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Class Bacillariophyceae
Class Bacillariophyceae
Diatoms are photosynthetic, unicellular algae that are known for their geometric shapes. They lack flagella but possess ornate shapes that allow them to float in the upper (sunlit) zone of water. Their abundance and efficiency amounts for a 20-25% of the world's photosynthesis (Malviya et al. 2016). Diatomaceous earth, the fossilized remains of diatoms, is composed of diatom "glass" frustules that can be used to puncture the exoskeleton and dehydrate / kill insects. A few species of diatoms (e.g. Pseudo-nitzschia) produce a neurotoxin called domoic acid, which causes amnesiac shellfish poisoning in sea animals and humans.
Ecology and form
Ecology and form
Photosynthetic, unicellular, or colonial algae
Possess both chlorophyll a & chlorophyll c
Also possess secondary pigments, such as fucoxanthin, which gives diatoms and brown color
Absorbs blue-green to yellow-green light spectrum
Diatoms possess silica in their cell walls
They are constructed like a Petri plate, with two halves called frustules
Centric forms are radially symmetrical, having a circular or triangular form
Pennate forms are bilaterally symmetrical, forming a pin shape
Geologic Age
Geologic Age
Ordovician(?) - present
Lifecycle
Lifecycle
Mostly asexual
Diplontic life cycle in rare cases
Below: line drawing of a pennate (left) and centric (right) diatoms
Diatomaceous earth
Diatomaceous earth
The fossilized remains of diatoms, also called diatomite
It is composed of diatom "glass" frustules
When ground into a powder, it can be used to puncture the exoskeleton and dehydrate/kill insects
Diazoplasts
Diazoplasts
Diazoplasts are nitrogen-fixing organelles found in certain species of diatoms
These organelles have their own DNA, similar to chloroplasts and mitochondria. This suggests an endosymbiotic origin from a nitrogen-fixing bacterium or archaea (Schwartz et al., 2024).
Above: diatomaceous earth with the silica remains of diatoms
Above: Marine diatoms. Photo by Wim van Egmond, Micropolitan Museum,
Above: an assortment of diatom frustules
Questions for Thought
Questions for Thought
Why are diatoms so successful?
What is primary productivity?
Additional Resources
Additional Resources
└Biogeochemical anatomy and ecosystem dynamics of a large phytoplankton bloom north of the Hawaiian Islands (Foreman et al., 2025)
New way to track toxic algae that threaten shellfish industries (Phys.org 16Dec2025)
└Characterizing seasonal persistence of low levels of Pseudo-nitzschia australis in the Gulf of Maine with environmental DNA (Greenlee et al., 2025)
A fungus that kills toxic algae threatening human health (Phys.org 17Dec2025)
└Algophthora mediterranea, gen. et sp. nov.: Novel dinoflagellate- and diatom-infecting generalist marine chytrid from the Mediterranean Sea (Pou-Solà et al., 2025)
Tiny diatoms, big climate impact: How microscopic skeletons rapidly shape ocean chemistry (Phys.org 5Nov2025)
└Rapid transformation of biogenic silica to authigenic clay: Mechanisms and geochemical constraints (Zhao et al, 2025)
Baltic diatoms remained genetically stable for millennia—then humans came into play (Phys.org 26Sep2025)
└Multi‐Millennial Genetic Resilience of Baltic Diatom Populations Disturbed in the Past Centuries (Schmidt et al., 2025)
Scientists uncover extreme life inside the Arctic ice (Phys.org 10Sep2025)
└Zhang et al. (2025) Ice gliding diatoms establish record-low temperature limits for motility in a eukaryotic cell
Tiny ocean partnership between diatoms and N2-fixing bacteria reveals secrets of evolution (Phys.org 29Aug2025)
└Grujcic et al. (2025) Stepwise genome evolution from a facultative symbiont to an endosymbiont in the N2-fixing diatom-Richelia symbioses
Diatoms bind uranium inside and out, raising food chain concerns (Phys.org 7Aug2025)
└He et al. (2025) A multiscale investigation of uranium(VI) interaction with a freshwater diatom species
Study uncovers technologically appealing trick used by microalgae to manipulate light (Phys.org 23Jul2025)
└Dyakov et al. (2025) Talbot effect in nanostructured diatom frustules
Algae bloom ends along Southern California coast, but marine mammals remain at risk (Phys.org 6Jun2025)
Study resolves diatom tree of life, revealing rapid speciation 170 million years ago (Phys.org 3Jun2025)
└Alverson et al. (2025) Phylogenomics reveals the slow-burning fuse of diatom evolution
After 7,000 years without light and oxygen in Baltic Sea mud, researchers bring diatoms back to life (Phys.org 28Mar2025)
└Bolius et al. (2025) Resurrection of a diatom after 7000 years from anoxic Baltic Sea sediment
Diatom surprise could rewrite the global carbon cycle (Phys.org 17Jul2024)
└Kumar et al. (2024) Mixotrophic growth of a ubiquitous marine diatom
Hundreds of Sea Lions Are Dying. Is a Diatom Bloom to Blame? (NY Times 21June2023)
Decline of diatoms due to ocean acidification (25May2022 Nature)
What causes diatoms to become toxic? (Univ. of California - Santa Cruz 2017)
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