Scanning Electron Microscope: Potentials in the Morphology of Microorganisms Abstract. Morphologic characteristics related to ecology and evolutionary sequences, and to specific, generic, and familial relations, can now be determined with the scanning electron microscope. These detailed characteristics will help to establish a more natural faunal classification and will enable more accurate ecologic and biostratigraphic correlations. Detailed study of microorganisms has always been hampered by our inability to observe minute structures of test morphology. The imaging capability of the scanning electron microscope now enables the researcher to observe effectively many of the fine details upon which faunal classifications are based; the microscope also provides information on and permits illustration of features that have not been observed previously (see cover). This microscope became commercially available in 1966 and differs from ' the transmission instrument in many ways. The depth of field (500 times that of a light microscope) and high resolution (ten times that of a light microscope) are complemented by a very wide range of magnifications (x 50 to greater than xX 100,000); development and operation of this microscope have been discussed (J, 2). Specimens can be observed directly within a few minutes of being mounted; troublesome delays, due to faulty replicas, normally experienced by the transmission electron microscopist are completely eliminated, and suitably prepared whole specimens are used rather than thin replicas. The following technique is used for the study of various microorganisms such as foraminifers, diatoms, radio- larians, and ostracodes. Specimens are mounted with a diluted solution of tragacanth containing a small amount of glycerin. The mounts are allowed to dry at room temperature before being placed on a rotary turntable in a high-vacuum coating unit. Two coating runs (coating angles, 45° and 10°) are made, and the specimens are coated with gold to a thickness of 300 A; conductive coatings of 100 to 200 A also have been used. Photographs are taken at an accelerating voltage of 25 kv. Scanning time required to record the pictures, with a single-line scan, is 20 seconds. Panatomic-X 35-mm film is used. Morphologic characteristics of specimens are then studied in detail. The thinnest conductive coatings applicable to microorganisms have not yet been determined. Pease et al. (2) have photographed living insects without conductive coatings, but I was un- successful with foraminifers, radiolarians, diatoms, ostracodes, and dinoflagellates without use of conductive coatings. Much higher contrast in image of the specimen is possible if nonmetallic specimens are coated. Expensive thin-film monitoring equipment is necessary to determine ac- curately the minimum thickness of coating that can be applied. New techniques incorporating high-vacuum evaporation are being devised for the deep penetration of metallic molecules within individual pore structures. Beam angle, beam collimation, and metal-par- ticle speed, together with a variable substrate temperature, are a few of the many important aspects of highvacuum evaporation now being investigated. Differences in spinal development and shell strengthening with depth in the marine environment are exemplified by Globigerina bulloides dOrbigny (Figs. 1 and 2). Strong spinal development in the apertural region is present in both specimens shown, but the specimen collected at 200 m (Fig. 1) dis- plays distinct thickening and orientation of “flying buttress” support spines, which are evident in both the apertural and sutural regions. The specimen of G. bulloides collected at 50 m (Fig. 2) is characterized by extremely long, nar- row, pointed spines lacking the thickening mentioned. Most specimens of planktonic Foraminifera collected at depths greater than 100 m exemplify some form of primary Fig. 1. Globigerina bulloides (d@’Orbigny); Recent, Scotian Shelf at 200 m. Note narrow elongate spines on periphery, and thickened “flying buttress” spinal development in the apertural region (x 400). 1318 bilamellar shell growth and secondary thickening. Bilamellar growth is shown in spinal thickening originating at the base of each spine and progressing to the apical end. Secondary growth (usually layering) consists of thick calcite crusts deposited over the primary shell. SCIENCE, VOL. 158