Young coral beat the heat with metabolic rewiring

Corals have been under a lot of stress lately. These marine invertebrates—relatives of jellyfish—are pretty picky when it comes to temperature. That’s why rising ocean temperatures over the past few decades have led to several dramatic coral bleaching events, in which coral loses the algae partner it relies upon for nutrients and turns white—meaning death for the coral will come soon. But new research published in PLOS Biology suggests that young corals may show some resilience to elevated temperatures (2024, DOI: 10.1371/journal.pbio.3002875).

Ariana Huffmyer, a marine invertebrate biologist at the University of Washington and lead author on the paper, says that when corals reproduce, they generally go through a larval stage in which “they swim around the reef until they find a spot they want to settle, and they metamorphose into the [mature coral] polyp.” To survive that process, those larvae need energy—much of which comes from photosynthetic algae symbionts that live inside the coral cells. But the connection between the coral and algae is delicate.

Mikhail Matz, a coral genomicist at the University of Texas at Austin who wasn’t involved in this research, says the relationship between corals and algae is similar to that of humans and cows. “It’s a very strong exploitative relationship where coral basically farms the algae and exploits their photosynthetic capacity while preventing them from growing,” he says. “It’s not a mutually beneficial thing. They are not friends, corals and algae.”

Huffmyer says that under normal conditions, “the coral host limits the amount of nitrogen that is available to the symbionts,” to prevent the algae population from growing out of control. Nitrogen is essentially a fertilizer for these algae symbionts. Under the stress of elevated temperatures, corals produce excess nitrogen waste that algae can take advantage of. Some scientists hypothesize that when algae growth is fueled by that excess nitrogen, it can reduce the amount of glucose the coral receives from the algae.However, Huffmyer found that under elevated temperatures, the larvae of one type of coral, Montipora capitata, undergoes metabolic rewiring. The larvae begin investing more energy in ammonium assimilation and urea metabolism to hold that extra nitrogen away from the algae, thus keeping the algae population in check.Using carbon-13 stable isotope metabolomic analysis, Huffmyer was able to show that this rewiring helps maintain the level of glucose the algae give back to the coral, which the authors hypothesize prevents bleaching.

That metabolic rewiring is a complex process at the heart of a complex relationship, and there are still many questions about this mechanism. But Matz says the paper is a rigorous study further highlighting how some corals can manipulate their relationship with algal symbionts. He also says the metabolic rewiring uncovers “a fundamental thing about what corals do when they are under stress with their symbionts” that researchers weren’t aware of before.

Huffmyer says that these data show some amount of adaptability but that most species of coral are still highly sensitive to temperature. “We have to make a strong and decisive decision to address climate change aggressively, or we will lose the reefs as we know them today,” she says.