Diatom Cell Wall Composition
Imagine a tiny glass box — that is the most direct way to picture a diatom. These are single-celled algae that live in water, and their most striking feature is the cell wall, which is not made of the cellulose you find in plants. Instead, it is made of silica (silicon dioxide, SiO2), the same material that makes up sand and glass.
Why silica? Diatoms live in aquatic environments where silicon is naturally dissolved in the water. They actively take up dissolved silicic acid (Si(OH)4) and polymerise it into solid, hydrated silica inside their cells. The result is a rigid, transparent and intricately patterned shell called a frustule.
The Frustule: A Two-Part Box
The frustule is not a single piece. It consists of two overlapping halves, like a Petri dish or a soap box:
- The larger, upper half is the epitheca (the lid).
- The smaller, lower half is the hypotheca (the box).
The epitheca overlaps the hypotheca like the two halves of a soap dish. This overlapping structure matters for how diatoms reproduce — when the cell divides, each daughter cell keeps one half and builds a new, smaller half to complete it, which is why diatom cell size tends to shrink across generations.
The Organic Coating
Silica alone would be brittle, so the frustule carries a thin coating of organic material — mostly polysaccharides and proteins. This organic coating:
- Protects the silica from dissolving in water.
- Provides a template for the precise, ornate pattern of pores and ridges unique to each diatom species.
These pores are functional, not decorative — they allow nutrients, gases and waste to pass through the otherwise impermeable silica wall.
The Precise Statement
Diatom cell wall = hydrated silica (SiO2, with water of hydration, written SiO2·nH2O) plus an organic coating of polysaccharides and proteins.
The cell wall is a frustule made of two overlapping halves (epitheca and hypotheca), composed mainly of amorphous, hydrated silica, with an organic coating that provides structural integrity and controls how the silica is deposited.
Why This Matters for Exams
Two points come up often:
- Silica, not cellulose — diatoms are the only major group of algae with a silica cell wall. This is a defining feature.
- The frustule resists decay — when diatoms die, their silica frustules sink and accumulate over millions of years to form diatomaceous earth, a sedimentary deposit used as a filtering agent, a mild abrasive in toothpaste, and even as an insecticide (its sharp silica particles damage insect exoskeletons).
A common mistake is to call the diatom cell wall "glass" or "sand." The material is chemically similar (silica), but the correct biological term is frustule, and the chemical form is hydrated silica (SiO2·nH2O), not pure quartz. Do not confuse it with the calcium carbonate shells of foraminifera or the cellulose walls of green algae — each group has its own distinct wall composition.