In this proposal we aim to investigate the 3D modulation of Mg in calcitic biomineralized tissues of the red algae. We have found via aberration corrected HRTEM that varying concentrations of Mg exists within these biominerals in an alternating manner. However EDS in TEM is not near as sensitive as XRF and cannot pick up small varientions. Moreover, TEM cannot reveal 3D modulations but mainly only 2D small slices. We would like for the first time to study these modulations in 3D. These modulations have implications in understanding the process of biomineralization (transformation of a Mg-rich amorphous precursor to a crystalline phase) as well as have strong affects on the mechanical properties. Such unprecedented 3D structural information due to nanotomography imaging coupled with Mg XRF will make it possible, for the first time, to localize the elemental distributions within the sample in 3D.