Scanning Electron Microscopy of Phytoplankton: Achieving High-Quality Images Through the Use of Safer Alternative Chemical Fixative

Abstract

line and width, shape of the valve ends, the polarity of the transapical axis, the size of the central interspace, and the length of cell end overlap (Tomas et al., 1997). While many phenotypic markers of the genus can be properly identified under light microscopy, species-specific identification is extremely tedious, time consuming and most often requires the eye of a taxonomic expert (Lim et al., 2012). The defining features are too small to be accurately confirmed via light microscopy (Tomas et al., 1997) because of the limited number and quality of the lenses it uses, as well as the wavelength of light it utilized for illumination. Scanning electron microscopy uses electrons instead of photons to view phytoplankton in depth, making it capable of capturing those modest variations between species of the same genus. Due to the nature of SEM, the sample must be completely dry before placement within the microscope, unless variablepressure or environmental SEM is employed. Biological samples are saturated in water, so when they are dehydrated they begin to decompose and shrivel up into unidentifiable masses. This is especially problematic when working with marine life, because the loss of sea water is often leaves salt crystals in the outer membrane, impeding observation of some features. Marine diatoms are more stout than some other phytoplankton because they are protected by a hard, siliceous cell wall called a frustule (Tomas et al., 1997). These protect the diatoms from heavy alterations during electron microscopy processing, and are usually still identifiable under SEM observation. Some flagellates, which are soft bodied, 1 Department of Marine Sciences at the University of Hawai’i at Hilo