Studies in the Alga Chlamydomonas reinhardtii Led to the Development of a Powerful Technique to Study Brain Function

The green unicellular alga Chlamydomonas reinhardtii (Figure 1-22b), which swims using its two long flagella, is widely used in studies of the structure, function, and assembly of this organelle. In part because of the powerful genetic techniques now available, Chlamydomonas is also used in studies of chloroplast formation and photosynthesis. The Chlamydomonas genome (see Table 1-2) encodes many more proteins than do those of yeasts, including flagellar proteins and proteins needed to build a chloroplast, organelles not found in yeasts.

One important outcome of the use of this experimental organism came from studies of phototaxis, the behavior in which an organism moves toward or away from a source of light. Chlamydomonas needs to move toward light to undergo photosynthesis and thus generate the energy it needs to grow and divide, but light that is too intense repels it, as it causes damage to the chloroplast. Studies of Chlamydomonas phototaxis led to the discovery of two proteins in its plasma membrane that, when they absorb light, open a “channel” in the membrane that allows ions such as Ca2+ to flow from the extracellular medium into the cytosol, triggering phototactic responses. As detailed in Chapter 22, recombinant DNA techniques have been used to express one such protein in specific neurons in the mouse brain, allowing investigators to activate just one or a few cells in the brain using a point source of light. Thus studies on this humble alga have led to the development of an important experimental system—optogenetics—for the study of brain function.