Effects of H+ concentration on amorphous mineral granules: Structural stability and elemental mobilization

Phosphate-rich amorphous mineral granules (AMG) have been studied in a number of organisms, and show different physical and chemical properties according to their organic and mineral composition. We studied AMG isolated from the hepatopancreas of the land crab Ucides cordatus, which were subjected to different pHs in order to mimic the possible effects of H+ on these structures. We used scanning and transmission electron microscopy (SEM, TEM), energy-filtering transmission electron microscopy (EFTEM), energy-dispersive X-ray analysis (EDXA) and nuclear magnetic resonance (NMR). TEM showed that granules were structurally disrupted when subjected to pH 5. The granules contain a soluble fraction that is rich in orthophosphate, which was the most abundant form of phosphate, although pyrophosphate and glucose-6-phosphate were also detected by 31P NMR analysis. The redistribution of elements in the structure and pH conditions is discussed, focusing on their possible implications for AMG structure, function and dynamics.