Researchers Witness Coral Cells Engulfing Algae For First Time

Scientists have observed coral cells engulfing photosynthetic algae for the first time. It is a crucial step in understanding their all-important symbiotic relationship.

Coral reefs are important biodiversity hotspots in the world's oceans, the researchers of a new study, published in Frontiers in Marine Science, said. However, they have been placed under great stress in recent decades because of anthropogenic warming.

In particular, corals' important symbiotic relationship with microscopic algae called dinoflagellates has been strained.

"Coral cells take up the algae and provide them with shelter and the building blocks for photosynthesis. In return, the algae provide the corals with nutrients that they synthesize," study senior author, Professor Noriyuki Satoh of Okinawa Institute of Science and Technology (OIST) Graduate University, explained in the university news release.

But because of stressors such as warming temperatures, acidification and pollution, corals expel the algae and end up dying in mass bleaching events.

Stony corals from the Acroporidae family are said to be the most common type of coral in tropical and sub-tropical reefs, the university noted. They're also vital to building coral reefs because they grow fast and provide calcium carbonate skeletons. However, they're also particularly susceptible to such bleaching events, so understanding the "cellular and molecular mechanisms" of their symbiosis with algae is key to coral reef preservation.

One problem in this effort is coral cells are said to be "notoriously difficult" to culture, OIST noted, so previous studies used the cells of related creatures instead.

"To date, however, almost no in vitro experimental systems have been devised to illuminate such mechanisms," the researchers wrote.

In their study, the scientists from Japan successfully achieved this feat, culturing the cell lines of larvae from the stony coral Acropora tenuis in petri dishes. The researchers then added the dinoflagellates and, sure enough, they were able to observe the process of the coral cells engulfing the algae for the first time.

In the video of the "world-first" observation, one can see the coral cells reaching for the dinoflagellates and engulfing them. The process of in vitro symbiosis took just about 30 minutes. Within 24 hours, about half of the corals already incorporated the algae "in a reproducible manner."

Once harboring the algae, the coral cells became "round and less mobile," the researchers noted. The symbiosis state remained for about a month, then the coral cells died.

The researchers' work not only shows the first time that coral cells engulfed algae but also provides clues as to how the symbiotic relationship between them began in the first place, the researchers said. Importantly, it also sheds light on how we can possibly save coral reefs in the future.

"Knowledge obtained in future studies using this system may advance our understanding of coral biology associated with bleaching, thereby providing cues for methodological improvements of coral reef preservation," the researchers wrote.