Diatoms are microscopic (2 µm to 2 mm) single-celled or colonial eukaryotic algae found in all aqueous habitats and can also occur as endosymbionts in dinoflagellates and foraminifers, as well on sea turtles. 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 convert (fix) inorganic carbon (carbon dioxide) to organic compounds and evolve oxygen. These photosynthetic eukaryotes are responsible for about 20% of photosynthesis on Earth and 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
Diatoms are encased in distinctive, porous silica shells, called frustules, that cause them to sink rapidly when they die, carrying fixed organic carbon to the deep ocean. 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 Chrysophyceae or Bolidophyceae. Most recent estimates range from 12,000 to 30,000 specie 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 growing harmlessly upon other plants. 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
pennate diatom (Light microscope, oil, 1000X)
Micrograph by Larysa Johnston
Because of their complex evolutionary history, diatoms have a “mix-and-match genome” that provides them with a range of potentially useful attributes, such as a rigid silicified cell wall, the presence of vacuoles for nutrient storage, fast responses to changes in ambient light, resting stage formation, proton pump-like rhodopsins, ice-binding proteins, and a urea cycle. In general, planktonic diatoms seem well-adapted to regimes of intermittent light and nutrient exposure; however, they are particularly common in nutrient-rich regions encompassing polar as well as upwelling and coastal areas (10), highlighting their success in occupying a wide range of ecological niches and biomes.11
Diatoms are very efficient at nutrient utilization and thus successful organisms in various environments  but can be readily adapted into prolific cultures; in these artificial conditions they often outcompete other algae in mixed cultures. They are also relatively resistant to various pathogens. Hence, they are highly regarded for their potential in producing feed or as a sustainable source of valuable substances, especially biofuels.
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.
(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
growing on stalks (Light microscope, oil, 1000X)
Micrograph by Larysa Johnston
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
- Robert Edward Lee. Phycology
- Joseph Seckbach, Patrick Kociolek. The Diatom World
- Lynn Margulis, Michael J. Chapman. Kingdoms & Domains: An Illustrated Guide to the Phyla of Life on Earth
- Moselio Schaechter (editor). Eukaryotic Microbes
- S. Blair Hedges, Sudhir Kumar. The Timetree of Life
- Joseph Seckbach. All Flesh Is Grass: Plant-Animal Interrelationships
- John D. Wehr, Robert G. Sheath. Freshwater Algae of North America: Ecology and Classification
- Daniel C. Abel, Robert L. McConnell. Environmental Oceanography: Topics and Analysis
- Amar N. Rai, Birgitta Bergman, Ulla Rasmussen. Cyanobacteria in Symbiosis
- Detlef R. U. Knappe, R. C. Belk. Algae Detection and Removal Strategies for Drinking Water Treatment Plants
- Malviya S, Scalco E, Audic S, et al. Insights into global diatom distribution and diversity in the world’s ocean. Proceedings of the National Academy of Sciences of the United States of America. 2016;113(11):E1516-E1525. doi:10.1073/pnas.1509523113.
- Epibiotic Diatoms Are Universally Present on All Sea Turtle Species. Nathan J. Robinson, Roksana Majewska, Eric A. Lazo-Wasem, Ronel Nel, Frank V. Paladino Lourdes Rojas, John D. Zardus, and Theodora Pinou PLoS One. 2016; 11(6): e0157011.