Eva-Maria Zetsche, Ahmed El Mallahi & Filip J. R. Meysman. Diatom Research, 31:1, 1-16, 2016.
A recent publication by Zetsche et al. (2016) highlights the difficulties in imaging aquatic organisms such as diatoms: “Diatom cells are for the large part transparent, and since transparent substances or objects, by definition, do not absorb light in appreciable quantities when suspended in water, these entities are hard to discriminate and detect by microscopy techniques that rely on intensity information alone”.
“Piper (2011) suggested that interference-based contrast microscopy reveals the shape and structure of cells more clearly, as it improves the plasticity and contour sharpness.” Digital holographic microscopy (DHM) is in fact an interference-based approach and Dr. Zetsche and her co-authors are able to show that with DHM “the structural organization of diatoms is more clearly determined, in terms of cellular components, shapes and features.”
Ovizio’s qMod, a differential digital holographic microscopy camera for classical microscopes, is one of the instruments which offers a tool for the improved discrimination of living and dead diatoms (success rate >95%). Possible future applications can be the live-dead discrimination of microscopic aquatic organisms, as well as improved species identification. “Certain species of diatoms are frequently used to assess the water quality of rivers and lakes as well as coastal areas (Anton-Garrido et al. 2013, Kelly et al. 2009, Sabater et al. 2007). DHM may facilitate the live-dead differentiation of cells and thus improve these monitoring procedures” (Zetsche et al. 2016).
Figure: (a) Hologram of a cleaned frustule of Stauroneis sp. (University of Gent, Belgium) as obtained with an Ovizio digital holographic microscope. This hologram contains both light-intensity information (b) as well as phase information (c) representing the optical path length (OPL) of the object. (d) The OPL of an object is more clearly visualized with false coloring of the phase information. (Taken from Zetsche et al. 2016)