Diatoms are a major group of algae and are among the most common types of phytoplankton. These unicellular organisms inhabit oceans, freshwater, and soils across the globe. Their ecological significance is substantial, as they generate a large portion of the world’s oxygen and form the base of many aquatic food webs. The defining characteristic of a diatom is its intricate cell wall, a shell of silica that dictates the diatom’s form and interaction with its environment.
The Diatom Frustule
The cell wall of a diatom is called a frustule. This rigid structure is composed of biogenic silica, a form of hydrated silicon dioxide similar to opal, which gives it a glass-like quality. This results in a shell that is both strong and transparent. The frustule’s construction is often compared to a petri dish, as it consists of two main parts that fit together. The intricate, species-specific patterns on the surface of this silica shell are a hallmark of diatoms, leading to immense variation among species.
Anatomy of the Frustule
The frustule is composed of two overlapping halves known as thecae. The larger, older half is the epitheca, which acts as the “lid,” fitting over the smaller, younger half, the hypotheca. Each theca consists of a flattened plate called a valve and a series of connecting bands known as girdle bands, which form the side walls. The combined set of girdle bands is referred to as the cingulum.
This structural arrangement is related to diatom reproduction. During asexual cell division, the two thecae separate, and each new cell retains one half of the parent frustule. This parental half then becomes the epitheca for the new cell, which synthesizes a new hypotheca inside it. The girdle bands provide flexibility for the cell’s contents to expand before division occurs.
Symmetry in Diatom Forms
Diatom shapes are organized into two main groups based on the symmetry of the frustule, a primary characteristic used for their classification. The two types are centric diatoms and pennate diatoms. Centric diatoms exhibit radial symmetry, meaning their valves are structured around a central point. When viewed from the top, they often appear circular, triangular, or polygonal.
Pennate diatoms, on the other hand, are characterized by bilateral symmetry. Their valve shape is elongated and organized along a central axis, often described as boat-shaped or elliptical. The distinction between centric and pennate forms often correlates with the diatom’s lifestyle; many planktonic species are centric, while many benthic species are pennate.
Microscopic Frustule Features
The surface of the diatom frustule is perforated by an array of pores and other structures that are often species-specific. The most common features are pores called areolae, which are organized in intricate patterns. These are not simple holes but complex chambers within the silica wall. Areolae are arranged in rows, forming lines known as striae, and the frustule may be thickened between these rows with silica ribs called costae.
Many pennate diatoms possess a distinct feature known as a raphe, a long slit that runs along the central axis of the valve. This structure is directly associated with the diatom’s ability to move. Other microscopic features can include long silica spines or other projections that extend from the frustule surface.
Functional Roles of Diatom Structure
The frustule’s architecture serves multiple functions. The rigid silica shell provides protection against mechanical stress and grazing by predators. The transparency of the silica and arrangement of pores also play a role in light harvesting, channeling light toward the chloroplasts for photosynthesis and protecting the cell from harmful UV radiation.
The pores, or areolae, are the primary sites for exchanging nutrients, gases, and waste products with the surrounding water. The size and arrangement of these pores can control the diffusion of molecules, effectively acting as a sieve. For pennate diatoms with a raphe, this structure enables their motility. By secreting mucilage through the raphe, the diatom can glide along surfaces. The frustule’s shape and density also influence buoyancy, affecting its ability to remain in sunlit waters.