Diatom Cell: Structure, Function, and Importance

Diatoms are single-celled algae that exist in nearly every aquatic environment on Earth, from oceans and lakes to moist soils. These microscopic organisms, classified as protists, are a major component of phytoplankton. Their significance stems from a unique cellular structure that sets them apart from other algae. The specialized features of diatoms give them a substantial influence on global processes.

The Intricate Glass Shell of Diatoms

At the heart of every diatom is its cell wall, a delicate and ornate structure called a frustule. This shell is composed of biogenic hydrated silica, essentially glass that the organism builds itself. The frustule consists of two overlapping halves that fit together much like a petri dish. The larger, upper half is the epitheca, while the smaller, lower half is the hypotheca, and they are held together by silica strips called girdle bands.

The frustule’s architecture is diverse and is a primary feature used to classify diatom species. Shapes fall into two categories: centric diatoms are radially symmetric, while pennate diatoms are bilaterally symmetric and elongated. The surface of the frustule is covered in intricate, species-specific patterns of pores and ribs.

These detailed patterns are not merely decorative; they are arranged to allow the diatom to interact with its environment. Some pennate diatoms possess a slit-like groove called a raphe, which is involved in movement. The combination of strength and porosity in these silica structures results in a shell that is both protective and functional.

Life Within the Frustule

Contained within the protective frustule is the living protoplast, where the cell’s functions occur. Like other eukaryotic organisms, a diatom cell possesses a nucleus containing its genetic material, mitochondria for energy production, and a large central vacuole that takes up a significant portion of the cell’s volume. The cytoplasm is situated as a layer along the inner surface of the frustule, surrounding this vacuole.

A defining feature of the diatom cell is its chloroplasts, the sites of photosynthesis. These organelles give diatoms their characteristic golden-brown color due to pigments like chlorophylls and fucoxanthin. Energy captured through photosynthesis is stored not as starch, like in plants, but as a carbohydrate called chrysolaminarin.

Diatom Reproduction and Life Cycles

Diatoms exhibit a distinctive reproduction method linked to their rigid frustule. The primary mode is asexual cell division, where the epitheca and hypotheca separate. Each half then becomes the epitheca for a new daughter cell. Each new cell synthesizes a new, smaller hypotheca within its inherited valve.

This method has a significant consequence: one of the daughter cells is always smaller than the parent cell. Over successive generations, the average cell size within a population decreases, a phenomenon known as the MacDonald-Pfitzer rule. To counteract this shrinking and restore the population to its maximum size, diatoms engage in sexual reproduction.

Sexual reproduction is triggered by environmental cues once cells reach a small size. The process involves the formation of gametes that fuse to create a zygote. This zygote develops into a wall-less cell called an auxospore, which expands considerably before forming a new, full-sized frustule. Once complete, the cell can resume asexual division.

Diatoms in Global Ecosystems

Despite their microscopic size, diatoms have a large impact on the planet’s ecosystems. They are primary producers in marine and freshwater environments, responsible for a substantial portion of global photosynthesis. It is estimated that diatoms generate about 20 to 50 percent of the oxygen produced on Earth each year, fixing vast amounts of atmospheric carbon dioxide.

As producers, diatoms form the base of many aquatic food webs. They are a food source for zooplankton, which are then consumed by larger organisms such as fish and shellfish. The productivity of many of the world’s fisheries is directly linked to the abundance of diatom populations.

Diatoms also play a part in global biogeochemical cycles. Their requirement for dissolved silica to construct their frustules makes them regulators of the world’s silicon cycle. When diatoms die, their heavy silica shells cause them to sink, carrying their stored carbon from the surface to the deep ocean in a process known as the biological carbon pump.

Diatoms and Human Innovation

The properties of diatom frustules persist long after the organism dies. Over millions of years, the fossilized remains of diatoms have accumulated in deposits known as diatomaceous earth. This porous, silica-rich material has many industrial applications. Its abrasive nature makes it a component in products like toothpaste and metal polishes, while its high porosity is utilized for filtration of swimming pools and beverages.

The fossil record of diatoms preserved in sediment cores provides useful data. By analyzing the species composition and abundance in different layers, paleoclimatologists can reconstruct past environmental conditions, such as water temperature and salinity. In forensic science, identifying specific diatom species in water samples can aid investigations, like determining the location of a drowning.

The nanometer-scale structures of diatom frustules have captured the attention of nanotechnology researchers. Scientists are exploring their use in advanced applications, including:

• Drug delivery systems
• Optical biosensors
• Components in solar cells
• Components in batteries

The frustule’s architecture offers a blueprint for developing new materials and technologies.