Lichtgedanken 04

S C HW E R P U N K T 18 For a long time you have been researching the internal clock of green algae—what are you currently working on? We are investigating the properties and functions of cryp- tochromes. These are proteins that exist in many organisms and act either as receptors, which forward light information to the endogenous oscillator, or are themselves a part of the internal clock. In the case of the single-celled flagellate green alga Chlamydomonas reinhardtii , there is one plant-based and one animal-like cryptochrome, amongst others. Interestingly, the plant cryptochrome does not just transmit light informa- tion; it also appears to be part of the internal clock itself. How did you come to these conclusions? We know from previous experiments that Chlamydomonas cells that are placed in permanent darkness react to light pulses only during the subjective day and swim towards them, but not during the subjective night. This circadian rhythm is known as photoaccumulation. We recently re- duced the expression of the plant cryptochrome from Chla- mydomonas to around ten percent and discovered that the circadian behavior of the photoaccumulation in these green algae changed: the period extended to around 28 hours and the behavior became arrhythmic after a few days. This sug- gests that the plant cryptochrome plays a decisive role in controlling the internal clock of Chlamydomonas within the endogenous oscillator. And humans also have cryptochromes? Yes, animal-like cryptochromes have been identified as part of the endogenous oscillator for the circadian clock in mice and humans. Thus, they directly affect the sleep/wake cycle. Do we need light for our internal clock? The internal clock is synchronised using the light-dark cycle. If you expose the organism to permanent low light or perma- nent darkness, a human will, for example, experience a period of around 25 hours during one sleep-wake cycle. This means that, at some point, humans miss a day. Jürgen Aschoff dis- covered this already in the 1960s when he conducted experi­ ments observing the sleep-wake cycle of people in bunkers The internal clock It is not just our willpower that determines whether we are awake or asleep. Our sleep-wake cycle is governed by an internal clock, which is entrained by the rhythmic change of daylight and darkness. Prof. Dr Maria Mittag is investigating the internal clock of tiny green algae. In this interview, the botanist talks about what single-celled organisms have in common with humans from the view of a chronobiologist, how the internal clock works un­ der free-running conditions in constant darkness, and why this topic earns a Nobel Prize. INTERVIEW: JULIANE DÖLITZSCH over several days. Students often participated in such experi­ ments. In parallel, they were often also studying for exams, but had to sleep at some point. Depending on the organism, the period may be slightly longer or shorter than 24 hours un- der constant conditions. Thus, an internal clock determines the daily routine for most organisms. How great is the influence of light on the day-night rhythm? In principle, every journey into a different time zone rep- resents a manipulation of our internal clock. Once we have spent two to three days in the USA or Australia, we overcome the jet lag and adapt to the new light-dark cycle, as our inter- nal clock resynchronizes. This applies to both humans and green algae. Green algae know what time it is in Australia? Of course. There were some people who were sceptical of the idea of an internal clock and they conducted experiments. Thus, colonies of bees were once flown to the USA because the researchers believed that they would simply continue with their rhythm once there. But, after a few days, they had been adapted to the new light-dark conditions and had synchro- nized their internal clock accordingly. It would also have been possible to simply keep them in Germany and expose them to a reversed dark-light cycle to test the same thing, as chronobi- ologist Colin Pittendrigh rightly noted. Algae have also been sent to outer space, as scientists believed that their rhythm may be dependent on the earth’s gravitational pull. Howev- er, the rhythm of the photoaccumulation of Chlamydomonas continued in space, as Dieter Mergenhagen and his wife Elke demonstrated. This confirms that an internal clock continues to dictate things, even at microgravity. In recent years, the Nobel Prize for Medicine or Physiolo­ gy has been awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young. The three Americans have been researching chronobiology since the 1980s. Why have they now been honoured with a Nobel Prize? Almost every organism on earth is subject to light-dark cycles and their adaptation to these is associated with many bene- F E AT U R E