Ichthyocarbonates collected from the intestine differ in composition, morphology, and dissolution rate among regions and from post-excretion ichthyocarbonate

Increasing anthropogenic CO₂ emissions underscore the urgent need to advance our understanding of Earth's marine carbon cycle to more accurately predict future climate conditions. Climate change is driving rapid alterations in the marine carbonate system, notably resulting in declines in both pH and the saturation state of carbonate minerals like aragonite and calcite. Marine teleost fish, which are known to be part of the biological pump, also produce magnesium rich carbonate minerals (“ichthyocarbonate”) in their intestines which are excreted to the marine environment. Therefore, marine teleost fish serve as a critical link between the biological and carbonate pumps in the ocean. All~13,000 species of marine teleost fish species likely produce CaCO₃, and the group is estimated to be one of the largest CaCO₃ producers in the ocean. Even though fish are strong acid-base regulators, it has been shown that an increase in temperature and in partial pressure of CO₂ results in higher production rate of ichthyocarbonate. Because marine teleosts contribute to both the biological pump and the marine inorganic carbon cycle, understanding the physiological responses of fish to a changing ocean environment of ichthyocarbonate is pivotal to accurate predictions of ichthyocarbonate production in the face of climate change. Prior work suggests that ichthyocarbonates are unusual with regards to chemistry and morphology, both characteristics which affect their fate post excretion. The aim of this study was to understand how ichthyocarbonates change elementally and morphologically as they move through the 4 regions of the marine fish intestine – anterior, mid, posterior, and rectal segments and compare these results with complementary measurements conducted on excreted ichthyocarbonates.  We assessed the mol%MgCO₃ content, dissolution rate, organic matter content, and morphology of ichthyocarbonate from all 4 regions of the intestine and excreted ichthyocarbonates. We found that the mol%MgCO₃ is overall high, ranging from 74 to 56%, and decreases significantly as the ichthyocarbonate moves through the intestine, a trend opposite to previously documented Mg²⁺ concentrations in intestinal fluid. Also unexpectedly, the dissolution rate of ichthyocarbonate from the more distal regions was higher than anterior ichthyocarbonate, despite having lower mol%MgCO₃. Previous investigation of inorganic magnesium-rich carbonate minerals indicates that with increasing mol%MgCO_3, dissolution rate increases. However, mol%MgCO₃ is not an overarching control on dissolution rate of ichthyocarbonate, and increased organic matter content has been shown to strongly reduce dissolution rates, a plausible explanation for our observed relationship. To test this, organic matter content was assessed on ichthyocarbonate from all intestinal regions, and indeed, ichthyocarbonate collected from the rectal segments contained lower amounts of organic matter than ichthyocarbonate in more proximal segments, however it evaded significance. Finally, the morphology of ichthyocarbonate crystallites was heterogenous throughout all intestinal regions, but heterogeneity decreased as the ichthyocarbonate progressed through the regions of the intestine.