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Caption : figure s1. the s. apsteinii lopadolith studied by tomography, visualised by integrated differential phase contrast imaging.4, 12 a) the tip of the steel insect pin; b) epoxy resin used as a fixative; c) a thicker section near the rim of the lopadolith. scale in mrad.

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Caption : figure s2. powder x-ray diffraction patterns from synthetic and natural calcite, aragonite and strontianite samples, and s. apsteinii coccoliths. sr/ca ratios were determined by edx.

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Caption : figure s3. additional views of the tomographic reconstruction of the s. apsteinii lopadolith. a-f) volume rendering of the calcium signal; g-l) volume rendering of the strontium signal; m-r) overlay of the strontium and calcium reconstructions with calcium rendered in red and strontium rendered in green; s-x) slices through the tomogram approximately half way through the lopadolith. volume renderings b, h, n and d, j, p are cut along the same plane as slices s, t and u, v respectively, showing the internal structure of the lopadolith. volume renderings e, k, q present a top-down view and f, l, r a base-up view of the lopadolith. the 10 μm scale bar applies to all images.

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Caption : figure s4. colourblind-sensitive images from the tomographic reconstruction of the s. apsteinii lopadolith a-f), g-l), and s-x) as figure s3; m-r) overlay of the strontium and calcium reconstructions with calcium rendered in orange and strontium rendered in blue.

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Caption : figure s5. example raw xrf spectrum from a single pixel (count time 0.02 s/pixel, step size 60 nm) from the tomography data collection. the ca kα (red) and sr kα x-ray (green) lines are marked.13

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Caption : figure s6. example raw xrf spectrum from a single pixel (count time 0.75 s/pixel, step size 60 nm) from figure 3. 

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Caption : figure s7. a) example raw xrf spectrum from a single pixel (count time 0.5 s/pixel, step size 100 nm, blue dot in image) from the centre of a lopadolith that was imaged in 2d on a silicon nitride window, with the ca kα (red) and sr kα lines marked. b) example raw xrf spectrum from a single pixel of the silicon nitride window (yellow dot, corner of image) behind showing the minimal contribution of the window to the overall spectrum. the peak at 2.958 kev is argon (yellow) due to the air gap between the sample and the xrf detector. image: ca signal image of the xrf map used to select pixels for the single pixel spectrum examples. dots show the position from which the example spectra were taken (blue: a, yellow: b).  

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Caption : figure s8. ca and sr xrf maps from an additional specimen mapped in 2d with both red/green and blue/green overlaid images. sr is green in both maps, ca is either red or blue (50 nm steps, 0.1 s count time at 18 kev). in this specimen, there was a low sr rim as seen in others, but no specific stripes.

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Caption : figure s9. alternative colour scheme maps for the images shown in figure 4 (bottom row) of the main text. in the top row, ca is in magenta and sr in yellow. in the bottom row, ca is in blue and sr in green.

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Caption : figure s10. alternative colour scheme maps for the overlaid ca and sr colour maps in figure 5 of the main text, showing the presence of stripes and the variability between specimens for lopadoliths from cells grown at different concentrations of sr in the media (left hand side). in these images, the ca signal is in pink and sr signal in yellow, or ca in blue and sr in green (gradients next to c). images in a) correspond to image c) in figure 5. images in b) correspond to g) in figure 5. images in c) correspond to k) in figure 5 and images in d) correspond to o) in figure 5 of the main text. 

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Caption : supplementary video 1: movie showing the 3d reconstruction of the lopadolith with the calcium (red) and strontium (green) signals separately and together. 

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Caption : supplementary movie 1: movie showing the 3d reconstruction of the lopadolith with the calcium (orange) and strontium (blue) signals separately and together.