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SEM of a saltwater diatom. Diatoms are single-celled photosynthetic algae. They have mineralized cell walls that contain silica and provide protection and support. The diatoms form an important part of the plankton at the base of the marine and freshwater food chains. Magnification: 5,830x and the calibration bar is 10 um. (KEYSTONE/SCIENCE SOURCE/TED M. KINSMAN)