Tracking coral probiotics unlocks symbiotic secrets

With coral reefs under extreme stress worldwide due to climate change, effective tools are needed to protect corals from devastating bleaching events. KAUST researchers, in collaboration with scientists in Brazil and Australia, have shown how certain bacterial species included in lab-designed coral probiotics actually localize within coral tissues, boosting resilience from inside the creatures themselves^[1].

Similar to developing probiotics for the human gut, the concept of boosting coral health using inoculations of beneficial bacteria is a long-standing research area at KAUST. Corals are symbiotic organisms, meaning they partner with algae to stay alive, sharing resources and trading nutrients. Under stress, these symbiotic partners compete for limited resources, and the corals may expel the algae and turn white, hence why it is called “coral bleaching.” Without their algal symbionts, the corals may die within a few weeks.

Previous studies have shown that certain combinations of bacterial species can boost coral resilience to heat stress and bleaching, improve nitrogen cycling and mitigate the effects of toxic compounds and pathogens in corals.

“Probiotics essentially provide personalized medication for corals,” says Ph.D. candidate Pedro Cardoso, who worked on the project under the supervision of KAUST’s Raquel Peixoto. “By selecting bacteria with specific traits, we hope to create tailormade probiotics for particular corals at specific reefs. While we do this, we will better understand precisely how these probiotic bacteria support the corals: do they stay in the water, or are they being taken up into the coral tissues and acting as true endosymbionts?”

For this latest study, Cardoso and colleagues used a previously tested group of seven bacterial strains, using fluorescent probes to track the movement of the bacteria after inoculation into the coral Pocillopora damicornis. Their results reveal that two bacterial species, Halomonas sp. and Cobetia sp., were enriched within the coral. These two species were found in the epidermis (the outer tissue of the coral) and the gastrodermis — the lining of the coral’s gastrovascular cavity, where the algae also live. This suggests that these bacterial species live in direct symbiosis with the corals.

“These insights are hugely valuable for understanding how our probiotic tools work, while also improving our basic knowledge of coral symbiosis,” says Peixoto. “This study will support our ongoing trials of probiotics in the reefs off the coast here at KAUST.”

“That these bacteria are found in the gastric tissue means that they may also interact with the symbiotic algae that live there,” notes Cardoso. “Future experiments will provide more insights.”