Current Biology︱藻类与珊瑚共生关系独立于光合作用，助于对抗珊瑚白化

Discovery about coral-algal symbiosis could help coral reefs recover after bleaching events

Author︱Holly Ober

Editor-in-Chief︱Sizhen Wang

Editor︱Jiaxue Zha

Finding that algae can establish symbiosis in coral without photosynthesis could help fight coral bleaching.

Corals are keystone species for reef and marine ecosystems but coral bleaching due to increased ocean temperatures is killing them. Corals belong to a group of animals called Cnidarians, which also includes sea anemones and jellyfish, that receive some of their nutrients through a symbiotic relationship with photosynthetic algae living inside their cells. High ocean temperatures cause a breakdown in the symbiosis—algae are expelled—resulting in a ‘bleached’ coral without algae. If symbiosis is not initiated within a few weeks the coral will starve to death.

A new study finds that although photosynthesis by algae is a key part of the symbiotic relationship it is not required to initiate symbiosis. The discovery adds to the little-understood relationship between cnidarians and algae at the molecular level and offers insight into how to jump start the symbiotic relationship between the two organisms after a bleaching event. strategies that might prevent warmer oceans from breaking the symbiotic relationship between the two organisms and saving what remains of the world’s corals.

Cnidarians form a mutualistic symbiosis with photosynthetic algae from the dinoflagellate family Symbiodiniaceae that live inside of their host cells. The algae perform photosynthesis, fix carbon dioxide into sugars, and then give that to their hosts. Some hosts, like some species of coral, are completely dependent on the food they receive from their algal symbionts and will die without it.

In return the algae receive nutrients like nitrogen and phosphorus from the prey that the host catches. Photosynthesis is a key part of this symbiotic relationship but it was not known if this symbiosis can form without photosynthesis. It was assumed that photosynthesis was required but this had never been rigorously evaluated before.

Tingting Xiang, an assistant professor of biological sciences at the University of North Carolina at Charlotte, and UC Riverside chemical and environmental engineering assistant professor Robert Jinkerson, led a team to make the first mutants in Symbiodiniaceae algae, isolate photosynthesis mutants specifically, and use these mutants to investigate symbiosis with cnidarians

“We hypothesized that we could make photosynthesis mutants in this species of algae but no one had ever made any mutants in these organisms before,” said Jinkerson. “We were very excited to be able to generate six photosynthetic mutants and then use those mutants to start to probe the symbiosis between these algae and their hosts.”

The team introduced the mutant algae into seawater tanks that contained sea anemones (Exaiptasia pallida) that had not yet established symbiosis with any algae. After just one day the algae could already be found within the sea anemone’s tentacles, even without photosynthesis.

To learn if the algae could survive in sea anemone host tissue without photosynthesis for longer periods of time, the researchers infected some sea anemones in darkness with mutant and non-mutant algae and kept them in darkness for six months. Even after six months, algal cells were still observable in the sea anemone’s tissues. The group also tested four other species of algae known to form symbiotic relationships with the sea anemones and found that they too could initiate symbiosis in the dark.

Jinkerson, Xiang, and their colleague in Japan Dr. Masayuki Hatta then introduced the unmodified algae in darkness into a tank containing juvenile polyps of a stony coral, Acropora tenuis. The algae infected the coral successfully in the dark. Unexpectedly the algae were able to proliferate in the coral tissues without photosynthesis, something not observed in the sea anemones.

Finally, to learn if the pattern held true for the third member of the cnidarian group, the researchers added the algae to a darkened tank of upside-down jellyfish Cassiopea xamachana polyps. Once again, the algae infected the polyps, though not quite as successfully as in the sea anemone and coral.

Symbiosis establishment can proceed without photosynthesis in coral, jellyfish, and sea anemone hosts, but different aspects of the relationship, such as proliferation of the algae without photosynthesis, is dependent on the specific host–algae relationship.

“Our study highlights the power of forward genetic approaches to probe cnidarian Symbiodiniaceae symbiosis and provides a promising platform to answer key questions in symbiosis and ultimately develop strategies to save corals” said Xiang.

The discovery that photosynthesis is not essential to begin symbiotic relationships is a step toward finding ways to help cnidarians survive climate change.

“Time is of the essence regarding the protection of the coral reefs, and our hope is that these mutants will allow ourselves and others to increase the overall pace towards this goal,” said Russo.

 Legend: Photosynthesis shapes the symbiotic relationships between cnidarian hosts and Symbiodiniaceae algae. Jinkerson et al. show that infection, proliferation, and maintenance can proceed without photosynthesis during symbiosis establishment but the ability to do so depends on specific host-symbiont pairs.

（Data Figure Source: Current Biology）

 Legend: A fluorescence image of a few coral Acropora juvenile polyps hosting the photosynthesis mutant ora1 that was used in this paper, shown as red dots. Green color is the endogenous green fluorescence from corals.

（Data Figure Source: Xiang’s lab）

Jinkerson and Xiang were joined in the research by Joseph A. Russo, Casandra R. Newkirk, Andrea L. Kirk, Richard J. Chi, Mark Q. Martindale, Arthur R. Grossman, and Masayuki Hatta. The open-access paper, “Cnidarian-Symbiodiniaceae symbiosis establishment is independent of photosynthesis” is published in Current Biology and is available.

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