The world's biologically richest marine ecosystems, and some say the most beautiful, depend on the mutually beneficial relationship between corals and the single-celled algae that inhabit their tissues. Excess heat cleaves this alliance, and thousands of other species fall in its wake. 

Dr Patrick Buerger of Australia's CSIRO and Professor Madeleine van Oppen of the Australian Institute of Marine Science are seeking solutions to the enormous bleaching events that have recently killed a large fraction of the world's corals. They have now succeeded in producing microalgae in the laboratory that can handle the heat, and shown that when these take up residence in corals, both flourish in the temperature.

The team cultured 10 cell lines of Symbiodiniaceae and exposed them to steadily rising temperatures. The microalgae were initially genetically identical, but as the cells replicated, mutations crept in, a few of which made for greater heat tolerance. Over four years, those most suited to the conditions thrived and took over. In Science Advances, Buerger and van Oppen describe successfully inoculating some strains of heat-hardened Symbiodiniaceae into coral at the larval stage.

"Once the microalgae were reintroduced into coral larvae, the newly established coral-algal symbiosis was more heat tolerant compared to the original one," Buerger said in a statement.

“We don't know if the changes are genetic or epigenetic,” van Oppen told IFLScience. Nevertheless, the team are starting to understand what makes them better suited to a warmer world.

"We found that the heat tolerant microalgae are better at photosynthesis and improve the heat response of the coral animal,” van Oppen said. "These exciting findings show that the microalgae and the coral are in direct communication with each other."

Inevitably, the same process happens in the wild, but so far isn't keeping up with the rate of warming.

“We can grow the algae much faster than they do inside corals, so we get more cell divisions, and can place them under more directed selection,” van Oppen said.

The team are also working on breeding corals for heat tolerance independently of their inhabitants, but van Oppen told IFLScience there is an advantage to focusing on microalgae.

“This particular symbiont can form a partnership with many coral species, whereas for corals we would need to breed each individually,” she said.

Nevertheless, this Symbiodiniaceae species does not combine with all coral species, so it would not provide a universal cure on its own. Van Oppen acknowledged there is a long way to go. The team don't yet know if the symbionts will be maintained in the wild or if coral seeded with them will prefer other partners outside heat waves. Moreover, she admits scaling up to produce enough microalgae to seed entire reef systems is a daunting task. Still, it might be the best hope we have.