When Lauren Speare first saw the results, she knew she was onto something important. In a tank of sick corals in Summerland Key, Florida, the tiny predators she had added were doing exactly what she had hoped: hunting down a deadly disease and stopping it cold.
Speare, an assistant professor at Georgia Tech, has spent years studying a microscopic menace called Vibrio coralliilyticus — a pathogen that causes massive tissue loss and death in stony corals across the Caribbean. The infection spreads fast. In lab tests at Mote Marine Laboratory's Elizabeth Moore International Center for Coral Reef Research and Restoration, every untreated coral went white with bleaching within just 48 hours.
But the corals treated with Speare's tiny warriors told a different story. More than half showed no signs of bleaching beyond the spot where the disease had started. The infection stopped spreading. The corals lived.
The warriors in question are called Halobacteriovorax — predatory bacteria so small that Speare had to filter them through a mesh fine enough to catch only them. Unlike harsh chemical treatments, these bacteria work like a living probiotic, hunting and consuming the pathogens causing the infection without damaging the surrounding reef.
"Disease is a major driver of death for these corals, and with sea surface temperatures continuing to rise globally, we anticipate that rates of disease will only increase," Speare said. "Predatory bacteria function like a living probiotic, fighting coral disease without harsh side effects. This could be a targeted way to protect and treat our most vulnerable coral reefs."
The idea started with Speare's mentor, Rebecca Vega Thurber, a professor at the University of California, Santa Barbara, who discovered that these predatory bacteria naturally live inside some coral colonies. Speare hypothesized that they might be keeping coral ecosystems healthy by preying on harmful pathogens. The challenge was catching them: the bacteria are scarce, appearing only when infection strikes and disappearing shortly after.
"It's a bit like trying to catch a mountain lion in the act of hunting a deer," Speare said. "If we aren't looking at the right time, we might miss it entirely."
To test her theory, Speare spent six weeks at the Summerland Key facility, developing what she calls a "pathogen- sticker" — a small agar patch that mimics how infections spread in the wild. Once corals got sick, she poured the predatory bacteria directly into their tanks. The treatment was simple: just add water.
Because of that straightforward delivery method, Speare believes the approach could easily scale up to treat natural reefs, not just lab corals. Her next step is moving from aquariums to actual ocean environments — and trying to understand why the bacteria aren't already preventing more outbreaks in the wild.
"If this is such an effective way to control pathogens, why isn't this system preventing disease outbreaks on reefs already?" she asked. "We need to understand what's limiting this natural defense system — and that's what we're digging into now."
For Caribbean corals facing a perfect storm of warming waters and spreading disease, the answer could matter more than ever.