Tracking the growing rings in biogenic aragonite from fish otolith using confocal Raman microspectroscopy and imaging

Abstract

Fish otoliths pose increasing interest due to their potential of rendering information about environmental changes, underlined in their non-linear time-dependent biogenic crystal growth. Otoliths are acellular, metabolically inert and continuously grow in a complex process which still needs to be understood. In the present work confocal Raman microspectroscopy (CRM) and imaging is employed to investigate the growth pattern in otoliths from Sparus aurata of Mediterranean provenance. CRM signal acquired from otolith sagittal section is exploited to associate it with the periodic growth increments denoted as rings. Raman signal collected from the core to the margins with micrometer spatial resolution invariably revealed characteristic signal of aragonite. Relative intensity variability was observed particularly for the lattice modes, indicating changes in crystalline orientation relative to incident laser. Bands associated with organic fraction were absent in the 90-1840 cm-1 spectral range. Daily growth rings were further studied using the Raman mapping of main aragonite bands intensities at 1083, 704 and the lattice modes in the 100-350 cm-1 range. The spectral intensity pattern closely follows the daily growth pattern. Traces of toxic or heavy metals incorporated in biogenic carbonate mineral were spuriously detected in the mapped areas, according to the position and width of the Raman bands of witherite (BaCO3), strontianite (SrCO3), along with the main aragonite and trace of its calcite polymorph.