I grew up in Aachen in a family coming from Westphalia with a large number of medical doctors, including both parents, both grandfathers, brother, and a number of cousins and uncles. My youth was extremely traditional. I was sent to a humanistic gymnasium with many years of Latin and Greek and I hated to translate boring reports about Roman and Greek wars. I disliked this school with the exception of the small topics Chemistry, Physics, Biology, Art and Music, most of them quite unimportant for that ancient school type. But playing flute in the orchestra and fooling around on motorcycles was great fun outside school.

In parallel, I became fascinated with the great maritime explorers including da Gama, Columbus, Vaspuchi, Magellan, Cook, Joshua Slokum and many others. Later, near the end of school, I focused on arctic expeditions with my greatest heroes Nansen and Amundsen. I learned from them what you need to explore new territories: courage, very accurate preparation, a huge amount of patience, and certainly self-confident, knowledgeable and reliable co-workers you can trust in difficult situations. However, when I finished school I realized that there is no territory left on this planet that remains undiscovered, with the possible exception of deep-sea areas. Thereafter, I became interested in astronomy but I was convinced that my mathematical skills were underdeveloped. But, discovery seemed not to be the only route to satisfaction. To create something new that never existed before appeared to me a challenging alternative and as a consequence, I began to study chemistry in Münster where I was born. After two years I moved to Munich to focus on biochemistry.

As a student at the Ludwig Maximilian University I was fascinated by Feodor Lynen, a great enzymologist and unconventional character. Shortly after my last exam he died and I started my masters work in the lab of one of Lynen’s most successful students, Dieter Osterhelt, at that time a young Director at the Max Planck Institute for Biochemistry. Dieter had discovered the function of bacteriorhodpsin as a light-driven proton pump, which was the birth of the microbial rhodopsin research. The years in this institute were the most stimulating time for me. I characterized the just identified light-driven proton pump Halorhodopsin. Dieter, until today, remains my only real academic teacher. We also had a lot of fun skiing and mountaineering in the Alps, and in the summer, sailing and occasionally racing on the many lakes.

After my PhD I tried to do my research independently as soon as possible and decided to work on the photoreceptors in green alga Chlamydomonas. After a one-year visit to the physics department in Syracuse, NY, to focus on quantitative photobiology with Ken Foster (a late Max Delbrück graduate student) I started my own group again at the MPI for Biochemistry to identify the phototaxis photoreceptor. The biochemical work failed despite my intense experience with microbial rhodopsins. The reason was that Chlamydomonas contains > 20 sensory photoreceptors and at least eight of them rhodopsins. But, in parallel I started electrophysiology on Chlamydomonas with my 2nd graduate student Hartman Harz supported by Hans-Dieter Lux, a director at the MPI for Psychiatry in the next-door building. We characterized photocurrents in Chlamydomonas and proposed that they are carried by rhodopsin and due to the very fast current rise that the rhodopsin and the channel form a single protein complex. A few years later we extended the proposal by saying that the rhodopsin and the channel are one protein. Also, from my private perspective, I had a great time in Munich where I met my wife, we had three children together and we all are loving and fighting each other to this day — now with two grandchildren supporting us in both directions.

Chlamydomonas research was neither the most cited nor the best funded research but I got a position at the University Regensburg where a number of new colleagues worked on eukaryotic model organisms and Chlamydomonas was appreciated. We made slow progress with our electrophysiological studies but after many biochemical failures Suneel Keteriya identified two large rhodopsin cDNAs in the Katzusa Chlamydomonas cDNA data base. Our interest was to verify our proposal and we proved the function of both new rhodopsins in a collaboration with Georg Nagel. He was the one who proved in Xenopus oocytes that both proteins are indeed light-activated ion channels that we named Channelrhodopsin-1 and -2 (ChR1 and ChR2). We already found that ChR2 is better expressed than ChR1 with 10x larger photocurrents and the key experiments on the way into neuroscience was the demonstration that ChR2 expresses well and is fully functional in human kidney cells (HEK).

Not as a big surprise, mostly young researchers have undertaken the adventure to go with ChR2 into application which should also be considered as optogenetic pioneers. They expressed ChRs in a number of cells, tissues and animals, namely Zhuo-Hua Pan in the retina of mice, Karl Deisseroth in hippocampal neurons, Stefan Herlitze in chicken embryos, Alexander Gottschalk in C. elegans, and Hiromu Yawo in mouse brain slices. After their key publications in 2005/6 the field exploded and many superb scientists applied and improved optogenetic systems worldwide and my lab contributed with photoreceptor characterization and molecular improvements over the past 15 years in Berlin where I am working since 2005. Berlin somehow suits us, chaotic and stimulating with its everyday struggles. I am grateful to all the wonderful companions who have travelled with me on more or less bumpy roads during the past 35 years.

20 May 2021 Hong Kong