habitats or recover from bleaching events faster than those that rely on vertical transmission (from parent to offspring).
These findings suggest that corals in the Indo-Pacific region have more flexibility and resilience to climate change than those in the Atlantic Ocean, where symbiont diversity is lower and symbioses are more stable and specific.
However, this does not mean that Indo-Pacific corals are immune to global warming or other threats, such as pollution, overfishing and disease.
The researchers caution that even the most resilient corals have their limits and may not be able to keep up with the rapid pace of environmental change.
Therefore, it is crucial to protect and conserve coral reefs worldwide by reducing greenhouse gas emissions and implementing effective management strategies.
By understanding the biology and ecology of corals and their symbionts, scientists hope to find ways to enhance their resilience and restore their health.
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