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Bacterial compound shows promise for coral disease protection, restoration efforts
By Michelle Z. Donahue
New research in Florida seeks to learn more about how young corals react to the presence of a protective bacterial probiotic previously only tested on adult corals, and whether the probiotic may serve a greater role in coral restoration efforts.
The probiotic, a beneficial and protective strain of bacteria known as MCH1-7, was found by Smithsonian scientists in 2018. Researchers collected and isolated the substance from a wild coral colony that showed resistance to stony coral tissue loss disease (SCTLD). MCH1-7 has since been found to produce a variety of marine antibiotics, including korormicin and marinocine. Currently being tested as a treatment for wild corals in southern Florida and the Keys, the hope is that the probiotic could eventually be used to pre-treat healthy colonies to protect them from infection.
MCH1-7 also produces another compound that has been under investigation at SMS, albeit from a totally different source. Tetrabromopyrrole, or TPB, stimulates coral larvae to metamorphose and affix themselves permanently to the sites where they will each grow into a multi-polyp coral colony. Until recently, SMS biologist Dr. Jennifer Sneed has primarily been looking at TBP produced by Pseudoalteromonas bacteria living in biofilms on crustose coralline algae (CCA) that form on hard surfaces of the seafloor.
Intrigued by MCH1-7’s ability to produce TBP, Sneed and postdoctoral researcher Dr. Alyssa Demko set about to investigate whether the probiotic may have as-yet-undiscovered benefits for juvenile corals. And because MCH1-7 produces both defensive antibiotics and a powerful coral settlement cue, it could be a useful one-two punch in coral disease work as well in conservation and restoration.
“If TBP is a natural settlement cue, and if bacteria that also produce this compound protect corals from disease, it makes sense that larvae would settle where those compounds are being produced,” Sneed said. “More of them would survive to be able to recognize the compound.”
Sneed and Demko’s investigations are looking whether MCH1-7 may also be protective of juvenile corals, including larvae of grooved brain coral larvae (Diploria labyrinthiformis) and great star coral (Montastraea cavernosa). If young larvae do benefit from pre-treatment with probiotics, then prophylactic treatment in lab-based restoration operations could give them a defense boost before being attached to permanent sites on wild reefs.
“We don’t know much about whether newly settled larvae are susceptible to the disease,” Sneed said. “So part of these investigations were also to look at if larvae and recruits can become infected.”
The summer’s trials also boosted evidence for TBP’s general usefulness as a settlement cue for every species that encounters the compound.
TPB prompted settlement in all eight species of coral SMS received for testing, including pillar coral (Dendrogyra cylindrus), grooved brain coral (Diploria labyrinthiformis), great star coral (Montastraea cavernosa), mountainous star coral (Orbicella faveolata), and boulder brain coral (Colpophyllia natans). Larvae were contributed by the Florida Aquarium, Nova Southeastern University, the National Oceanic and Atmospheric Administration (NOAA) and SECORE International.
“TBP is very broadly active, and induces brooders as well as spawners from all different genera and families,” Sneed says. “It consistently induces settlement with everything we’ve tested – it’s very cool.”
Sneed is optimistic that TBP could prove to be a powerful tool for coral restoration efforts. SMS has been working with SECORE to streamline their methodology for creating field-ready coral settlement tiles. Coral larvae settle down and metamorphose into polyps on these small ceramic structures. Once established, the young coral colonies can be more easily transported to other lab or field sites for research and reef rebuilding projects.
Currently, SECORE researchers must put the tiles in special pens in the ocean for four to eight weeks to allow CCA (and their TBP-laden biofilms) to grow on them naturally. But before they can be introduced to larvae, the tiles must be cleaned carefully by hand to remove other accumulated debris and obstructions, a painfully slow and labor-intensive process.
If the slow-growing CCA could be eliminated from the equation by treating tiles directly with TBP, the overall process could be sped up considerably, Sneed said.
As usual when something seems too good to be true, TBP does have a significant limitation: it is an unstable molecule and breaks down relatively quickly in both lab and storage conditions. This limits its usefulness for transporting it to remote locations for work with corals in the field.
In an effort to uncover a more stable molecule that could also easily induce coral settlement, Sneed and SMS head scientist Dr. Valerie Paul collaborated with Georgia Tech chemist Dr. Vinayak Agarwal, who created nearly a dozen chemically similar molecules. But it quickly became clear that coral larvae could only be tempted to settle down in the presence of the genuine article.
Another wrinkle is that in the lab, in the presence of TBP, larvae refuse to settle on the ceramic tiles. They seem to prefer the plastic sides and bottom of the wells Sneed uses to run the experiments.
Demko and Sneed also tried mixing TBP into sodium alginate hydrogels and smearing that substance directly on the tiles. Unfortunately, the larvae still settled on the plastic wells instead of the tiles. So the scientists took a different tack: mixing the larvae themselves into the hydrogels.
Incredibly, this didn’t kill the larvae. Even more incredibly, many managed to wriggle free of the gel—only to settle out onto the sides of their plastic enclosures.
Sneed suspects TBP may be sticking to the plastic, or that the dishes themselves may be attractive to corals for some as-yet-unknown reason. She plans on future rounds of experiments in settlement chambers made of different materials.
Though much work remains to investigate TBP and how to use it most effectively, Sneed is hopeful that it proves to be a beneficial tool for lab study of young corals.
“If we can use the compound for restoration, that’s huge,” Sneed says. “But even without that, TBP is useful to get young corals to settle out, which we can use to monitor and study their growth and other aspects of their development.”