Uncovering the role of biomineralisation in otolith trace element uptake by microstructural and microchemical analysis
Otolith biomineralisation was examined in order to uncover its role in trace element uptake within otolith structures by microstructural and chemical analysis. The mechanism of otolith elemental incorporation, the orientation of aragonite and the organic matrix in otoliths of four marine species were investigated in conjunction with the otolith growth patterns and the distribution of trace and minor elemental composition. Results for otoliths of Platycephalus bassensis, Chrysophrys auratus, Trachurus novaezelandiae and Lates calcarifer suggest a common biomineralisation mechanism for marine species, with consistent results across all species. The aragonite orientation, as mapped by electron backscatter diffraction (EBSD), revealed the aragonite c-axis was preferentially aligned parallel with the otolith growth axis and perpendicular to the insoluble organic matrices orientation, as determined by Raman microspectroscopy, in the otoliths of Platycephalus bassensis, Chrysophrys auratus, Trachurus novaezelandiae and Lates calcarifer. The orientation results indicate that otolith mineral growth is directed and controlled by the organic matrix, which acts as a template for the mineral growth, constraining the aragonite orientation and alignment. The organic rich zones (D-zone bands) within the otolith were composed of both insoluble and soluble organic matrices in all four species, with the insoluble matrices bridging the gap between mineral rich zones (L- zone bands) so as to retain crystal orientation across the otolith. Optimisation of electron probe microanalysis (EPMA) quantitative mapping achieved both high spatial resolution (<3 μm) and two dimensional visualisation of the fine scale Sr and S distributions in the otoliths with minimal otolith damage. The otolith growth patterns showed a clear association with the distribution of Sr and S, with D-zone bands exhibiting elevated concentrations of both elements as compared to the L-zone bands. Further examination by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) showed that incorporation of Mg and Ba appears independent of both the S distribution and the growth patterns in all four species. The results suggest that element incorporation into the otolith is linked to the organic composition in the endolymph during mineralisation, and the organic matrices may in part assist the uptake of Sr. The study revealed that whilst the otolith biomineralisation is directed and templated, with aragonite orientation constrained by the insoluble organic matrices, this process has no direct influence over trace element uptake. The association between the distribution of Sr, S and the organic matrices within the otoliths suggests however that the soluble organic matrices may play a role in trace element incorporation, particularly Sr incorporation. These findings may therefore have significant implications for the interpretation of otolith Sr chemistry.
Otoliths, biomineralisation, biochemistry, trace element analysis
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