Label-free non-invasive quantitative measurement of lipid contents in individual microalgal cells using refractive index tomography

Abstract

Microalgae are promising candidates for biofuel production due to their high lipid content. To facilitate utilization of the microalgae for biofuel, rapid quantification of the lipid contents in microalgae is necessary. However, conventional methods based on the chemical extraction of lipids require a time-consuming destructive extraction process. Here, we demonstrate label-free, non-invasive, rapid quantification of the lipid contents in individual micro-algal cells measuring the three-dimensional refractive index tomograms. We measure three-dimensional refractive index distributions within Nannochloropsis oculata cells and find that lipid droplets are identifiable in tomograms by their high refractive index. In addition, we alter N. oculata under nitrogen deficiency by measuring the volume, lipid weight, and dry cell weight of individual cells. Characterization of individual cells allows correlative analysis between the lipid content and size of individual cells.

Three-dimensional label-free observation of individual bacteria upon antibiotic treatment using optical diffraction tomography

“…In addition, the RII of 0.185 mL/g is known as a typical value for proteins [42,46]. Because many bacteria have proteinaceous organelles [48], it is reasonable that the typical value of RII for proteins could be used in our analysis. The temporal changes in the cellular dry mass are calculated as the total sum of the concentration at each voxel multiplied by its volume.…”

Holotomography: refractive index as an intrinsic imaging contrast for 3-D label-free live cell imaging

“…Potentials in biological and medical applications: From spatial RI distributions, HT technology can retrieve various quantitative parameters. Certain molecules such as lipid (Jung et al 2018;Kim et al 2016b) or metal nanoparticles (Kim et al 2018a;Turko et al 2013) have distinctly high RI values, which can be addressed by measuring 3-D RI maps. RI values of a solution are linearly proportional to its concentration (Yoon et al 2017).…”

“…Nonetheless, the spatial distribution RI values can provide limited morphological information about subcellular organelles. For example, nucleus membrane, nucleoli, lipid droplets, and vacuoles can be easily identified from RI distributions because they have distinct RI values different from neighboring environments (Barer 1953;Jung et al 2018;Kim et al 2016b;Schürmann et al 2016). Furthermore, the gradient of RI can also be utilized for further discriminating subcellular structures .…”

Deep-learning-based label-free segmentation of cell nuclei in time-lapse refractive index tomograms

“…Optical diffraction tomography (ODT) is one of the 3D QPI techniques, which reconstructs the 3D refractive index (RI) distribution of a sample from multiple 2D holographic images measured at various illumination angles [6][7][8] . Due to its label-free and quantitative imaging capability, ODT has been utilized in various topics of studies including microalge 9 , hematology 10 , infectious diseases 11 , and yeast study 12 . In addition, RI is an intrinsic property of materials governing light-matter interaction (i.e., scattering potential) and the reconstructed tomogram provides abundant morphological information about cells 13 .…”