Fractionation of the Gulf toadfish intestinal precipitate organic matrix reveals potential functions of individual proteins

Implications of dietary carbon incorporation in fish carbonates for the global carbon cycle

Marine bony fish are important participants in Earth's carbon cycle through their contributions to the biological pump and the marine inorganic carbon cycle. However, uncertainties in the composition and magnitude of fish contributions preclude their integration into fully coupled carbon-climate models. Here, we consider recent upwards revisions to global fish biomass estimates (2.7-9.5×) and provide new stable carbon isotope measurements that show marine fish are prodigious producers of carbonate with unique composition. Assuming the median increase (4.17×) in fish biomass estimates is linearly reflected in fish carbonate (ichthyocarbonate) production rate, marine fish are estimated to produce between 1.43 and 3.99 Pg CaCO3 yr-1, but potentially as much as 9.03 Pg CaCO3 yr-1. Thus, marine fish carbonate production is equivalent to or potentially higher than contributions by coccolithophores or pelagic foraminifera. New stable carbon isotope analyses indicate that a significant proportion of ichthyocarbonate is derived from dietary carbon, rather than seawater dissolved inorganic carbon. Using a statistical mixing model to derive source contributions, we estimate ichthyocarbonate contains up to 81 % dietary carbon, with average compositions of 28-56 %, standing in contrast to contents <10 % in other biogenic carbonate minerals. Results also indicate ichthyocarbonate contains 5.5-40.4 % total organic carbon. When scaled to the median revised global production of ichthyocarbonate, an additional 0.08 to 1.61 Pg C yr-1 can potentially be added to estimates of fish contributions to the biological pump, significantly increasing marine fish contributions to total surface carbon export. Our integration of geochemical and physiological analyses identifies an overlooked link between carbonate production and the biological pump. Since ichthyocarbonate production is anticipated to increase with climate change scenarios, due to ocean warming and acidification, these results emphasize the importance of quantitative understanding of the multifaceted role of marine fish in the global carbon cycle.

The role of marine fish-produced carbonates in the oceanic carbon cycle is determined by size, specific gravity, and dissolution rate

Rising CO2 emissions have heightened the necessity for increased understanding of Earth's carbon cycle to predict future climates. The involvement of marine planktonic species in the global carbon cycle has been extensively studied, but contributions by marine fish remain poorly characterized. Marine teleost fishes produce carbonate minerals ('ichthyocarbonates') within the lumen of their intestines which are excreted at significant rates on a global scale. However, we have limited understanding of the fate of excreted ichthyocarbonate. We analyzed ichthyocarbonate produced by three different marine teleosts for mol%MgCO3 content, size, specific gravity, and dissolution rate to gain a better understanding of ichthyocarbonate fate. Based on the species examined here, we report that 75 % of ichthyocarbonates are ≤0.91 mm in diameter. Analyses indicate high Mg2+ content across species (22.3 to 32.3 % mol%MgCO3), consistent with previous findings. Furthermore, ichthyocarbonate specific gravity ranged from 1.23 to 1.33 g/cm3, and ichthyocarbonate dissolution rates varied among species as a function of aragonite saturation state. Ichthyocarbonate sinking rates and dissolution depth were estimated for the Atlantic, Pacific, and Indian ocean basins for the three species examined. In the North Atlantic, for example, ~33 % of examined ichthyocarbonates are expected to reach depths exceeding 200 m prior to complete dissolution. The remaining ~66 % of ichthyocarbonate is estimated to dissolve and contribute to shallow water alkalinity budgets. Considering fish biomass and ichthyocarbonate production rates, our results support that marine fishes are critical to the global carbon cycle, contributing to oceanic alkalinity budgets and thereby influencing the ability of the oceans to neutralize atmospheric CO2.

Comparison of the organic matrix found in intestinal CaCO3 precipitates produced by several marine teleost species

Marine bony fish poses the unique ability to hydrate from imbibed seawater. They accomplish this, in part, by the precipitation of inorganic carbonate mineral in their intestine, which lowers luminal osmotic pressure and allows for water uptake. It has recently been described that in the Gulf toadfish (Opsanus beta) this Ca(Mg)CO₃ precipitation occurs under the regulation of an organic matrix. To date no investigations have aimed to determine if this phenomenon applies more generally to marine fish. Here, intestinally derived precipitates were collected from gray snapper (Lutjanus griseus), white grunt (Haemulon plumieri), European flounder (Platichthys flesus), as well as Gulf toadfish, and their matrices were extracted. The ability of these matrices to regulate CaCO₃ production was determined using an in vitro calcification assay, which revealed that the matrix derived from each of the tested species increased precipitation at low concentrations, while inhibiting it at higher concentrations in full agreement with the earlier studies on toadfish. Matrix extracted from European flounder precipitates was then analyzed by mass spectrometry, leading to the identification of over 50 unique proteins. When the identities of these proteins were compared to previous investigation of toadfish precipitate matrix, nearly 35% were found to overlap between the flounder and toadfish analyses, suggesting conserved mechanisms of precipitation control. The effects of using different sodium hypochlorite (NaOCl) solutions during precipitate purification on the resulting organic matrix are also discussed.