Characterization of the protein matrix of cod otoliths
Otolith microchemistry can be very useful in identifying fish populations and reconstructing fish movements. Recent attempts have been made to evaluate otoliths as proxies of ambient transition metal levels, but findings have been inconsistent, varying by metal, species, and experimental method. Some of the difficulty with obtaining definitive answers stems from an incomplete understanding of the biological controls governing transition metal speciation in otoliths. Metals may be incorporated into the calcium carbonate crystal structure, trapped in crystalline interstitial spaces, or associated with the protein matrix. Metal binding to the protein phase may be inferred from its structural and biochemical properties but has not been observed previously. Inherent difficulties with the extraction of metal-binding proteins in their native state from the calcium carbonate phase make them extraordinarily difficult to measure.
A method has been developed that facilitates the extraction of otolith proteins without total disruption of transition metal-binding. Chelating agents such as EDTA, used in otolith decalcification, can demetallate the proteins if the demineralization is allowed to reach equilibrium; however, if the reaction is halted prior to equilibration, intact metal-protein complexes can be obtained. Using such an approach, we have established that between 70% and 100% of Cu and 40% to 60% of Zn found in whole otoliths are associated with the soluble portion of the protein matrix. Mn was not observed to be associated with the protein, indicating that it is either weakly bound or that no protein-bound Mn is present. Our results, combined with knowledge of the biological control of these metals, suggest that otoliths are unlikely to be reliable indicators of Cu and Zn exposure, but may provide a useful and unique insight into fish growth and physiological development.
We have also partially characterized the protein matrix of the otolith itself, and developed analytical methods which will facilitate further study into the nature of this complex system. We have observed multiple water-soluble proteins over a large mass range, many of which are glycosylated. This work will aid in furthering the understanding of otolith development, which is essential if trace metal signatures in otoliths are to serve as bioindicators.