Diatoms are microscopic, single-celled organisms found in virtually every aquatic environment on Earth, from oceans and rivers to moist soils. These tiny organisms are a form of algae, and they play a significant role in global ecology. They are primary producers that use sunlight to generate their own food, a process commonly associated with plants. However, diatoms are not classified within the Kingdom Plantae.
The Classification: Why Diatoms Are Not Plants
The biological classification of diatoms separates them from the plant kingdom based on fundamental differences in cellular and organizational structure. Diatoms belong to the class Bacillariophyceae and are placed within the eukaryotic supergroup known as the Stramenopiles, or heterokonts.
The most significant distinction is that true plants are multicellular organisms with specialized tissues and organs. Diatoms, conversely, are typically unicellular, existing as single cells or sometimes forming simple colonies. They lack the complex, differentiated structures that define the Plantae kingdom.
While diatoms perform photosynthesis, this shared function is not the sole determinant for classification. Their photosynthetic pigments also differ from those in true plants, possessing chlorophylls a and c but typically lacking chlorophyll b. The lack of specialized reproductive structures is another factor that excludes diatoms from the plant kingdom.
The Unique Feature: The Silica Frustule
A defining physical trait of diatoms is their specialized cell wall, called the frustule. This intricate external shell is not composed of cellulose, but is instead made of opaline silica, which is hydrated silicon dioxide, essentially a form of glass. The frustule is a highly ornate structure that provides mechanical protection to the cell within.
The structure resembles a small, overlapping box, consisting of two main parts: the epitheca (the slightly larger top half) and the hypotheca (the slightly smaller bottom half). These two halves, or valves, fit tightly together and are connected by a series of girdle bands. This overlapping arrangement allows the cell to expand slightly during growth and reproduction.
The frustule’s surface is covered with a variety of pores, ribs, and delicate patterns, which are often used by scientists to identify different species. Diatoms come in two general shapes: centric diatoms, which are radially symmetrical and appear circular, and pennate diatoms, which are bilaterally symmetrical and appear elongated or boat-shaped. Some pennate forms possess a specialized slit called a raphe, which secretes a mucilage that allows the organism to glide along surfaces.
Global Impact and Ecological Function
Although diatoms are microscopic, their sheer numbers and metabolic activity give them a significant influence on the planet’s environment and climate systems. They are the dominant primary producers in many aquatic food webs, transforming light energy into chemical energy through photosynthesis. This energy forms the base of the food chain, sustaining zooplankton, aquatic insects, fish, and marine mammals.
The photosynthetic process affects the Earth’s atmosphere. Diatoms are estimated to generate between 20 and 30 percent of the oxygen in the air we breathe. This means that roughly every fifth breath is a product of diatom activity in the oceans and waterways.
Diatoms are important players in the global carbon cycle, largely through a process known as the biological pump. They efficiently fix carbon dioxide from the atmosphere, incorporating the carbon into their organic material. When diatoms die, their heavy silica shells cause them to sink rapidly to the ocean floor, transporting and sequestering this captured carbon to deep-sea sediments for geological timescales.
Over millions of years, the accumulation of these fossilized silica frustules has formed thick deposits. This material is commercially harvested and processed into diatomaceous earth. Diatomaceous earth has many industrial uses, including as a natural filtration aid for water and beverages, a mild abrasive in polishes, and a component in insulation materials.