Chlamydomonas is a genus of single-celled green algae inhabiting freshwater environments worldwide. These microscopic organisms are notable for their movement, a sophisticated behavior that enables the cell to navigate its aquatic home and seek out optimal conditions, particularly light for photosynthesis. The study of Chlamydomonas provides insight into the fundamental mechanics of cellular movement and sensory systems, revealing how a seemingly simple organism achieves complex navigational feats.
Chlamydomonas’s Engines: The Flagella
The primary structures responsible for Chlamydomonas’s movement are two whip-like appendages called flagella, which extend from the front end of the cell. At the core of each flagellum is an intricate arrangement of microtubules known as the axoneme, which provides the structural backbone for movement. This entire structure is anchored to the cell’s membrane by basal bodies, which also play a part in the growth and organization of the flagella. This internal machinery allows the flagellum to bend and produce the powerful strokes necessary for swimming, and the organelles also serve as sensors.
How Chlamydomonas Swims
Chlamydomonas propels itself through water using a distinctive, breaststroke-like motion of its two flagella. During this forward motion, the flagella execute a power stroke that pushes water away from the cell, followed by a recovery stroke. The process is powered by motor proteins, the most prominent of which are dyneins, which generate the force required to bend the microtubules of the axoneme.
The cell’s movement is not limited to a single forward gear. Chlamydomonas can alter the shape and frequency of its flagellar beat to change direction or even swim in reverse. The “reverse” motion involves a different waveform where the curvature of each flagellum is modified. As the cell swims forward, it also typically rotates about its long axis, spinning like a corkscrew at a rate of about one to two times per second. This rotation is a feature of its navigation, allowing it to systematically scan its environment.
Synchronized Swimming: Coordinating Two Flagella
For Chlamydomonas to swim efficiently in a straight line, its two flagella must beat in a synchronized fashion. If they beat out of sync, the cell would tumble erratically. Since the organism is a single cell, it lacks a nervous system to send timing signals to each flagellum. Instead, synchronization is achieved through a physical principle known as hydrodynamic coupling.
The movement of one flagellum through the water creates fluid flows that directly influence the motion of the second flagellum. This coupling is enhanced by a subtle rocking motion of the cell body itself. As the flagella beat, they exert forces that cause the cell to oscillate slightly. This rocking helps to mechanically link the two flagella, pulling them into a stable, synchronized rhythm. This physical solution ensures the two motors work together for effective propulsion.
Following the Light: Chlamydomonas’s Guiding System
Chlamydomonas possesses a sophisticated guidance system to navigate toward or away from light, a behavior known as phototaxis. The cell’s primary light-sensing structure is a single, reddish-orange organelle called the eyespot, or stigma, located near the cell’s equator. The eyespot does not form an image but functions as a directional light detector, absorbing photons and initiating a biochemical signal.
As the cell swims forward with its characteristic helical rotation, the eyespot periodically scans the environment. The light signal received by the eyespot varies depending on the cell’s orientation relative to the light source, providing the information needed to determine the direction of the light. Based on this input, the cell can adjust the beating pattern of its flagella to steer. By slightly increasing the force of one flagellum over the other, the cell can turn its trajectory toward a light source (positive phototaxis) or away from light that is too intense (negative phototaxis).