Lichtgedanken 03

S C HW E R P U N K T 34 Chlamydomonas reinhardtii Tiny, nimble and quick: the single-celled green alga Chlamydomonas reinhardtii measures just 10 micrometres—a good 10 times thinner than a sheet of paper. But with their two flagella, these algae can really speed up. Taking its size into account, C. reinhardtii swims as much as 12 times faster than a human being. This tiny alga is usually in a hurry because of changes in the light. Depending on the incidence and intensity of the light, it seeks out the best position for photo- synthesis and energy generation . These microalgae live in fresh water all over the world and, in addition to bodies of water, they also occur in damp soil. To perceive light, C. reinhardtii uses a large number of photoreceptors. Some of these are located in what is called the eyespot , which is on the »equator« of the single-celled organism and is coloured orange by carotenoids. Work on these receptors has opened up a new field of research—optogenetics—in which particular proteins from Chlamydomonas are put to use in neurobiology as »mo- lecular light switches«. The alga has a pronounced day-night rhythm, which is controlled by an »internal clock« , similar to the way this operates in humans. Even if the algae are kept in the dark for an extended time period, this rhythm continues to work, with a period of around 24 hours, which means that the algae »know« when it is actu- ally day or night. Chronobiologists call this a circadian rhythm and distinguish between a subjective day and a subjective night. If the green algae register light pulses during their subjective day, they swim towards the light. If light pulses are shone on them during their subjective night, however, they do not react. C. reinhardtii is also interesting for research because this simple organism can be used to study many fundamental processes of developmental biology and physiology. As the green alga only has a haploid (single) chromosome set , targeted manipulations of the microalga’s genetic material can be imme- diately studied. C. reinhardtii was one of the first green algae for which the genome was fully sequenced, which underlines its importance as a model organism. Its genetic material comprises more than 15,000 genes, some of which correspond to plant genes, while others show similarities with animal genes. What is more, the green alga is increasingly important for biotechnological processes . For example, it is used to produce biodiesel or alternative ener- gy sources such as hydrogen. C. reinhardtii is also the first plant organism that is known to digest cellulose, i.e. other plants, in order to generate ener- gy. If its environment contains too little carbon dioxide, which it needs for photosynthesis, the microalga forms enzymes that enable it to break down cellulose instead. This makes the alga an interesting organism for the ge- neration of biofuels from plant waste containing cellulose. As early as 2014, the phycology section of the German Botanical Society named Chlamydomonas reinhardtii »alga of the year« . Doctoral candidate Prasad Aiyar studies cultures of the single-celled green alga Chlamydomonas reinhardtii .