Coral reefs are massive geological structures consisting of calcium carbonate produced by coral animals, which possess dense populations of photosynthetic algae from the family Symbiodiniaceae within their tissues. Due to global warming and other environmental changes, corals in the Atlantic Ocean have declined substantially over the past years. However, corals in the Pacific and Indian Oceans are more resilient.
By examining several species of symbiotic algae that corals need in order to survive and grow, a team of researchers led by the Pennsylvania State University has recently discovered that the mutualistic relationships between corals and their symbionts in the Indo-Pacific region are more flexible than those in the Atlantic, thus making them more resilient to climate change in the long term.
When environmental stressors, such as rising ocean temperatures, cause the relationship between algae and corals to break down, a phenomenon known as “coral bleaching” occurs. Although corals can recover after bleaching, many times prolonged and intense periods of stress can lead to massive coral mortality.
“Coral bleaching not only affects the corals themselves, but also entire ecosystems of organisms – from invertebrates, like sea urchins and spiny lobsters, to vertebrates, like fish and sea turtles,” said senior Todd LaJeunesse, a professor of Biology at Penn State. “It’s important to study the biology of corals and their symbionts so we can predict how they will respond to future environmental changes, especially ocean warming.”
However, not all corals and their symbionts respond in the same way to environmental stressors. Scientists have extensively studied the vast diversity of corals and their unique attributes, but less attention has been given to how the diversity of symbiont species may also contribute to coral resilience.
“Scientists previously lumped all the symbionts into a few broad groups,” said LaJeunesse. “My lab’s work over the past several years has been to describe individual species of symbiont so we know what we’re dealing with. Without this information, you really can’t adequately study the ecology, physiology, and biogeography of corals,” LaJeunesse explained.
By collecting and analyzing coral samples from both the Atlantic and the Indo-Pacific regions, the experts discovered that, while some of their symbiont species are specialists, meaning they can associate with only one or a few species of coral hosts, others are generalists, being able to associate with a wide variety of coral host species.
Moreover, the investigation revealed that, while corals in the Atlantic – particularly in the Caribbean – rely mostly on specialist symbionts, Indo-Pacific corals associate largely with generalists. Thus, the lack of flexibility among Caribbean corals may make them more sensitive to environmental stressors, while the more flexible partnerships among Indo-Pacific corals and their symbionts make them more resilient to environmental changes.
By sequencing the DNA of symbiotic algae collected from coral samples across the Indo-Pacific, the researchers identified and described five species of symbionts from the genus Cladocopium, which are capable to associate with a variety of host coral species.
“The organisms that we described are widespread, and as oceans warm, these thermally tolerant generalists are likely to expand to new coral communities. Recognizing these distinct species enables informed research into their ecology,” concluded lead author Caleb Butler, a graduate student in Biology at Penn State.
The study is published in the Journal of Phycology.
Corals are sensitive organisms that can be affected by various environmental factors, but some coral species and individuals are more resilient than others. Several factors contribute to coral resilience, which can help corals withstand stressors, recover from damage, and adapt to changing conditions.
A diverse gene pool allows for a greater range of traits and adaptations, enabling some coral individuals or species to better cope with environmental stressors.
Corals have a symbiotic relationship with microscopic algae called zooxanthellae, which live within their tissues. This relationship is crucial for the coral’s survival, as the algae provide energy through photosynthesis. Some corals can switch to different types of algae that may be more resilient under certain conditions.
The physical structure of a coral colony can influence its resilience. For example, branching corals may be more resilient to certain stressors, like wave action, while massive corals may fare better under thermal stress.
Corals reproduce both sexually and asexually. Sexual reproduction allows for genetic recombination, increasing genetic diversity and the chances of adapting to new conditions. Asexual reproduction, like fragmentation, can help corals recover quickly from disturbances.
Corals can adapt to changing environmental conditions over time through natural selection. For example, some corals have evolved to tolerate higher temperatures or lower pH levels.
Coral reefs that are well-connected to other reefs through ocean currents can receive an influx of new coral larvae, helping to maintain genetic diversity and support recovery after disturbances.
A healthy and diverse ecosystem surrounding the coral reefs can provide additional resilience. For example, herbivorous fish can help control algal growth, which can otherwise smother corals.
Efforts to protect and conserve coral reefs, such as implementing marine protected areas, reducing pollution, and promoting sustainable tourism, can help bolster coral resilience by reducing stressors and promoting recovery.
Overall, coral resilience is a complex interplay of multiple factors that help these organisms withstand, adapt to, and recover from various stressors. Protecting and conserving coral reefs and their surrounding ecosystems is critical to maintaining their resilience and overall health.