Algae and Coral Have Been BFFs Since the Dinosaur Age

A new study shows that the relationship between coral polyps and zooxanthellae that produces colorful coral reefs began 160 million years ago

Coral and its symbiotic algae Todd C. LaJeunesse

Coral isn’t really a plant, an animal or a mineral. Instead, it’s more of a complex relationship between all three. Researchers believed that the colorful mash-up first took place about 60 million years ago, but Mary Halton at the BBC reports that new research suggests the matchmaking took place 100 million years earlier and was strong enough to survive the events that doomed the dinosaurs. Scientists have also discovered that the algae that cohabitates with coral is much more diverse than previously thought, suggesting hundreds if not thousands of species should be reclassified on the tree of life.

To understand the new research you need to know a little bit about how coral operates. The animal part of the system, the coral polyp, floats through the sea until it latches onto a rock or other hard surface. There, it begins excreting a calcium carbonate skeleton that protects the polyp within, eventually producing a colony of coral that can grow into large structures. Different types of coral—elkhorn, brain, tube and staghorn, among others—produce different shapes. Most of those carbonate reefs are bone white on their own, but coral gets its beautiful colors from photosynthetic algae called zooxanthellae, which live within the cells of the coral animal. In that symbiotic relationship, the algae get a protected place to live and in return the coral gets the glucose and amino acids it needs to survive, along with a nice pop of color.

Traditionally, almost all of the algae living in coral were lumped into one genus called Symbiodinium. But in a new study published in the journal Current Biology, a team of international researchers took a closer look at the genetics of the genus, finding a huge amount of biodiversity in the zooxanthellae. According to a press release from Saudi Arabia’s King Abdullah University of Science & Technology, the team used various genomic and phylogenic techniques to re-classify the algae species. Using a genetic analysis technique known as the molecular clock, which looks at the number of mutations a genome picks up over time, the international team also found that the oldest zooxanthellae evolved around 160 million years ago, more than doubling the age of the coral/algae symbiotic relationship. According to a press release from Oregon State University, the team suggests that the genus Symbiodinium should actually be subdivided into 15 genera, including hundreds or thousands of species of zooxanthellae.

The research isn’t just about moving species around in the tree of life. The longevity of the symbiotic relationship gives scientists new insight into the resilience of coral reefs. “Our recognition of the true origin of those microbes that give corals life is major revelation,” lead author Todd LaJeunesse of Pennsylvania State University tells the BBC’s Halton. “They are way older than was previously estimated. Meaning that [this partnership has] been around for a hell of a long time!”

Knowing that coral and algae have been living together for so long helps explain why corals across the world are so diverse. It also has big implications for conservation since coral reefs are one of the habitats most susceptible to climate change and have already experienced serious disturbances due to warming oceans, with half of Australia’s Great Barrier Reef dying since 2015. Understanding the differences in the algae will help researchers understand variations in things like heat tolerance, susceptibility to pollution and how well each species recovers from bleaching events, in which environmental stressors cause the zooxanthellae to temporarily flee the coral polyp.

“Until now, much research on these algae attempted to compare apples to apples, but we now know that often we are comparing apples to oranges considering how divergent some of these species are,” co-author John Parkinson of Oregon State University says in the release. “Our work will help researchers to think more objectively about the comparisons they are making in experiments.”

The fact that coral reef systems survived the event that finished off the dinosaurs and other global upheavals is encouraging. LaJeunesse tells Halton that it's likely the coral/algae mashup will survive in some form for a long time to come. As resilient as it is though, the system won’t make it through the current changes in our oceans unscathed, and will likely see species loss and the collapse of functional coral ecosystems, which support about a quarter of all marine life.

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