A new study Led by the Leibniz Center for Tropical Marine Research (ZMT) has shown how fish are vitally important to the inorganic carbon cycle of the oceans. More often thought of in terms of their place in the food chain, fish also act as “carbon regulators” storing carbon within their bodies as they grow and eventually exporting it to deep seas through their sinking feces and carcasses. They also make carbonates from marine salts within their guts and excrete them at high rates.
Key to this is the fact that marine fish live in a salty environment that has a higher osmotic pressure than their blood. As a result, their bodies are deprived of water. “To hydrate, the fish drink seawater, but they have to get rid of the large amounts of calcium and magnesium contained in it,” explains Mattia Ghilardi, a marine biologist at the ZMT and lead author of the study. “These are precipitated in the gut in the form of carbonate crystals and released into the sea within the feces.”
As part of the study, the team examined a wide variety of fish species to determine how characteristics and environmental changes control the amount and composition of excreted carbonates. The team retrieved carbonates produced by fish from Palau, Australia and the Bahamas while keeping them within aquaria in conditions natural to their home reef. They then compiled an extensive database of carbonate excretion rates from more than 380 coral reef fishes from 85 species, including damselfishes, surgeonfishes, parrotfishes, wrasses, snappers and groupers. They confirmed that fish carbonate excretion is proportional to their metabolism. High temperatures, for example, boost metabolism and carbonate excretion. Fish with forked or lance-shaped tails are more active and thus excrete more carbonates than fish with rounded tails. Interestingly, the length of the gut also plays a role: like temperature, it affects the amount and mineralogical composition of the excreted crystals.
The new findings will enable researchers to more reliably calculate the amount and composition of carbonates produced by fish communities in different marine regions and worldwide. “With our study, we pave the way for more accurate predictions about the role of fish in the marine carbon cycle under changing conditions such as climate warming and overfishing,” says co-author Sonia Bejarano, reef ecologist at the ZMT. “Such predictions are very important for management and decision-making processes aimed at maintaining ecosystem functions.”
More information: Mattia Ghilardi et al, Temperature, species identity and morphological traits predict carbonate excretion and mineralogy in tropical reef fishes, Nature Communications (2023). DOI: 10.1038/s41467-023-36617-7
Journal information: Nature Communications