Figure 2.

Fluorescence and ptychographic x-ray images of a second unsectioned frozen-hydrated Chlamydomonas alga. X-ray fluorescence images (a) show the distribution of S, P, K, and Ca within the plunge-frozen cell. The 5.2 keV x-ray ptychographic phase contrast image (b) shows unlabeled subcellular structures, including a big pyrenoid (Py) on the top. Because the fluorescence and ptychographic image data are recorded simultaneously, the various images are in perfect registry allowing for (c) which is a color composite image overlay of K, P, and Ca on top of the ptychographic image. With this method, the electron-dense spherical structures in (b) can be identified as polyphosphate bodies (Ph) that contain polyphosphate complexed with calcium⁵⁸. Again, the granule (labeled Ca) seems to be different from other polyphosphate bodies as it contains less potassium, which suggests that this granule might undergo degeneration or aging.