Coral cells engulfing algae for the first time

In a world first, scientists in Japan observed individual stony coral cells engulfing single-celled photosynthetic algae.

The microscopic algae, known as dinoflagellates, were engulfed by cells grown from the stony coral, acropora well, scientists reported in the journal Frontiers in marine sciences.

“Dinoflagellates are essential for keeping corals healthy and alive,” said Professor Noriyuki Satoh, lead author of the study and head of the Marine Genomics Unit at the Graduate University of the Okinawa Institute of Science and Technology. “Coral cells absorb algae and provide them with shelter and the building blocks for photosynthesis. In return, the algae provide the corals with the nutrients they synthesize.

However, in recent decades this essential relationship has been strained. Driven by pollution, acidification and rising ocean temperatures, stressed coral cells push out microscopic and colored algae during massive bleaching events, resulting in huge expanses of dead white reefs.

Stony coral Acroporidae family – the most common type of coral found in tropical and subtropical reefs – are particularly susceptible to these bleaching events. These fast growing corals form calcium carbonate skeletons and therefore play a key role in the construction of coral reefs.

“For the conservation of coral reefs, it is vital for us to fully understand the partnership between the hard corals and the algae that live inside these animals, at the level of a single cell”, explained the co-prime Professor Kaz Kawamura of the University of Kochi. “But until recently, it was very difficult to achieve.”

Coral cells are notoriously difficult to grow, so scientists previously had to rely on experimental systems of other closely related sea creatures, such as sea anemones, to study the mechanism by which dinoflagellates enter and exit. cells.

It wasn’t until April 2021 that the research team took a big step forward, reporting in Marine biotechnology that they had successfully cultivated different cell lines from hard coral larvae, acropora well, in Petri dishes.

For this study, the scientists focused on a coral cell line called IVB5. Many cells of this lineage have properties similar to endodermal cells in shape, behavior and genetic activity. Importantly, in whole coral organisms, it is the endodermal cells that engulf the algae.

Scientists added the dinoflagellate, Brevium minute, to a Petri dish containing the IVB5 coral cells.

About 40% of the coral cells in the culture quickly formed long finger-like projections that stretched out to contact the dinoflagellates. The algae were then “swallowed”, in a process of about 30 minutes.

“It was incredible to see, it was almost a dream! said Professor Satoh.

Over the next two days, the algae inside the cell was either broken down into fragments or successfully enclosed in membrane-bound bags, called vacuoles, inside the cells. For researchers, this indicates how the relationship may have started millennia ago.

“It may be that the ancestors of the coral originally gobbled up these algae and broken it down for food. But over time, they evolved to use the algae for photosynthesis instead,” the co suggested. -first author, Dr Satoko Sekida of the University of Kochi. .

Researchers are now using electron microscopes to get more detailed images of how coral cells engulf dinoflagellates. They are also working on genetic experiments to identify the genes of the corals involved.

At this stage, the coral cells containing the algae live for about a month before dying. In the near future, the team hopes to achieve a stable culture where coral cells and dinoflagellates can reproduce together.

“It would be very exciting because then we can ask new questions, like the reaction of corals when they are under stress,” said Professor Satoh. “It could give us a more complete understanding of how bleaching occurs and how we can mitigate it.”