How Diatoms Reproduce
Diatoms can reproduce in two different modes, sexual and asexual. They also may, but not necessarily, pass through a seed-like phase known as the resting spore.
Diatoms have a unique "shrinking division" mode of asexual reproduction. After cell division, the two valves of the test separate. Each forms the epivalve of a daughter cell, and new hypovalves are secreted within each of the parent valves. The result is one cell that is the same size as the parent cell, and one cell that is slightly smaller. Due to the rigidity of the test material, growth of the cell is impossible once the test is secreted. Thus, the average diatom size gets progressively smaller with each round of replication.
Very small diatoms may switch to a sexual mode of reproduction. The sexual reproduction mode allows for growth of the zygote to relatively large size. It is the escape hatch for diatoms from the ever-shrinking asexual mode.
In times of low nutrients, poor sunlight, or other stresses, diatoms may form metabolically inactive spores called resting spores. These spores have lots of stored energy in the form of photosynthetic products and tough thickened cell walls. They sink to the bottom of the sea to rest. If they can return to favorable conditions, the cells may return to normal cell functioning.