July 14 (UPI) -- For the first time, scientists have witnessed coral cells enveloping dinoflagellates, single-celled, photosynthetic algae.
Scientists in Japan added individual cells cultured from the stony coral, Acropora tenuis, to Petri dishes and watched as they swallowed algal cells whole.
In the ocean, algae and coral forge a mutualistic partnership. Algae gain shelter, while coral gets sustenance by usurping some of the energy algae harvests from sunlight via photosynthesis.
The first-of-its-kind research -- detailed Wednesday in the journal Frontiers in Marine Science -- could help scientists better understand this symbiotic relationship and why it sometimes goes wrong.
Over the last two decades, marine ecologists have observed an increase in large coral bleaching events. Coral bleaching occurs when environmental stressors -- pollution, acidification and rising ocean temperatures -- trigger a breakup of the symbiotic relationship between coral and algae.
Stony coral, one of the most common corals among tropical and subtropical reefs, are especially susceptible to bleaching.
"For coral reef conservation, it's vital for us to fully understand the partnership between stony coral and the algae that live inside these animals, at the level of a single cell," co-first author Kaz Kawamura said in a press release.
"But until recently, this was very hard to achieve," said Kawamura, a professor at Kochi University.
For the last several years, Kawamura and his colleagues have been working to culture different coral cell lines, deploying the same methods used to culture sea anemone cells.
Finally, last spring, scientists successfully cultured a cell line called IVB5 with properties similar to endodermal cells, the cells responsible for engulfing algae.
When scientists added dinoflagellate, Breviolum minutum, to Petri dishes containing IVB5 cells, roughly 40 percent of the coral cells formed long, finger-like projections to reach out and begin swallowing up algal cells whole.
"It was amazing to see -- it was almost a dream!" said senior author Noriyuki Satoh, professor at the Okinawa Institute of Science and Technology Graduate University.
In the days following the breakthrough, researchers watched as the algae cells were enclosed in membrane-bound sacs, called vacuoles.
Some of the algae cells were broken down into fragments inside the coral cells, hinting at the ancient origins of the symbiotic relationship.
"It may be that originally, the ancestors of coral engulfed these algae and broke them down for food," said co-first author Satoko Sekida, researcher at Kochi University. "But then over time, they evolved to use the algae for photosynthesis instead."
Researchers are now using electron microscopes to gain more detailed images of the envelopment process. The team is also working on experiments to identify the genes corals use to regulate the algae absorption process.
Currently, the coral cells can survive in Petri dishes for about a month, but researchers are hoping to culture coral and algae cells capable of living and reproducing together indefinitely.
"This would be very exciting as then we can ask new questions, like how the corals react when placed under stress," said Satoh. "This could give us a more complete understanding of how bleaching occurs, and how we can mitigate it."