Coral larvae are picky about where they attach and settle down. One of the ways they decide is by "smelling" chemicals in the water that are associated with healthy reefs .
Now, researchers at UC San Diego's Scripps Institution of Oceanography and Jacobs School of Engineering have developed a gel using nano-particles that slowly release some of coral larvae's favorite "smells." When the researchers applied the gel, called SNAP-X, to surfaces in lab experiments it increased coral larvae settlement by up to 20 times compared to untreated surfaces. SNAP-X could help overcome a major bottleneck in reef restoration efforts at a time when climate change is challenging the health of coral habitats . The gel is applied to surfaces as a coating and releases the coral-attracting chemicals for up to a month.
The research, published today in Trends in Biotechnology, was conducted with funding from the Defense Advanced Research Projects Agency's Reefense program, which aimed to develop self-healing, hybrid biological and engineered reef-mimicking structures for coastal protection.
"Coral are animals, and their larvae are selective about where they are going to attach because once they do, they're stuck there," said Daniel Wangpraseurt , the study's senior author and a marine biologist at Scripps with a previous appointment in UC San Diego's Department of Chemical and Nano Engineering. "With SNAP-X, we created a material that releases chemical cues that tell coral larvae this is a good place to live."
Coral reefs are severely threatened by ocean warming caused by climate change. They are projected to decline by 70-90% at 1.5 degrees Celsius (2.7 degrees Fahrenheit) of warming relative to preindustrial times, and by 99% at 2°C (3.6°F). The decline of coral reefs, with their kaleidoscopic beauty and estimated $375 billion in economic value, is something Wangpraseurt's Coral Reef Ecophysiology and Engineering Lab at Scripps is working tirelessly to prevent.
"I'm over hearing that corals are dying — I'm more interested in what we can do about it," said Wangpraseurt. "My lab's approach is to combine marine biology with physics and bioengineering to come up with new solutions."
A significant hurdle for scientists working on coral reef restoration is getting coral larvae to settle on degraded reefs or to attach to human-created structures that might not "smell" like home to the larvae. For coral to reproduce , adults spawn gametes that form planktonic larvae that drift in the currents and settle on a suitable substrate if environmental conditions are favorable.
One of the major bottlenecks in coral reef restoration is ensuring that reefs become self-sustaining and can reproduce naturally. Degraded reefs often lack suitable settlement substrates and instead of providing the chemical signals that encourage coral larvae to settle and grow, these damaged environments frequently emit deterrent cues that inhibit coral recruitment.
Scientists have long known that certain types of crusty algae, known as crustose coralline algae , release chemicals that encourage baby corals to attach to surfaces. However, translating this knowledge into practical solutions to boost coral settlement had remained elusive.
Wangpraseurt and his lab wanted to develop a substance that could deliver these chemical cues over an extended period of time in the ocean to accelerate reef recovery efforts.
"If you just throw these chemical cues in the ocean they dissipate very quickly, making it hard for coral larvae to find their source," said Samapti Kundu, a postdoctoral researcher at Scripps who worked on the project during her time at UC San Diego's Department of Chemical and Nano Engineering, and the study's first author. "We needed to develop something like an extended release drug delivery system that would slowly release these settlement cues in the ocean."
The team solved this problem by encapsulating chemical compounds extracted from crustose coralline algae in nanoparticles made of silica, the main mineral in sand. The researchers then suspended those nanoparticles in a liquid gel that would solidify like Jell-O when exposed to ultraviolet (UV) light. This combination meant the team could paint or spray the substance onto a surface and then cure it with UV light to make it stay put.
The resulting biomaterial, SNAP-X, releases the chemicals that encourage coral larvae settlement for up to one month — long enough to give coral restoration practitioners an excellent chance of timing its application with a coral spawning event.
In laboratory tests, SNAP-X increased coral settlement by up to six-fold compared to an untreated surface. In additional experiments that featured water flow that better simulated reef environments, larval settlement increased by up to 20 times.
"I think this material is a breakthrough that can hopefully make a big contribution to coral restoration," said Wangpraseurt. "Biomedical scientists have spent a lot of time developing nanomaterials as drug carriers, and here we were able to apply some of that knowledge to marine restoration. This paper highlights that if you bring together ideas from different scientific fields, you can create innovative solutions to tough problems like restoring coral reefs."
Notably, the experiments in the study were all conducted using one species of coral — the Hawaiian stony coral species Montipora capitata. More experiments are needed to show that SNAP-X can work with other corals from other regions. However, Wangpraseurt suggested that their material could be adapted to other species or areas by loading SNAP-X with coral settlement-promoting chemicals collected from suitable crustose coralline algae that are locally present.
Kundu and Wangpraseurt are also working to scale up such biomaterial solutions with a startup company called Hybrid Reef Solutions .
"We want these materials to be used and have a big impact. To me, this means we can't spend all our time on research — the business side needs development as well," said Wangpraseurt. "We have had tremendous support from Scripps Oceanography and the UC San Diego Office of Innovation and Commercialization . We are really excited to take this as far as it can go."
In addition to Kundu and Wangpraseurt, the study was co-authored by Linda Wegley Kelly of Scripps Oceanography, Natalie Levy of Scripps Oceanography and UC San Diego, Justin Chen, Timothy Noritake, Zahra Karimi, Shaochen Chen of UC San Diego, Zachary Quinlan of Scripps Oceanography and University of Hawai'i, Hendrikje Jorissen, Joshua Hancock, Crawford Drury of the University of Hawai'i, Simone Potenti of the University of Milan, Helena Willard of the University of Amsterdam, Luisa De Cola of the Mario Negri Institute for Pharmacological Research and the University of Milan, and consortium authors with the Rapid Resilient Reefs for Coastal Defense.
