Abstract
The study of the sinking phenomenon of diatom cells, which have a slightly larger specific gravity (~1.3) compared to that of water, is an important research topic for understanding photosynthetic efficiency. In this study, we successfully demonstrated the observation of the sinking behaviors of four different species of diatom using a homemade “tumbled” optical microscope. A homemade 1 mm3 microchamber was employed to decrease the effects of convection currents. In the microchamber, diatom cells were basically settled in a linear manner without floating, although some of the cells were rotated during their sinking. Sinking speeds of the four species of diatom cells, Nitzschia sp., Pheodactylum tricornutum, Navicula sp., and Odontella aurita, were 0.81 ± 5.56, 3.03 ± 10.17, 3.29 ± 7.39, and 11.22 ± 21.42 μm/s, respectively, based on the automatic tracking analysis of the centroids of each cell. Manual analysis of a vector between two longitudinal ends of the cells (two-point analysis) was effective for quantitatively characterizing the rotation phenomenon; therefore, angles and angular velocities of rotating cells were well determined as a function of time. The effects of the cell shapes on sinking velocity could be explained by simulation analysis using the modified Stokes’ law proposed by Miklasz et al.
Highlights
Direct observation of floating or sinking phenomenon of living cells or biomolecules is an attractive research subject (Malkiel et al, 1999; Xiao et al, 2012; Treguer et al, 2018; Ide et al, 2020; Shoumura et al, 2020)
For the approximate lengths of the apical, transapical, and pervalvar axes of Nitzschia sp. (NIT), PHE, Navicula sp. (NAV), and ODO, the average values of five individuals of four species of diatoms were recorded by digital holographic microscopy (DHM) observation (Fig. 1b; Umemura et al, 2020)
The diatom cell shape was lanceolate for NIT and PHE, narrow elliptic for NAV, and cylindrical for ODO
Summary
Direct observation of floating or sinking phenomenon of living cells or biomolecules is an attractive research subject (Malkiel et al, 1999; Xiao et al, 2012; Treguer et al, 2018; Ide et al, 2020; Shoumura et al, 2020). Malkiel et al (1999) successfully observed 10–20 μm spherical particles and 3 μm linear particles using a homemade PIV system Their purpose was to study the distribution of diatoms at various depths, not to measure the velocity of diatom motion. PIV is a valuable and powerful method to study diatom behaviors in water; it is not similar to direct observation of individual cells by optical microscopy. When we mounted a sample, such as a sealed Petri dish with diatom cell suspension, we could directly observe the floating behavior of individual diatom cells in the microscope. This method is a kind of miniature aquarium. For Odontella aurita, the approximate lengths of the apical, transapical, and pervalvar axes were 57, 9.2, and 10 μm, respectively
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