Diatoms are single-celled algae, often called microalgae, that populate nearly every aquatic environment on Earth, from oceans and lakes to damp soil. These organisms play a foundational role in the planet’s ecosystem, forming the base of many aquatic food webs and are responsible for a significant portion of the oxygen produced each year. Their appearance under a microscope is unique due to a highly structured cell wall.
The Defining Feature: The Silica Frustule
The most distinguishing characteristic of diatoms is the rigid, protective cell wall that encases the cell, known as the frustule. This structure is composed primarily of hydrated silicon dioxide, or opal, which is chemically similar to glass. The resulting appearance is often described as a microscopic glass house because of its colorless transparency and ornate structure.
The frustule is constructed like a small, two-part box, with one half slightly overlapping the other. The two main sections are called the valves or thecae, and the overlap is secured by silica bands known as girdle bands. The larger, overlapping half is termed the epitheca, and the smaller, inner half is the hypotheca.
The surface of this silica shell is covered in an incredibly intricate pattern of pores, ribs, and slits. These minute openings, called areolae and striae, are arranged in symmetrical designs that are species-specific and used by scientists for classification. These patterns allow the cell to exchange nutrients and waste with the external environment while maintaining its structural integrity.
Two Fundamental Forms: Centric and Pennate
Under magnification, diatoms are broadly classified into two major groups based on the overall geometry and symmetry of their frustules. This fundamental difference in shape dictates their macroscopic appearance and lifestyle.
The first group, the centric diatoms, exhibits radial symmetry, meaning they look the same when rotated around a central point. When viewed from the top, their shapes are typically circular, but they can also be triangular, polygonal, or cylindrical. Centric diatoms are frequently planktonic, meaning they drift passively in the water column.
The second major group is the pennate diatoms, which display bilateral symmetry, giving them a mirror-image appearance when divided lengthwise. These cells are generally elongated, often appearing under the microscope like boats or long, narrow ovals. Many pennate diatoms possess a feature called a raphe, a slit running along the length of the valve face. This structure allows the diatom to secrete mucilage, enabling it to glide along surfaces.
Viewing Diatoms: Size and Microscopic Detail
To fully appreciate the beauty and complexity of a diatom, a microscope is required because of their extremely small size. Individual diatom cells typically range from about 2 micrometers (µm) up to 500 micrometers in length, with the majority falling between 20 and 300 micrometers. The largest specimens are barely visible to the naked eye, while the smallest are far below the limit of human vision.
Viewing the fine details of the frustule’s architecture requires significant magnification and high resolving power. While basic compound microscopes can show the overall shape of the frustule at 100x or 400x magnification, higher powers, often up to 1000x, are needed to resolve the intricate pores and striations. The fine patterns on diatom frustules have historically been used to test the optical quality and resolution limits of microscope lenses.
When viewing a live diatom, the silica frustule appears transparent and colorless, but the living contents within the shell provide color. Diatoms contain chloroplasts, the organelles responsible for photosynthesis, which give the cell a distinctive yellowish-brown or golden-brown hue. This coloration results from the accessory pigment fucoxanthin, which masks the green of the chlorophyll. The contrast between the colorless, glass-like shell and the colored cell contents makes diatoms a spectacular subject for microscopy.