HMN 2026: How Fish-microbe partnership may influence ocean health by making carbon-trapping minerals

New research reveals a potential link between the gut microbes of a fish and global ocean processes, offering new insight into how marine ecosystems help regulate ocean chemistry and the marine carbon cycle. The study, titled “Symbiotic bacteria may support calcium carbonate precipitation in the Gulf toadfish,” is published in the journal PLOS Biology.

Microbial partners in fish calcium cycles

The study, led by former graduate student Anthony Bonacolta in the Department of Marine Biology and Ecology at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, found that symbiotic gut microbes may work in tandem with marine fish to produce a form of calcium carbonate that influences overall ocean health and serves as a key carbon sink. This process, long attributed primarily to fish physiology, may in fact depend on a previously unrecognized microbial partnership.

Bony fish, called teleosts, drink seawater to stay hydrated. Inside their intestines, they process excess calcium and carbonate ions and excrete them as solid pellets of calcium carbonate called ichthyocarbonates.

“This work suggests that the gut microbiome may play a broader role in both fish biology and global marine nutrient cycles,” said one of the study’s senior authors, Martin Grosell, Maytag Professor of Ichthyology and chair of the Department of Marine Biology and Ecology. “What was previously thought to be a process driven solely by the fish may actually reflect a close symbiosis between the fish and its gut microbial community.”

How the lab experiment worked

To conduct the lab experiment, the researchers exposed Gulf toadfish to different salinity levels—brackish (9 ppt), seawater (35 ppt), and hypersaline (60 ppt)—to test how changes in salinity affect ichthyocarbonate formation, which is known to increase as part of the fish’s normal osmoregulation process. Fish kept in low salinity did not produce ichthyocarbonates, while those in seawater and more so in high salinity did.

Samples were collected from different sections of the intestine, from the ichthyocarbonates themselves, and from surrounding water. DNA and RNA were extracted to study both the gut microbiome and gene expression in fish and associated microbes. Microbial communities were characterized using genetic sequencing, and gene expression analyses were used to identify potential roles in carbonate formation.

The surprising role of vibrios

They found that vibrios, particularly Photobacterium damselae subsp. damselae—were highly abundant in both the gut and associated ichthyocarbonates. These bacteria showed genetic potential for processes linked to ichthyocarbonate production, suggesting they may contribute to mineral formation alongside the fish host.

“Most life on Earth is microbial, driving nutrient cycles and ecosystem function while revealing new dimensions of biological diversity through symbiosis,” said Grosell. “The ocean is especially rich in these partnerships, and the toadfish–vibrio symbiosis potentially linked to calcium carbonate production is a striking new example.”

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