Nitrogen (N) is an essential macronutrient for microalgae. Shortage of N profoundly affects their growth dynamics, cell metabolism, productivity, and composition. Nevertheless, many free-living microalgae thrive in oligotrophic habitats, obviously due to the presence of long-term N reserves in their cells. Recently, purine microcrystals were nominated for the role of such reserves that was experimentally confirmed as such in several microalgal species. However, to the best of our knowledge, the presence of the purine crystals was not documented in ecologically important diatoms belonging to the genus Nitzschia at the time of this writing. Here, we report on the birefringent inclusions found by polarization microscopy in the cells of a freshwater Nitzschia strain isolated from a temporary waterbody in the Moscow Region. Based on the frustule morphology, the species was identified as N. acidoclinata. The Raman microspectroscopy study, a mainstream method of chemical fingerprinting of cell inclusions, revealed that uric acid was a major component of the inclusions. Aged cultures contained much more inclusions than younger, rapidly dividing ones. The inclusions were relatively stable in living cells albeit dissolved rapidly in the cells treated with aqueous fixatives. In the air-dried cells, the inclusions looked unchanged on the polarized light micrographs, but it was not possible to confirm their chemical composition with Raman spectroscopy. The findings are discussed in the context of the role of purine crystals as longterm N reserves in diatoms and their environmental fitness.