Diatoms are microscopic (2 µm to 2 mm) single-celled or colonial eukaryotic algae found in all aqueous habitats. Their name is derived from the Greek words dia ("through") and temnein ("to cut"), since their cells are divided into two halves or two box-like parts (valves). One of the most abundant type of phytoplankton, diatoms play an important role in the oceans where they fix large amounts of carbon dioxide and evolve oxygen. It is estimated that diatoms may evolve more oxygen than all the rainforests combined. They synthesize carbohydrates and omega-3 fatty acids that serve as a chief source of zooplankton food in the marine food chain 2. When decomposing, the largely intact cell wall sinks to the bottom and these flakes of silica are the main component in white sandy beaches 6,9. Today nanotechnologists are turning to diatoms to build a variety of devices, including drug delivery.4

The diatoms probably originated 200 million years ago from a scaly member of the Chrysophyceae or Bolidophyceae. About 200,000 species of diatoms in 250 genera have been described, including 70 fossil groups. Their extraordinary diversity might be due to rapid rates of horizontal gene transfer with many bacteria.4 Species are classified mostly by the shapes and patterns of their hard silica parts. Common diatom genera include Asterionella, Cyclotella, Fragilaria, Melosira, Navicula, Synedra, and Tabellaria. Diatoms occur as single cells (Cyclotella), in chains of cells resembling filaments (Melosira), or in colonies (Asterionella).

Diatoms comprise the main component of the open-water marine flora and a significant part of freshwater flora. Attached diatoms can be characterized by the brown scums found on various kinds of surfaces, as well as the fluffy brown growths caused by abundant epiphytic diatoms. Pennate diatoms are present in about equal numbers in marine and freshwater habitats, whereas the centric diatoms are present predominantly in the marine environment. 1

Cymbella pennate diatom
Cymbella pennate diatom (Light microscope, oil, 1000X)
Micrograph by Larysa Johnston


The diatom shell (test, or frustule) consists of two overlapping valves joined with girdle bands. The shell is composed of organic material with imbedded clear silica glass (silicon dioxide SiO2). In fact, diatoms can be described as living cells inside a glass house. Synthesis of siliceous cell walls requires less energy than the cell walls of other types of algae, a phenomenon that offers the diatoms a competitive advantage over other algae classes. To avoid sinking and survive, diatoms evolved a series of adaptations, most notably storing oil droplets, which are less dense than sea water. Many diatoms grow spines and other protrusions that help them to stay afloat.8,10 Diatoms require dissolved silica for growth. In fact, they are so good at the removal of silica from natural waters that they can reduce the concentration to less than one part per million, below the value detectable by chemical techniques 3.

Pennate diatom
Pennate diatom (Light microscope, oil, 1000X)
Micrograph by Larysa Johnston


The three great classes of diatoms are Coscinopiscophyceae (Centrales), or centric diatoms, Bacillariophyceae (Pennales, or raphid pennate, those with a slit opening called raphe), and Fragilariophyceae (araphid pennate, those without slit opening). Centric diatoms have radial symmetry, while pennate diatoms have bilateral symmetry.5 Diatoms with raphe are capable of relatively rapid and prolonged movement. Centric diatoms have oogamous sexual reproduction, while pennate produce ameboid gametes. Several types of self-fertilization occur within the diatoms.7

Gomphonema diatom colony
Gomphonema growing on stalks (Light microscope, oil, 1000X)
Micrograph by Larysa Johnston

Poisonous Diatoms

All is not beautiful in the diatom world, even though they live in protective shells, diatoms can be infected by viruses and toxin production by diatoms has adverse effect on the ecosystem and human health.2 For a long time, diatoms have been considered as the "good guys" of the plankton, serving as the base of the food chain, taking up carbon dioxide and releasing oxygen, until it was discovered that some species can produce neurotoxins. One of such neurotoxins, domoic acid, is produced by diatom species of Pseudo-nitzschia and Amphora coffeaeformis. The neurotoxin accumulates in shellfish and causes shellfish poisoning.6

Because plant growth depends on the availability of nutrients, where nutrients influx is high, diatom numbers can quickly reach astronomical proportions, up to 12,000,000 diatoms per milliliter of water. When this happens, the phenomenon is called a diatom "bloom." The presence of diatoms in drinking water sources is commonly associated with filter clogging and taste and odor problems.10


  1. Robert Edward Lee. Phycology
  2. Joseph Seckbach, Patrick Kociolek. The Diatom World
  3. Lynn Margulis, Michael J. Chapman. Kingdoms & Domains: An Illustrated Guide to the Phyla of Life on Earth
  4. Moselio Schaechter (editor). Eukaryotic Microbes
  5. S. Blair Hedges, Sudhir Kumar. The Timetree of Life
  6. Joseph Seckbach. All Flesh Is Grass: Plant-Animal Interrelationships
  7. John D. Wehr, Robert G. Sheath. Freshwater Algae of North America: Ecology and Classification
  8. Daniel C. Abel, Robert L. McConnell. Environmental Oceanography: Topics and Analysis
  9. Amar N. Rai, Birgitta Bergman, Ulla Rasmussen. Cyanobacteria in Symbiosis
  10. Detlef R. U. Knappe, R. C. Belk. Algae Detection and Removal Strategies for Drinking Water Treatment Plants