The dynamics of phototaxis in photosynthetic microorganisms

The motility strategies of M. pusilla are characterised for the first time, establishing a new variant of run-tumble motion: stop, run or reverse. This pattern bears remarkable similarities to the run-reverse motion of marine bacteria, suggesting the size of the organism dominates the choice of motility strategy. The phototaxis of M. pusilla is then described for the first time at both population and single cell scales. The proposed method of phototaxis - an extension of the run length when the cell is orientated towards the light stimulus - is verified with a series of jump-diffusion numerical simulations. The phototactic study was extended by demonstrating the first recorded intensity dependent step-up photophobic response of M. pusilla, followed by a stronger step-down response. During these responses cells switch from typically stationary behaviour to continuous swimming to escape the harmful environment. The step-up response can also be triggered chemically during an apparent cell death, leading to burst events where cells attempt to escape from a spherically diffusing source of pollutant radiating from a single cell. The similarities in these independent responses suggest there is a global avoidance strategy present in the organism to escape from harmful environments. Finally, a new experimental system is proposed to investigate phototaxis in more complicated optical landscapes and channel confinements using the model organism C. reinhardtii. Photoaccumulation to two Gaussian stripes is observed to impede the transport through a channel, opening up the question of what influence phototaxis could have in porous media. In whole, the motility and phototactic behaviour of the most globally dominant pico-eukaryote M. pusilla has been investigated and characterised for the first time, as well the discovery of an apparently universal avoidance strategy from a variety of harmful environments