Direct deposition of crystalline aragonite in the controlled biomineralization of the calcareous tubeworm

Abstract

Although space delineation is a well-accepted requirement for biologically controlled biomineralization, the actual location of the mineralizing compartment within marine invertebrates has only recently been determined. We observed that the biomineralization was compartmented within the collar region of the metamorphosing larvae of Hydrodies elegans at its earliest possible time, i.e., at the post-metamorphic stage. We have also found that these highly regulated compartments contained aragonite crystals, as detected by EBSD and confirmed by electron diffraction TEM. Within these compartments, the metamorphosed larvae maintained a pH 9, at the pKa for CaCO3 formation. This model describes how biomineralization is a space delineation event in which calcium carbonate formation is an intracellular phenomenon.

Significance statement: Using the tubeworm, a popular invertebrate model for marine research, we propose a unifying mechanism of calcification that emphasizes the importance of specialized mineralizing compartments in providing delineated space for controlled biomineralization within a multicellular organism. Through the use of correlative microscopy, we have demonstrated the functional observation of organisms under fluorescent microscopy can be directly followed by performing structural analysis using the extreme edge of electron microscopy, specifically SEM-EBSD and FIB-TEM. Unlike conventional fluorescent observations that require the use of fixation artifacts that are inevitable in electron microscopy, this novel method of understanding both the function and structure of any calcifying organism avoids such artifacts, while still providing a higher structural resolution.