• Figure 1.
    Download figureOpen in new tabFigure 1. Movement of individual Synechocystis cells under different light regimes.

    Displacement over a 5 min time-frame was measured 1 min after the onset of illumination. The mean resultant length from a Rayleigh test (r) and the number of tracked cells (n) are shown. See also Video 1. All data were obtained with the same cells from a single continuous experiment. (a) Schematic diagram to illustrate the optical set-up for a light gradient projected onto the agar surface (as in b) and oblique directional illumination (as in c–e). (b) Cells moving in a white light gradient from 0–20 µmol photons m–2 s–1 over a 165 µm interval from the top to the bottom of the plot, showing no significant directional bias. (c) Illumination from an oblique RGB LED light source from the right, at intensity 10 µmol photons m–2 s–1. (d) Illumination from a similar light source placed orthogonally to that in (b). (e) Simultaneous illumination from both oblique light sources. (f) Correlation of cell movement with light direction, as a function of time after applying the light. The shading indicates the time window plotted in b–e. The y-axis shows the mean resultant length from a Rayleigh test (r) where 0 indicates random displacements and 1 indicates maximal clustering in the direction of illumination. LED, light emitting diode.

    DOI: http://dx.doi.org/10.7554/eLife.12620.003

    Figure 2.
    Download figureOpen in new tabFigure 2. Estimated transmission spectrum of a single motile Synechocystis cell.

    Estimate obtained by scaling and converting the absorption spectrum for a suspension of cells from a moving Synechocystis colony, as detailed in Materials and methods. Pigments (including chlorophyll: Chl and phycocyanin-coupled phycocyanobilin: PCB) are assumed to be evenly distributed within the thylakoid region as shown in the diagram, and the estimated transmission is for a narrow beam of light passing straight through the center of the cell, with an optical path length through the thylakoid region of 1 μm. The spectrum represents a minimum estimate for transmission through the cell, since the estimate assumes a homogeneous distribution of pigments within the thylakoid region. In reality, inhomogeneous distribution of membranes and pigments will tend to decrease absorption due to enhanced self-shading (Duysens, 1956).

    DOI: http://dx.doi.org/10.7554/eLife.12620.005

    Figure 5.
    Download figureOpen in new tabFigure 5. Model for control of positive phototaxis in Synechocystis.

    Directional illumination of the cell produces a sharply focused and intense spot of light (resembling a photonic nanojet) at the cell periphery on the opposite side from the light source. The focused spot is perceived by photoreceptors in the cytoplasmic membrane (for example PixJ1) triggering signal transduction via CheY-like response regulators that locally inactivates the T4P motility apparatus, dispersing T4P components including the extension motor PilB1. Consequently, patches of the motor proteins can only form on the side of the cell facing the light source. Pili are extended and retracted at this side of the cell, which therefore moves towards the light. T4P, Type IV pili.

    DOI: http://dx.doi.org/10.7554/eLife.12620.011

  • Video 1. Motility of Synechocystis cells under different illumination regimes.

    The video gives a schematic overview of the experimental set-up, followed by movement of cells in a projected light gradient, and with oblique illumination from two orthogonal directions, and then from both directions simultaneously. In each case, the raw video data is followed by the same movie clip with the tracks of cells superimposed. Time in minutes is indicated.

    DOI: http://dx.doi.org/10.7554/eLife.12620.004

  • Video 2. Effects of a highly-focused laser spot on directional motility in Synechocystis.

    Cells are imaged by fluorescence from the photosynthetic pigments, and are moving towards an oblique LED light at the bottom of the frame: note the focused light spot at the rear edge of each cell. The superimposed red spot indicates the position of the laser, and time in min is shown at the top left. LED, light emitting diode.

    DOI: http://dx.doi.org/10.7554/eLife.12620.008