When exposed to high levels of phosphate after experiencing phosphate deficiency, microalgae can synthesize granules rich in intracellular phosphate. The understanding of the mechanism of production and nature of these granules remains incomplete. Here we studied phosphate distribution and chemistry in Chlamydomonas reinhardtii and Desmodesmus armatus using scanning transmission X-ray microscopy (STXM), to reveal the X-ray absorption spectroscopy (XAS) characteristics of phosphorus, supported by transmission electron microscopy (TEM). While P K-edge XAS is not very sensitive to subtle changes in different forms of phosphate-based compounds, the analysis points to the possibility that the phosphate polymer present in the granules is phytic acid (inositol hexaphosphate) which has not been observed previously. STXM analysis of the phosphorus (P) in the granules and the surrounding cytoplasm revealed there were no major differences in the P chemistry of the granules formed in C. reinhardtii and D. armatus and confirms that phytic acid is present in the granules. This new understanding of phosphate storage mechanisms in algae may enable directed strategies to enhance environmental removal and capture of P to mitigate eutrophication of waterways and recover a P resource.

Phosphorus Storage in Microalgae: STXM and XAS P K-Edge Investigation