Scientists have found evidence that tiny microorganisms living inside fish may help drive important processes affecting the world’s oceans.
The study, led by former University of Miami graduate student Anthony Bonacorta, suggests that gut bacteria and marine fish work together to produce calcium carbonate, a mineral that plays an important role in ocean chemistry and ocean carbon cycling. The findings point to a previously overlooked partnership that can influence how the ocean stores carbon and maintains its overall health.
Researchers have long believed that this mineral production is primarily controlled by the fish themselves. New research shows that microbes living in the fish gut may also be essential participants in this process.
Symbiosis between fish and microorganisms
Teleost fish, known as teleosts, constantly drink seawater to maintain proper hydration. When processing seawater, excess calcium and carbonate ions are removed from the body and released as solid calcium carbonate pellets called ichthiocarbonate.
“This study suggests that the gut microbiome may play a broader role in both fish biology and global ocean nutrient cycling,” said Martin Groesel, Maytag Professor of Ichthyology and Director of Marine Biology and Ecology, one of the study’s senior authors. “What was previously thought to be a process solely caused by fish may actually reflect a close symbiosis between fish and their gut microbial communities.”
Testing fish in different salinity conditions
To investigate this process, the researchers conducted laboratory experiments using Gulf of Mexico toads exposed to water of different salinity. Fish were reared in brackish water (9 ppt), normal seawater (35 ppt), and hypersaline water (60 ppt).
The scientists wanted to understand how salinity affects the production of ichthiocarbonate, which is known to increase as fish adapt to higher salinity conditions through normal osmoregulation.
The results showed a clear difference. Fish living in low-salinity waters did not produce ichthiocarbonate. Fish kept in seawater produced them, and production increased even more in high-salinity environments.
Evidence from the fish gut microbiome
The research team collected samples from multiple regions of the fish’s intestines, the ichthiocarbonate itself, and the surrounding water.
DNA and RNA analysis allowed scientists to examine both the microbial communities living within the fish and the patterns of gene activity in the fish and associated microorganisms. Gene sequencing was used to identify the microorganisms present, while gene expression studies helped uncover potential biological functions related to calcium carbonate production.
The researchers found that Vibrio bacteria, especially Photobacterium damselae subsp.damselaewas highly abundant in both the intestinal tract and ichthiocarbonate. Genetic evidence suggests that these bacteria have abilities associated with ichthiocarbonate production, indicating that these bacteria, together with their fish hosts, may directly contribute to mineral formation.
Impact on ocean health and carbon cycle
The discovery highlighted how microorganisms can influence large-scale environmental processes.
“Most life on Earth is microbial, driving nutrient cycling and ecosystem function, while uncovering new aspects of biological diversity through symbiosis,” Grossell said. “The ocean is particularly blessed with such partnerships, and the toad-Vibrio symbiosis, potentially linked to calcium carbonate production, is a surprising new example.”
The discovery provides new insights into the relationship between marine animals, their microbiomes, and global processes that help regulate ocean chemistry and carbon storage.
This research was supported by start-up funds from the University of Miami and project PID2023-152522NB-I00 funded by the Spanish Ministry of Science, Innovation and Universities.
