Читать книгу Diatom Microscopy - Группа авторов - Страница 15

In differential interference contrast (DIC) microscopy, two cross-polarized light beams pass through a specimen and then combine to form interference fringes indicated by the thickness and birefringence of features of interest. DIC microscopy has been used in forensic medicine to characterize the morphology of diatoms, reproductive biology to characterize sperm cells and hematology for the early discovery of cancer cells [1.46]. This method provides high-resolution images with good contrast without halos around structures, even when imaging thick samples, in contrast to phase contrast microscopy. However, tiny changes or fine structures, which are below the resolution limit, might be invisible when imaged by conventional photography. Until 1980s, electronic contrast enhancement capabilities of video cameras were found to visualize 25-nm diameter micro-tubules by DIC microscopy with the resolution nearly an order of magnitude smaller than the diffraction limit [1.1, 1.59]. In the natural environment, phytoplankton and bacteria live in close proximity. Highresolution video-enhanced differential interference contrast (VEDIC) microscopy has been used to characterize the relationship between marine diatoms and the bacterium L. monocytogenes. As shown in Figure 1.5, the relationship between listeria and the benthic diatom Navicula sp. is parasitic in nature, whereas the relationship between the listeria and the planktonic diatom Phaeodactylum tricornutum is protocooperative (protocooperative: the co-cultivation of planktonic diatom with any of the L. monocytogenes strains can enhance the growth of both the diatom and the listeria cells.) [1.65].

Figure 1.3 Quantitative phase image of diatom cell recorded using 20 x/0.45 microscope objective. Shown here are the (a) intensity and (b) phase images of the complex transmittance. (c) Illustration of simulated Rheinberg filter located at the rear focal plane of a microscope condenser lens. The filter comprises eight red point sources located around a center green point source. (d) Resulting image from simulated Rheinberg illumination. (e) Filtering implemented in the DFT domain, for each of the independent point sources in the filter. From [1.17] with permission of OSA.

Figure 1.4 QPIs results showing a variety of diatom samples; (a) diatom recorded using a 63 × 1.3 NA microscope objective (scale bar = 5 μm); and (b) diatom cell recorded using a 20 × 0.4 NA microscope objective (scale bar = 10 μm). The color filters used in each case are illustrated in the bottom corner of the image. From [1.18] with permission of OSA.

Figure 1.5 Images illustrating the relationship between the bacterium L. monocytogenes strain P and the benthic diatom Navicula sp.: (a) first stage, (b) second stage, (c) third stage, and (d) control. The scale bars = 10 μm. From [1.65] with permission of Springer Nature.