by Anthony M. Bonacolta, Tristan Kravitz, Rocío Mozo, Lydia J. Baker, Rachael M. Heuer, Martin Grosell, Javier del Campo Marine fish play a significant yet understudied role in the oceanic carbon cycle through the production of magnesium-rich calcium carbonate (CaCO 3 ) precipitates known as ichthyocarbonates. These deposits form in the gut of marine teleost fish in response to salinity, serving as part of their osmoregulation strategy. Through this, marine fish may contribute as much as 9.04 Pg of CaCO 3 per year in global new carbonate production, being equivalent to or potentially higher than the production by coccolithophores and pelagic foraminifera. Despite their ecological relevance, the biological mechanisms driving ichthyocarbonate precipitation remain to be fully resolved. Intriguingly, bacteria are consistently found in intimate association with ichthyocarbonate precipitates. Given the widespread capacity of prokaryotes to mediate CaCO₃ precipitation, this association points