Feeding 'Supplements' to Corals Could One Day Help to Regrow The Great Barrier Reef

Corals found in the world's reefs have their own microbiomes, and scientists are working to figure out how to feed them probiotic'supplements,' in the hopes of saving them for future generations.

A swimming larva floating in the water is the beginning of a newborn coral's existence. When the larva has grown large enough, it sinks to the bottom – or, if it's lucky, a thriving reef. It begins to clone itself after it has landed.

Shallow-water corals, which are made up of a variety of organisms, are essentially colonies of tiny animals working together with a marine algae called zooxanthellae, which feeds the coral and aids in the production of calcium carbonate, which forms reefs over thousands – or even millions – of years.

While the connection between corals and zooxanthellae is widely recognized, scientists are just now beginning to investigate how much corals rely on other bacteria.

Coral microbiomes may be upset by changes in their environment, just like the delicate balance of bacteria in our own guts: ocean acidification, marine warming, bleaching episodes, and other stressors, the majority of which are caused by anthropogenic climate change and other human activities.

While climate action is essential to halt the havoc that widespread fossil fuel usage is doing on the planet's ecosystems, conservation biologists are also searching for solutions to fix the damage that has already been done, such as rebuilding coral reefs.

Stressors are increasing in the Great Barrier Reef's (GBR) waters, while tanks full of young corals are being nurtured on land in adjacent Townsville in a bid to ensure the reef's future.

These corals are being given probiotics in the hopes that one day, utilizing aquaculture, researchers will be able to produce millions of healthy corals, allowing for large-scale restoration operations on the GBR that are presently out of our reach.

Lone Hj is a microbial ecologist at the Australian Institute of Marine Science's 'Sea Simulator' lab, where he leads research on coral probiotics.

850 bacterium strains were recovered from six GBR coral species by Hj and her team: Acropora tenuis, A. millepora, A. hyacinthus, Platygyra daedalea, Porites lobata, and Goniastrea retiformis.

"Our coral bacterial culture collection covers a high taxonomic diversity, with more than 50 bacterial genera represented,"  Høj told ScienceAlert.

Scientists are testing a subset of the microorganisms in this enormous collection for features that might help human-reared coral thrive and flourish.

Antibacterial activity, biofilm development on coral mucus, and the generation of digestive enzymes are among the characteristics they're searching for in probiotic bacteria right now. These characteristics should help the coral keep its healthy microbiome when the probiotic treatment is finished. The detailed examination of the previous year's investigations is still continuing.

Coral spawning happens every year in the Sea Simulator lab, giving scientists the opportunity to conduct studies on the coral larvae. When the coral species A. tenuis and P. daedalea hatched in late 2021, the project's first test took place.

Following fertilization, the new'recruits' - larvae that settled at the bottom of the tanks – were exposed to several probiotic bacteria strains in a controlled environment. The researchers next looked at their development, the density of their symbionts (both algae and bacteria), their immunological responses, and their microbiome, particularly after the probiotic was removed.

Scientists are also planning for the upcoming coral spawning season, which is slated to begin in late 2022, so they may repeat the experiment. They'll utilize the same coral species and maintain all other parameters, but in the probiotic combination, they'll include a broad range of bacteria. 

According to Høj, the next trial will look at the probiotic's long-term impacts on lab-grown corals as they age, as well as the probiotic's potential for increasing the coral's resistance under stress. The researchers will also look at other delivery methods for the probiotic to young corals - it's not as easy as ingesting a pill with breakfast.

This discovery, according to Høj, might one day lead to probiotics for use in natural reef environments, albeit the feasibility of this remains to be shown. However, the probiotics now being developed are exclusively for use in laboratory and aquaculture, and only for GBR species.

While preventing human-caused climate change remains the most critical aspect in coral reef sustainability, it's comforting to know that other organisms are keeping an eye on these ecosystems.