A new study from the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Sciences shows that microorganisms that live in the guts of fish may play an unexpectedly important role in reshaping ocean chemistry and the global carbon cycle. The research team found that these bacteria may cooperate with the host fish to produce calcium carbonate minerals, thus constituting an important carbon sink pathway in the ocean, challenging the traditional understanding that "this process is driven only by the fish's own physiology."
The study, led by Anthony Bonacolta, a former graduate student at the school, focused on how bacteria in the fish intestines and the host work together to produce calcium carbonate, a key mineral. Calcium carbonate is not only deeply involved in chemical processes such as ocean acid-base balance, but is also regarded as an important form of carbon storage in the marine environment and has a long-term impact on the global climate.
Normally, bony fish (i.e. ray-finned fish) will continue to drink seawater in order to maintain the osmotic pressure balance of body fluids. During this process, their intestines actively remove excess calcium and carbonate ions and excrete them in the form of solid calcium carbonate particles, called ichthyocarbonates. For a long time, the scientific community has generally believed that fish-derived carbonates are entirely driven by the physiological regulatory activities of fish themselves. This study proposes that the newly discovered involvement of intestinal microorganisms may constitute an important "hidden link" in this process.
Martin Grosell, one of the senior authors of the paper, the Maytag Professor of Ichthyology and chair of the Department of Marine Biology and Ecology at the school, pointed out that this work prompts the need to re-examine the role of fish gut microbiota in fish biology and even global ocean nutrient cycling. He said that the mineral formation process that used to be regarded as "purely completed by the fish itself" is now more likely to be the result of a close symbiosis between a fish and its intestinal microorganisms.
In order to explore the possible functions of microorganisms, the research team used Gulf toadfish (scientific name Opsanus beta) as the experimental object to systematically examine the production of fish-derived carbonates under different salinity conditions. The experiment exposed fish to three environments: brackish water with low salinity (9‰), normal seawater (35‰), and hypersaline water with high salinity (60‰). Previous studies have shown that as environmental salinity increases, fish will increase the excretion of fish-derived carbonates during normal osmoregulation.
The results of this experiment show that toadfish in a low-salinity environment produce almost no fish-derived carbonate, while obvious discharge of carbonate particles can be observed under normal seawater conditions, and this production increases further in a hypersaline environment. The researchers collected samples from a variety of locations, including different segments of the intestine, the fish-derived carbonate particles themselves, and the water surrounding the fish, to analyze the microbial communities and gene expression in them. The team identified the microbial species in the samples through genome sequencing, and combined with gene expression analysis, inferred the potential metabolic pathways and functional characteristics of these microorganisms.
Analytical results showed that Vibrio microorganisms, especially Photobacterium damselae subsp. damselae, were abundantly present in fish intestines and fish-derived carbonate particles. Gene functional analysis suggested that these bacteria have characteristics and metabolic pathways related to calcium carbonate precipitation, and may be directly involved in mineral formation, rather than just passively inhabiting the intestinal environment. Based on this, the research team concluded that fish and their intestinal microorganisms are likely to jointly drive the production of fish-derived carbonates through synergistic interactions.
Grossel emphasized that the vast majority of life forms on earth belong to microorganisms, which drive nutrient cycles, support ecosystem functions, and continue to reveal new aspects of life diversity in a symbiotic manner. Symbiotic phenomena are particularly abundant in the marine environment, and the potential symbiotic relationship between toadfish and Vibrio bacteria in the formation of calcium carbonate adds a new representative case to this picture.
The researchers note that this finding provides new insights into how marine ecosystems influence ocean chemistry and the ocean's carbon cycle. If follow-up research can further confirm this mechanism, it means that a large number of microorganisms living in fish may be involved in affecting the carbon storage process and overall ocean health on a larger scale, and their role is far beyond previous understanding.