Diatoms are single-celled algae found in aquatic environments worldwide, encased in an ornate, glass-like shell known as a frustule. They are foundational to many aquatic food webs and contribute to global oxygen production. While many diatom species float passively, carried by currents, a significant number can actively move across surfaces. This self-propulsion allows them to navigate their immediate surroundings in a controlled manner.
The Structure for Movement
Diatom species are categorized into two main groups based on their frustule shape: centric and pennate. Centric diatoms are typically circular or triangular with radial symmetry and are generally non-motile. In contrast, pennate diatoms are elongated and bilaterally symmetrical, like a spindle or a boat. It is almost exclusively within this group that we find active movement.
This ability is tied to an anatomical feature called the raphe, a long slit that runs along the frustule. This structure is the physical portal through which the diatom interacts with its environment to generate motion. Not all pennate diatoms have a raphe, but for those that do, it is the defining characteristic that enables their gliding motility. The raphe is not a muscle but a channel for the cell’s propulsive mechanism.
The Gliding Mechanism
The gliding process begins with the secretion of mucilage, a sticky substance composed of polysaccharides, through the raphe slit. This material adheres to the substrate the diatom is on, such as a rock, grain of sand, or another cell. Once this connection is made, the diatom engages its internal motor to generate force.
Underneath the raphe, inside the cell, are bundles of actin filaments, which are part of the cytoskeleton. Myosin proteins, acting as molecular motors, move along these actin tracks. This action is transmitted through the cell membrane to the adhered mucilage on the outside, pushing it backward and propelling the cell forward.
This system can be visualized as a biological tank tread. The diatom lays down a sticky track of mucilage and then pulls itself along this track by pushing the material backward. This continuous process allows the diatom to glide smoothly across a surface. The movement is deliberate but relatively slow.
Why Diatoms Move
The ability to glide provides diatoms with survival advantages. One primary driver for this movement is phototaxis, the ability to move in response to light. This allows diatoms to position themselves for optimal photosynthesis, moving toward light when needed and away from it when the intensity becomes damaging.
Another reason for movement is chemotaxis, which is motion guided by chemical gradients. Diatoms can glide toward higher concentrations of scarce nutrients, such as nitrates and silicates. This is useful in biofilms, which are dense communities of microorganisms, where movement allows diatoms to find favorable positions, interact with other microbes, or escape predators.