Biogenic fish-gut calcium carbonate is a stable amorphous phase in the gilt-head seabream, Sparus aurata

Abstract

The main source of calcium carbonate (CaCO₃) in the ocean comes from the shells of calcifying planktonic organisms, but substantial amounts of CaCO₃ are also produced in fish intestines. The precipitation of CaCO₃ assists fish in intestinal water absorption and aids in whole body Ca²⁺ homeostasis. Here we report that the product formed in the intestinal lumen of the gilt-head seabream, Sparus aurata, is an amorphous calcium carbonate (ACC) phase. With FTIR spectroscopy and SEM imaging, our study shows that the fish-derived carbonates from S. aurata are maintained as a stable amorphous phase throughout the intestinal tract. Moreover, intestinal deposits contained up to 54 mol% Mg²⁺, the highest concentration yet reported in biogenic ACC. Mg is most likely responsible for stabilizing this inherently unstable mineral. The fish carbonates also displayed initial rapid dissolution when exposed to seawater, exhibiting a significant increase in carbonate concentration.

Figure 1. Infrared spectra of (a) mineral deposit from the hind-gut, and (b) an amorphous calcium carbonate standard.

The large water absorption peak around 3400 cm⁻¹, the broad partially split peak around 1440 cm⁻¹, the peak at 863 cm⁻¹ and the absence of a peak at 712 cm⁻¹ all indicate that the mineral phase in the fish hind-gut is ACC. Note that the slight shits in peak maxima and the extent of splitting of the main peak around 1440 cm⁻¹ show that the standard is probably more disordered than the fish ACC. The additional peaks in both spectra are due to additives that are probably involved in stabilizing the ACC or associated organic matter.

Figure 2. Infrared spectra of (a) the foregut, (b) the midgut and (c) the hindgut mineral deposits from one fish, and (d) the hindgut mineral deposits from another fish.

The peaks characteristic of ACC are present in (a), (b) and (c). In (d) the 860 cm⁻¹ peak is split with a sharper peak at 873 cm⁻¹ indicative of the presence of some crystalline calcite in addition to the ACC.

Figure 3. SEM image of ACC aggregates from the intestinal tract of S. aurata.

Figure 4. Alkalinity results after a 1-hour incubation in ambient (pH 8.1) seawater.