The Heikal Barzakh Objection: Deriving Protein-Mediated Biomineralization as a Universal Marine Boundary Process

The freshwater–saltwater interface is a natural phenomenon that remains not well understood. So far, explanations are mostly based on macroscopic physical processes, such as density gradient, difference in salinity, hydrodynamic mixing. Herein, HBT is introduced, which enables the formation and maintenance of natural aquatic boundaries on the basis of a biochemical process, biomineralization. The suggested process includes the identification of conchiolin, a structural organic matrix protein, as the main arbitrator of this event. Conchiolin serves like a molecular net to trap dissolved calcium and carbonate ions, it accumulates them in its 3D organic matrix where they undergo controlled nucleation and crystallization. Therefore, calcium carbonate undergoes precipitation in the form of highly oriented aragonite nanoneedles and not as randomly agglomerated mineral particulates. The build-up of these nanostructures generates biomineralized selective boundary layers, which minimiz e random ion diffusion and even help to sustain the physicochemical dissimilarity of fresh and salt water environments. This process has been substantiated by the recognized involvement of conchiolin in the ordered biomineralization of aragonite in pearl, molluskan shell and coral skeleton formation. When this process of biomineralization is generalized to include the formation of aquatic boundaries a biochemical underpinning is provided that complements the traditional physical model of freshwater–saltwater separation. The theory presented herein may provide insight into other types of natural boundary formation and find usefulness in biomimetic membrane design, selective filtration, environmental engineering, radiation protection, advanced functional materials, and tissue engineering.