The Anatomy of The Diatom
The actual protoplast of a diatom is quite similar to that of other algae. Organelles such as the nucleus, mitochondria, and prominent plastids (chromatophores) are typical of most diatoms. The real distinguishing feature of the diatoms is their shells. The valves are heavily embedded with silica (up to 71%). This glass-like wall reflects light creating intricate patterns that are striking and beautiful. The nucleus in diatoms is usually centrally located, migrating to specific sites in the cell as the diatom prepares for cell division. DNA in diatoms is often organized as a large number of very small chromosomes. Upon cell division these often appear as a band of chromasomes surrounding the mitotic spindle. In contrast to many animal mitotic spindles, the spindle fibers in diatoms are often organised as a tight cylender of parallel microtubules.
The shell of the diatom is actually two overlapping halves called valves. One half (epitheca) is a remnant from the previous cell division, while the younger half (hypotheca) is the newly formed side. The hypotheca overlaps the epitheca like two halves of a petri dish. The valve itself of the epitheca is called the epivalve and the girdle elements (cincture or pleura) are named the epicingulum. The prefixes are the same for the hypotheca. The silica shells of diatoms are formed by exocytosis from the protoplast. Since the valves and the girdle elements are rigid, the growth is a unidirectional single plane of growth. There are also slits in the shell that are used for movement called the raphe. Diatoms usually have only one or two raphes per cell. The slits are aided by minuscule pores for material exchange