Diatoms
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
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
Lifecycle
Mostly asexual
Diplontic life cycle in rare cases
Below: line drawing of a pennate (left) and centric (right) diatoms
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
Why are diatoms so successful?
What is primary productivity?
Additional Resources
Hundreds of Sea Lions Are Dying. Is an Algal 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)