Diatoms are a vast group of single-celled algae, forming a foundational part of aquatic food webs across the globe. These organisms are distinguished by a rigid, glass-like cell wall composed of silica, known as the frustule. The vast majority of diatoms do not possess flagella—the whip-like appendages commonly used for propulsion by other single-celled organisms. This general absence of flagella, however, is countered by a specific and biologically significant exception tied to their reproductive cycle.
The General Rule of Diatom Movement
The primary reason most diatoms lack flagella stems from the structure and composition of their cell wall. The frustule is a highly silicified, heavy outer casing that completely encases the cell during its vegetative state. This rigid shell is composed of two overlapping halves, much like a petri dish, which provides mechanical protection against grazers and hydrodynamic stress.
This dense, mineralized architecture makes the diatom cell substantially heavier than the surrounding water. Attempting to propel such a dense structure through water using a delicate flagellum would be highly inefficient. Therefore, the typical diatom is largely non-motile, spending its life cycle drifting passively within the water column.
The trade-off for this protective silica casing is the loss of independent swimming capability. Many genes related to flagella development, present in their evolutionary ancestors, have been lost or silenced in the diatom cell. This structural choice dictates a life strategy that relies more on environmental currents than on self-directed movement.
The Specialized Function of Flagella in Diatoms
The absence of flagella is broken exclusively during sexual reproduction. Flagella are found only on the male gametes of centric diatoms, a specific group that typically exhibits radial symmetry. This appearance is a temporary, specialized adaptation, and the flagella are absent in the main vegetative cells.
The male gamete is a small, motile cell, typically equipped with a single, smooth flagellum used for propulsion. This flagellum is essential because the female gamete is large and completely non-motile, a reproductive strategy known as oogamy. The flagellated male gamete must actively swim to locate and fertilize the stationary female gamete.
This sexual reproduction serves a crucial function in size restoration. Diatoms generally decrease in size with each round of vegetative cell division due to the nature of their rigid frustule. The fusion of gametes forms a zygote, which develops into an auxospore, allowing the cell to expand and restore its maximum size. The temporary, flagellated stage is a necessary mechanism to ensure the long-term viability of the species population.
Alternative Motility and Suspension
Since the majority of diatoms are non-motile, they rely on other mechanisms to maintain their position and move within their environment. Planktonic diatoms depend entirely on the natural turbulence and currents of the water column for transport and dispersal. To counteract the tendency of their heavy silica frustules to sink, they employ methods of buoyancy regulation.
One strategy involves adjusting their internal density by managing the storage of organic compounds. Diatoms regulate their sinking speed by altering the concentration of various internal substances, such as storing low-density lipids (like triglycerides) or changing the internal balance of ions and carbohydrates. This physiological control allows them to adjust their depth to reach optimal light and nutrient conditions.
A distinct group called pennate diatoms, which are bilaterally symmetrical, exhibit a form of active movement called gliding. This locomotion is not powered by flagella but is achieved by the secretion of an adhesive mucilage through a fine slit in the frustule called the raphe. This mucilage-driven mechanism allows the pennate diatom to crawl along submerged surfaces, such as sediment or rocks, enabling migration toward favorable light conditions or nutrients.