The top of the frustule, the epitheca, is perforated with many holes, arranged in a pattern characteristic of the species The holes permit the cell's plasma membrane to maintain close contact with the environment, and allows for the diffusion of materials into and out of the cell. Numerous pores also reduce the weight of the floating diatom Some diatoms also have longitudinal slots in the wall of the base, or hypotheca, through which the cell can make contact with a solid surface and crawl about. In the center of the frustule is the nucleus which contains the genetic materials and the chloroplasts, or photosynthetic organelles. After a diatom cell divides each new cell retains one part of the parent shell and builds a new part to fit into it. Diatoms cannot move under their own power, but have developed adaptations that keep them afloat.
Some diatoms have pairs of thin spines or setae projecting from the ends of the cells.
These setae fuse with other cells to form long chains, thereby increasing the buoyancy of the chained groups or colonies. Massive surface blooms of buoyant diatoms have been observed where the cold waters of polar seas converge with warm equatorial currents. The blooms are marked by a narrow line where the water changes from blue to green. These blooms are detectable by radar and even by satellites in orbit around the earth. When diatoms die, their hard, silicate shells remain in tact.