| Summary: | ABSTRACT: Optical diffraction tomography is an emerging label-free microscopic technique with its capability of label-free, quantitative, and rapid imaging of biological samples. In this work, we present the imaging and analysis of a living diatom Cylindrotheca sp. in seawater without using any pretreatment such as fluorescence staining. The 3D refractive index (RI) of a living diatom cell was measured, to which quantitative image analysis was perform to investigate subcellular parts of the diatom. Each part of the cell was well distinguished as RI values and distributions. From the analysis, RI values of frustules, protoplasm, vacuole, and chloroplast were estimated to be in the range of 1.352−1.388, 1.363−1.381, 1.388−1.395, and 1.403−1.436, respectively. Our results suggest that the present method will be a powerful tool not only for observing diatom cells but also for studying various cells and mesoscopic materials. • Subcellular parts of a living diatom cell was well visualized by digital holographic microscope. • Subcellular parts could be identified as differences of refractive indexes. • The observation was achieved without any pre-treatment of the living cell.
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