Pattern formation and hydrogen production in suspensions of swimming green algae

This thesis concerns two aspects of microorganism behaviour. Firstly, the phenomenon of bioconvection is explored, where suspensions of motile microorganisms that are denser than the fluid in which they swim spontaneously form concentrated aggregations of cells that drive fluid motion, forming intricate patterns. The cells considered herein orientate by gyrotaxis, a balance between a gravitational torque due to uneven starch deposits causing cells to be bottom heavy and a viscous torque due to fluid flow gradients, and phototaxis, biased movement towards or away from a light source. In Chapters 2 and 3, a stochastic continuum model for gyrotaxis is extended to include phototaxis using three physically diverse and novel methods. A linear stability analysis is performed for each model and the most unstable wavenumber for a range of parameter values is predicted. For two of the models, sufficiently strong illumination is found to stabilize all wavenumbers compared to the gyrotaxis only case. Phototaxis is also found to yield non-zero critical wavenumbers under such strong illumination. Two mechanisms that lead to oscillatory solutions are presented. Dramatically different results are found for the third model, where instabilities arise even in the absence of fluid flow. In Chapter 4, an experimental study of pattern formation by the photo-gyrotactic unicellular green alga species Chlamydomonas nivalis is presented. Fourier analysis is used to extract the wavelength of the initial dominant mode. Variations in red light illumination are found to have no significant effect on the initial pattern wavelength. However, fascinating trends for the effects of cell concentration and white light intensity on cells illuminated either from above or below are described. This work concludes with comparisons between theoretical predictions and experimental results, between which good agreement is found. Secondly, we investigate the intracellular pathways and processes that lead to hydrogen production upon implementation of a two-stage sulphur deprivation method in the green alga C. reinhardtii. In Chapter 5, a novel model of this system is constructed from a consideration of the main cellular processes. Model results for a range of initial conditions are found to be consistent with published experimental results. In Chapter 6, a parameter sensitivity of the model is performed and a study in which different sulphur input functions are used to optimize the yield of hydrogen gas over a set time is presented, with the aim of improving the commercial and economic viability of algal hydrogen production. One such continuous sulphur input function is found to significantly increase the yield of hydrogen gas compared to using the discontinuous two-stage cycling of Ghirardi et al. (2000)