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ABSTRACT Marine fish are likely one of the top producers of biogenic carbonate in the oceans. However, nothing is known about the production rate and composition of intestinal carbonate (ichthyocarbonate) excreted by mesopelagic fishes, which are small, fragile and account for up to 94% of global fish biomass. To address this knowledge gap, and associated uncertainty of global ichthyocarbonate production, we identified a model species residing at 350–430 m, depths relevant for mesopelagic fishes. The blackbelly rosefish (Helicolenus dactylopterus) lacks swim bladders and survives capture and transfer to the lab. Freshly collected blackbelly rosefish, maintained at 6°C, contained high amounts of intestinal ichthyocarbonate (0.4 g kg−1) and excreted ∼5 mg kg−1 h−1 ichthyocarbonate, in agreement with expectations based on allometric and thermal relationships for other species. Despite longer intestinal residence time, intestinal and excreted ichthyocarbonates are similar in crystallite morphology, composition and sinking rate, but have a higher dissolution rate than that produced by shallow water species at higher temperatures, ruling out strong effects of pressure and low temperatures on ichthyocarbonate formation and excretion. Considering allometric and thermal relationships, the metabolic rate of blackbelly rosefish is lower than that of other marine fish in general, and mesopelagic fishes in particular. Our observations support assumptions of ichthyocarbonate excretion by mesopelagic fishes, and suggest that thermal and allometric relationships for ichthyocarbonate excretion determined from shallow water species extend to fish populations at depth. 

Abstract Marine carbonate production and dissolution are important components of the global carbon cycle and the marine alkalinity budget. Global carbonate production by marine fish (ichthyocarbonate) has been estimated to be as high as 9.03 Pg CaCO₃ yr⁻¹; however, the fate of ichthyocarbonate is poorly understood. High magnesium concentrations in ichthyocarbonate would traditionally suggest rapid dissolution under current marine conditions, but a correlation between dissolution rate and mol%MgCO₃has not been observed. Here, we aim to determine the role of organic coatings on dissolution rates of ichthyocarbonate in marine environments. We applied a combination of petrographic, geochemical, and microCT approaches to assess the quantity and distribution of organic matter in ichthyocarbonate produced by two species of marine fish, the Gulf toadfish (Opsanus beta) and the Olive flounder (Paralichthys olivaceus). We show that organic matter, including external coatings and embedded organic material, is volumetrically significant, ranging from 8.5% to 32.3% of ichthyocarbonate by volume. Bleach oxidation of external organic matter coatings increased the dissolution rate of ichthyocarbonate by more than an order of magnitude, suggesting these coatings serve to reduce reactive surface area of the mineral fraction in ichthyocarbonate. Assuming that organic coatings do not influence sinking rates, external coatings extend the depth of ichthyocarbonate persistence in the water column by ∼12–15×. Therefore, organic coatings are an important determinant of the role of ichthyocarbonate in the marine carbon cycle.