Physical and Stochastic Aspects of Microorganism Behaviour
This thesis studies physical and stochastic aspects
From the point of view of
A large part of this thesis deals with the random walks of microorganisms. We study these active random walks both for single cells and those composed of individual organisms adhered together. The latter colonial random walkers are typified by choanoflagellates. We develop quantitative theories and use these to extract physical parameters.
The increasing ocean oxygen levels in the Precambrian era are thought to be an important factor
in the emergence of complex multicellular, animal life.
As a first step, we address this situation by studying the response of
We compare this continuous run-to-tumble with the run-and-tumble seen in bacteria by formulating a general model for persistent run-and-tumble. We find that although an optimal persistence does exist for a given tumble frequency, in the full parameter space there is a continuum of optimal solutions. We develop this model further by introducing finite tumble times.
Efficient uptake of prey and nutrients from the environment is an important component in the fitness of all microorganisms, and its dependence on size may reveal clues to the origins of evolutionary transitions to multicellularity. We examine these issues in depth for choanoflagellates, finding that in the absence of other requirements and in a homogeneously nutritious environment, the optimal strategy to maximise filter feeding is to swim fast which favours swimming unicells. In contrast, in large external flows, a sessile form becomes advantageous. Effects of prey diffusion are discussed and are also found to be advantageous for the swimming unicell.
Finally, we consider the switching between synchronous and anti-synchronous beating of flagella in the green alga