Optical properties of microbial cells ans sensing the light direction in phototaxis

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Sunlight, which drives photosynthesis, is the most important energy supplier for life on Earth. Photosynthetic cyanobacteria are the ancestors of chloroplasts and they are still very important primary producers in almost all habitats. Our current understanding of the physiology and the metabolism of cyanobacteria is largely based on studies of pure lab-grown cultures in suspension. Moreover, studies in recent years have mainly focused on the analysis of photosynthesis and biotechnological applications of these oxygenic phototrophs. We recently analysed the micro-optic properties of cyanobacterial cells and revealed the importance of these physical cellular characteristics for sensing light direction for phototaxis. In addition, there is emerging evidence that the optical properties of chloroplasts might be important for optimizing photosynthetic light harvesting in low light environments. The spherical cyanobacterium Synechocystis 6803 moves on surfaces using type IV pili towards or away from a light source. The single bacterial cell acts as a very effective micro-lens, focusing a sharp image of the light source close to the opposite (non-illuminated) side of the cell. This bright light spot locally triggers a so far unexplored signal transduction chain, which then controls movement of the cell in response to the light source. Our preliminary investigations with other microorganisms indicate that such micro-optic effects are not confined to spherical cyanobacteria or even to phototrophs. We hypothesize that these micro-optic effects may have important implications for light-sensing, interaction with the environment and UV photodamage. For this proposal, we wish to explore physical properties of cyanobacterial cells by new optical methods and to reveal the signal transduction pathway from the photoreceptors to the motility machinery. Probing optical effects at scales close to wavelength of light requires specialized physical expertise. Therefore we have developed a highly interdisciplinary project combining complementary skills in physics and (photo)biology. In a first approach we will use physical and biological methods to measure the optical properties of very small cells. More generally, we aim to develop a new way to probe molecular responses to optical stimulation at the single-cell scale with high optical resolution. This will be the base for the generation of a comprehensive model for directional light sensing in single-celled organisms.