Probiotics has become a buzzword among nutritionists and wellness gurus. But did you know that coral reefs benefit from probiotics too?

These probiotics might not come in a brightly colored bottle with a selection of different flavors, but corals rely on a diverse supply of microorganisms just like the rest of us. Coral probiotics come in the form of fish poo, but not all fish poo is equal when it comes to providing beneficial bacteria.

“Corallivorous [coral eating] fish are generally regarded as harmful because they bite the corals, sometimes harming the coral in the process,” Carsten Grupstra, a postdoctoral researcher at Boston University who studies the role of microbes in coral reef systems, told Newsweek. “But they kind of get a bad reputation.”

Fish in coral reef
Fish feces can act as a “coral probiotic” in coral reefs.
Cartsen Grupstra

In a study with researchers from Rice University in Texas, Grupstra and his team discovered that the feces from these fish actually contain an array of different microorganisms with the potential to support coral health. Even more surprising, the feces from so-called grazer fish—which are thought to keep reefs healthy by removing algae and detritus from the coral—contain a microbial assemblage that was harmful to the corals.

“It really surprised us that feces from grazer/detritivores had such obvious negative effects on coral health, compared to corallivore feces,” Grupstra said.

For the study, the team placed fecal pellets from each fish on a coral fragment for up to 24 hours to monitor the effects. “By comparing the effects of fresh feces to sterilized feces controls [with no microbes], we were able to test precisely how microbial communities in fish feces—and not the physical properties of the fecal pellet—affect coral health,” Grupstra said.

Corals are filter feeders, so they can take up nutrients from fish feces as well as beneficial microbes. However, reef fish are highly abundant and some defecate up to four times an hour, meaning that the corals can easily become smothered. Diseases may also be spread in the feces.

Carsten Grupstra
Carsten Grupstra, a postdoctoral researcher at Boston University, collects fish.
Cartsen Grupstra

“Our motivation was to start disentangling physical and microbial effects that fish feces may have on corals by using feces from two fish feces who we expected to have very different fecal microbial communities,” Grupstra said.

“How exactly these feces may affect coral health—how often do they serve as nutrition, do they smother coral polyps and do they transfer microbiota—requires more research attention before we can make confident claims about how fish feces generally affect coral reef health,” he said.

Although there are still a lot of questions, there were clear differences between the different fish species. The microbes in fresh grazer fish feces produced significantly more damage than the coral-eating fish feces when compared to their respective controls. And the coral-eating fish feces contained roughly twice as many beneficial bacteria and fewer disease-causing microbes than the grazer feces.

This complements previous research by the team, which found that coral-eating bacteria also help with the dispersal of the photosynthetic algae that live inside most coral structures and provide them with food.

“We found millions of live [algae] cells per milliliter of feces, suggesting that these fish may make these cells available for uptake by corals, similarly to how pollinators disperse microbes between flowers and how herbivores disperse beneficial fungi between plants,” Grupstra said.

“Findings from this [new] study grant insight into how corals acquire beneficial microbes, and the findings may be used down the line to help coral reef conservation,” he said.

Coral proiotics
Pictured are microbial colonies grown from fish feces.
Cartsen Grupstra

Coral reefs are among the most biodiverse habitats on Earth. They provide a home for a quarter of all of the fish species on this planet and protect coastlines from storms and erosion. However, these important and delicate ecosystems are increasingly under threat.

“Coral reefs are threatened by climate change, pollution and overfishing,” Grupstra said. “Fish have important functions on coral reefs, yet very little is known about how fish feces impact coral health.”

While this latest study may help inform conservation projects in the future, a lot of research still needs to be done to understand how these results apply to the wider coral reef ecosystem.

“The lab conditions [used in this study] are not representative of typical situations on the reef, and most feces may not damage corals,” Grupstra said. “[In nature] feces may more frequently get broken up by waves or water current and/or roll off corals as soon as they land on them.”

Coral reef with fish
Coral reef ecosystems are much more complex than a controlled laboratory environment, so it is not clear how fish feces would affect corals in their natural environment.
Cartsen Grupstra

He continued: “Little crabs, shrimp or brittle stars that live between coral branches may also manipulate or feed on the fish feces, resulting in it being broken up and preventing lesion formation. More work is needed to test how common it is for fish feces to cause lesions versus how often corals benefit from feces.”

Both Grupstra and his co-author, Adrienne Simoes Correa from Rice University, said that this study’s results may be striking but do not indicate that grazer fish are bad for the coral reef ecosystem. Nor do they indicate that coral-eating fish are always beneficial.

“The impact of both grazer/detritivores and corallivorous fishes can be beneficial in some ways and detrimental in others, depending on the species involved, their life stages/sizes and other factors,” Correa told Newsweek.

“If we want to leverage coral-fish interactions to increase reef resilience, we will need to carefully test and monitor the results of the actions we take,” she said.