Micro-optics and photosynthetic light-trapping in cyanobacteria

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Synechocystis is a cyanobacterium with spherical cells about 3 microns in diameter. It is capable of phototactic twitching motility. We recently investigated the basis for directional light perception in Synechocystis and made the unexpected observation that its cells act as very powerful spherical lenses, focusing a sharp image of a light source at the opposite edge of the cell. We also found that the cells of an elongated rod-shaped cyanobacterium can trap light by total internal reflection and therefore act as microscopic waveguides. These observations provided the key to understanding phototaxis in cyanobacteria, but they also have strong implications for photosynthetic performance, since micro-optic effects result in highly inhomogeneous distribution of light within the cell and also have the potential to greatly increase the optical path-length within the cell and therefore enhance the efficiency of photosynthetic light absorption. In this project we will explore the influence of micro-optic effects on photosynthesis in cyanobacteria. Since the effects are strongly dependent on the size and shape of cells we will use three model cyanobacteria: Synechocystis with spherical cells, Synechococcus with rod-shaped cells and the filamentous multicellular cyanobacterium Anabaena. In all three organisms, we will quantify the effects of lensing and internal reflection on light distribution within the cell, using the complementary techniques of fluorescence microscopy and photolithography. We will investigate whether micro-optic effects are influenced by specific features of the cyanobacterial cell surface, and we will quantify the influence of micro-optic effects on photosynthetic light absorption, photosynthetic performance and photoinhibition. The results will give a full picture of the significance of this hitherto unexplored aspect of photosynthesis, which may have implications from modelling the global ecosystem to enhancing the efficiency of photobioreactors.

The most immediate industrial beneficiaries will be biotechnological concerns interested in the exploitation of cyanobacteria for solar-powered production of biofuels and high-value products. Here, the design of efficient photobioreactors for growth of cyanobacteria (and other phototrophic micro-organisms) in mass culture is a major concern. The organisms need to be grown in dense cultures for efficient harvesting and use of space, but this can lead to very inefficient growth. For example a simple tank containing a dense culture of cyanobacteria works very inefficiently. When exposed to full sunlight, the cells in the surface layers will suffer photodamage from excess light, while cells in the interior of the tank will be shaded and suffer from light deprivation. At any one time, only a very small proportion of the culture will be in the correct light environment for efficient photosynthesis. Our preliminary results have revealed a previously unexplored aspect of the interaction of cyanobacteria with light, which will be fully characterised and quantified in our programme of research. Our research may lead to better solutions to the problem of efficient phototrophic growth in mass culture, for example by revealing adaptations that enhance photosynthesis in low light, or structures that maximise the efficiency of light absorption in cells growing within solid media or in dense biofilms. To maximise the applied potential of our research a continuous dialogue with relevant industrial concerns will be required, and the Pathways to Impact statement details plans for enabling this dialogue during the course of the project.