Diatoms belong to the classification of golden-brown/ yellow-green algae that use chlorophylls for energy production and contain the brown pigment fucoxanthin. Diatoms also make up a portion of heterogeneous organisms called phytoplankton, which are plankton of plant origin. Diatoms reproduce through sexual or asexual means and include at least 10,000 species with their own unique structures. Because of the large size of this group, many biologists prefer to classify diatoms as a distinct division of algae.

Diatoms possess some of the most beautiful and intricate structures in nature. Each diatom species possesses a characteristic shape; the body is called a frustule, and it is composed of two sections or halves called thecae. If the sections differ in size, the larger piece is called the epitheca and the smaller of the two is the hypotheca. Thecae fit together by overlapping and then bind with a material composed of silica. Diatoms are, in fact, unique in the biological world because they require silicon in their cell wall-as crystallized silica, Si(OH)₄ and some species require silicon for gene expression as well. Diatoms' exquisite frustules seem to disprove one maxim of biology, that there are no straight lines or 90° angles in the natural world.

Diatoms provide the microbial world with another unique feature: decreasing cell size with each new generation. In asexual reproduction, diatoms construct new theca inside the parent before the cell divides. Each successive generation produces smaller and smaller cells, in contrast to binary fission in bacteria, which produces two daughter cells that are replicas of the parent cell. Diatoms must find a way to return to their original size. When diatom cell size has diminished by about 30 percent, diatoms begin to reproduce sexually to form a resting cell called an auxospore. After the resting phase, a protoplast emerges from the auxospore and quickly expands to normal size before the cell builds a new rigid outer wall. Diatoms in this way provide a rare example in nature in which a protoplast plays an active role in a microbial life cycle. In bacteria, protoplasts form only when harsh environments damage cell walls, but the protoplasts never become part of bacteria's life cycle.

Diatoms are divided into the three following categories: (1) Centric diatoms, which inhabit marine waters and may be composed of either chains of interlocking frustules or free-floating planktonic cells; (2) pennate diatoms, which live in marine water or freshwater as well as the moisture on rocks or in soils; and (3) diatoms of the Triceratium species, which does not fit into either of the previous groups. Diatoms that stick to surfaces do so by secreting a mucus like substance called mucilage. Mucilage forms weak bonds between diatoms and various sur- faces so that the diatoms can glide across submerged surfaces rather than live anchored.

Diatomaceous earth contains a collection of fossilized frustules and has some value in industry. Diatomaceous earth has been used in toothpaste and in polishes because it is abrasive, and diatomaceous earth also serves as a low-cost material in large filters, such as swimming pool filters.

Diatoms produce a lethal neurotoxin called domoic acid, first discovered in mussels infected with the diatom Pseudonitzschia or the red alga Chondria armata. The neurotoxin can cause illness in people within 30 minutes to 24 hours after eating infected seafood. In severe cases, victims suffer permanent short-term memory loss in a condition called amnesic shellfish poisoning (ASP).

Marine and coastal animal populations have also suffered from domoic acid poisoning. In 1991, for example, pelicans fishing along the California coast began dying from a poisoning identified as domoic acid after eating anchovies. This incident provided the first solid evidence that domoic acid infection was not confined to the marine shellfish mussels, oysters, and razor clams; the poison could also be found in the nonmuscle tissue of anchovies, sardines, crab, and lobster. Since then, much of the research in domoic acid poisoning has been on seals and sea lions. The biologist Joe Cordaro of the National Marine Fisheries Service told the University of California-Santa Barbara Daily Nexus in 2003, "I've been recording numbers since 1998, and it [poisoning] seems to be happening on a yearly basis. Last year, over 1,000 animals came in [to the local Marine Mammal Care Center]." Domoic acid poisoning continues to threaten marine mammal health along the California coast.

Other algae that threaten the health of people or marine life consist of several species of green algae (Chlorophyta), gold-brown algae (Chrysophyta), and certain dinoflagellates; all have caused severe skin irritations and allergies in humans. Fishermen who handle infected catch have the highest risk of skin irritations, but marine biologists have not yet determined whether the symptoms arise from a toxin or an allergen or perhaps another compound altogether.