Coral

Coral symbiosis doesn’t need photosynthesis, after all

Jinkerson, R. E., Russo, J. A., Newkirk, C. R., Kirk, A. L., Chi, R. J., Martindale, M. Q., Grossman, A. R., Hatta, M., & Xiang, T. (2022). Cnidarian-Symbiodiniaceae symbiosis establishment is independent of photosynthesis. Current Biology. DOI: https://doi.org/10.1016/j.cub.2022.04.021

Coral reefs are some of the most beautiful and biodiverse ecosystems on the planet. Corals themselves belong to a group of animals called cnidarians, which also include jellyfish and sea anemones. Corals owe their success to their symbiotic (meaning “living together”) relationship with photosynthetic algae. The algae live inside coral tissues and use photosynthesis to turn light energy into food, then pass it off to the coral host. In turn, the algae receive a safe home and nutrients from the coral. This cooperative relationship is necessary for the continued existence of coral reefs, but factors such as heat and toxic chemicals can damage this relationship. For example, coral “bleaching” occurs when corals lose or reject their algal partner during stressful conditions. Often, the host coral dies shortly after.

Examples of healthy (right) and bleached (left) coral. Image source: Wikipedia, Copyright CC BY-SA.

How do corals tell friend from foe?

Because of the importance of symbiosis in reef health, biologists are interested in how the coral and algae establish and maintain their relationship. Seawater is full of other microbes, not just corals’ symbiotic partners. Corals therefore need a way to selectively take up and keep the symbiotic algae while rejecting other non-symbiotic organisms—and we still don’t know how they do this.

One possibility is that coral and algae “speak” to each other at a molecular level. The algae might produce certain chemicals that say “I’m a friend” so that coral allows it to stay and grow. A common hypothesis is that these chemical signals are byproducts of photosynthesis, but we don’t know for sure.

When does symbiosis work, and when doesn’t it?

Scientists recently tested whether symbiotic algae could colonize the host without photosynthesis. They tested multiple species of cnidarians—including a coral, a jellyfish, and a sea anemone—and many species of algae, called “Symbiodiniaceae.” In some experiments, the scientists raised animals in constant darkness to prevent photosynthesis. In other experiments, they used chemicals to block photosynthesis, and even created mutant algae that couldn’t photosynthesize. They then tested how successful the non-photosynthetic algae were in three stages of symbiosis: initial infection, growth, and long-term maintenance.

Experimental design of the study by Jinkerson and colleagues. Image source: Jinkerson et al. 2022. Copyright CC BY-NC.

In general, the researchers found that symbiosis does NOT necessarily require light or photosynthesis—but it depends on the specific pair of host and algae and algal partner. The initial infection stage can always proceed without photosynthesis, but in some cases the non-photosynthetic algae fail to grow and are lost over time. For the coral, every step of symbiosis proceeded with most of its algal partners.

Towards a better understanding of reef health

One important takeaway is that symbiosis is not one-size-fits-all. Different animals and different algae probably use different means to communicate and thrive. In many cases, algae must use molecular signals unrelated to photosynthesis, and this knowledge will help researchers in their search for those signals. Finally, the fact that symbiosis can continue without photosynthesis—when the coral gains no benefit—suggests that the algae might be taking advantage of the coral’s resources. As climate change continues to stress marine environments, coral reef symbioses will be challenged. Hopefully, a better understanding of the biology enabling these unique symbiotic relationships will help researchers understand how coral reefs will fare in the future.

 

Cover photo by Claudia Padilla Souza, Wikimedia. Copyright CC BY-SA.

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