UC Berkeley study finds algae parasitized coral cells to build reefs

Coral reefs may owe their existence not to a cooperative merger between algae and coral, but to a cellular heist. A research team at UC Berkeley has published findings in the journal Cell showing that algae colonized coral by parasitizing lysosomes — the digestive compartments inside cells — rather than being absorbed into specialized structures as scientists had long assumed.

Overturning a Long-Held Assumption

The study, led by Berkeley assistant professor of molecular and cell biology Phillip Cleves, challenges the prevailing view that corals absorbed algae into compartments called symbiosomes in a process analogous to how mitochondria and chloroplasts became permanent fixtures of animal and plant cells. Instead, the researchers argue that dinoflagellate algae evolved to resist the digestive enzymes inside lysosomes and turned these hostile compartments into homes.news.berkeley+1

"Parasites basically trick cells to do what they want. That's what I think is happening here," Cleves said. "The algae are hijacking the nutrient centers of the cells and acting like food that just never gets digested because they can fix carbon and make glucose from photosynthesis. I think about it as an everlasting gobstopper."phys

Once established inside the lysosome, the algae absorb carbon from the host cell and release glucose produced through photosynthesis, feeding the coral in return. This exchange enabled corals to thrive in nutrient-poor tropical waters and build the reef systems that support roughly a quarter of all marine species.phys

Genetic Experiments Reveal the Mechanism

Using coral raised in a custom saltwater nursery at Berkeley's Koshland Hall, the team isolated the symbiosome membrane in anemones and identified roughly 200 proteins, many of them normally found in lysosomes. Knockdown experiments showed that several lysosomal proteins are required for algae to survive inside these compartments.phys

Postdoctoral fellow Shumpei Maruyama and doctoral student Catherine Henderson then used CRISPR to knock out a bicarbonate transporter in the reef-building coral Galaxea fascicularis, demonstrating it is essential for symbiosis. That transporter supplies carbon dioxide to the algae, allowing photosynthesis to proceed even deep within a coral cell.sciencedirect+1

"We are now showing that the symbiosome is actually a type of phagolysosome, which is profound," Cleves said. "We think that this hijacking of the phagolysosome tells us how these algae are so promiscuous at evolving new symbioses with coral, anemones, jellyfish, clams and even flatworms."phys

Implications for Coral Bleaching

Because lysosomes are conserved across animals, the finding suggests that algae needed only to evolve resistance to digestion to colonize a wide range of marine hosts — explaining why the symbiont family Symbiodiniaceae appears in organisms from corals to clams. The work also opens a new line of inquiry into coral bleaching: dysfunction of symbiosomal proteins during heat stress may drive the breakdown of the relationship that keeps reefs alive.phys