Diatoms are a major group of single-celled algae, a type of phytoplankton, that inhabit nearly all aquatic environments. These microscopic organisms are responsible for producing a significant amount of the planet’s oxygen through photosynthesis, making them a foundational component of aquatic food webs. The number of diatom species is vast, with estimates suggesting there could be up to 100,000 different kinds.
The Glass House (The Frustule)
At the heart of every diatom is its unique cell wall, a structure called the frustule. This intricate casing is composed of hydrated silicon dioxide, chemically similar to opal or glass. The frustule is formed through biomineralization, where the diatom extracts silicic acid from the water to create its solid shell.
The frustule has a two-part construction, fitting together like a petri dish. The larger, outer half is the epitheca, and the smaller, inner half is the hypotheca. Each half consists of a flat surface called a valve and connecting silica strips known as girdle bands. These bands wrap around the cell’s midsection, holding the two halves together.
The complete structure is strong yet lightweight, offering protection from external pressures and predators. A diatom’s appearance depends on its orientation. Looking at the top or bottom valve is the “valve view,” while looking from the side reveals the overlapping girdle bands in the “girdle view.”
Architectural Diversity
The frustule design gives rise to a variety of forms, categorized into two groups based on symmetry. Centric diatoms exhibit radial symmetry, with valve faces structured around a central point. These forms are often circular, triangular, or elliptical in valve view and represent the most ancient lineage of diatoms.
The second group, pennate diatoms, displays bilateral symmetry with features arranged along a central axis, resulting in elongated or boat-shaped forms. This difference in body plan is a primary characteristic for classification and separates diatoms into two distinct evolutionary paths.
The frustule surface features a complex array of microscopic pores, ribs, and spines. These pores, called areolae, are chambers that can be covered by sieve-like plates. The specific pattern of these features is genetically determined and consistent within a species, serving as the primary basis for identification.
Structural Adaptations for Life
The frustule’s structure is directly linked to the diatom’s biological functions. The thousands of pores, or areolae, perforating the shell regulate the exchange of gases, nutrients, and waste products between the cell and its environment.
The frustule’s glass-like silica is transparent, allowing sunlight to pass through to the chloroplasts for photosynthesis. This transparency allows the diatom to produce energy while remaining protected within its shell.
Many pennate diatoms can move using a specialized feature called a raphe, a long slit along the valve face. By secreting mucilage through the raphe, the diatom glides along surfaces. This movement allows the organism to position itself for better light or nutrient access, an ability not shared by most non-motile centric diatoms.
Reproduction and the Structural Challenge
The rigid nature of the frustule presents a challenge during cell division. Diatoms primarily reproduce asexually through binary fission. The cell separates its two halves, the epitheca and hypotheca. Each half then becomes the larger epitheca for a new daughter cell, which synthesizes a new, smaller hypotheca inside it.
This division method means one of the two resulting cell lines becomes progressively smaller with each generation. The cell inheriting the original hypotheca builds an even smaller new one, leading to a gradual reduction in average cell size. This process is known as size diminution.
To counteract this shrinkage, diatoms use sexual reproduction. When cells reach a certain small size, they can be triggered to produce gametes. The fusion of gametes forms an unprotected cell called an auxospore. This auxospore expands to the species’ full, original size before forming a new frustule, resetting the cycle of size reduction.