Physical and Stochastic Aspects of Microorganism Behaviour
type: CreativeWork
description: This thesis studies physical and stochastic aspects of microorganisms. From the point of view of $\textit{physics}$, the studies in this thesis are motivated by the goal of gaining biological insight using the machinery of physics and mathematics. From the point of view of $\textit{biology}$, the studies in this thesis focus primarily on choanoflagellates, eukaryotes that are the closest living unicellular relatives of animals. This choice of model organism was motivated by the important biological question of the origin of multicellularity. Why was it that single-celled organisms evolved to become multicellular? In particular, we study closely the species $\textit{Salpingoeca rosetta}$, which has the ability to form colonies that resemble true multicellular organisms. 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 $\textit{S. rosetta}$ to oxygen gradients. We find that $\textit{S. rosetta}$ displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighbourhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version of classical run-and-tumble chemotaxis. We compare this continuous run-to-tumble with the run-and-tumble seen in bacteria by formulating a general model for persis…
abstract: This thesis studies physical and stochastic aspects of microorganisms. From the point of view of $\textit{physics}$, the studies in this thesis are motivated by the goal of gaining biological insight using the machinery of physics and mathematics. From the point of view of $\textit{biology}$, the studies in this thesis focus primarily on choanoflagellates, eukaryotes that are the closest living unicellular relatives of animals. This choice of model organism was motivated by the important biological question of the origin of multicellularity. Why was it that single-celled organisms evolved to become multicellular? In particular, we study closely the species $\textit{Salpingoeca rosetta}$, which has the ability to form colonies that resemble true multicellular organisms. 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 $\textit{S. rosetta}$ to oxygen gradients. We find that $\textit{S. rosetta}$ displays positive aerotaxis. Analysis of the spatial population distributions provides evidence for logarithmic sensing of oxygen, which enhances sensing in low oxygen neighbourhoods. Analysis of search strategy models on the experimental colony trajectories finds that choanoflagellate aerotaxis is consistent with stochastic navigation, the statistics of which are captured using an effective continuous version of classical run-and-tumble chemotaxis. We compare this continuous run-to-tumble with the run-and-tumble seen in bacteria by formulating a general model for persis…
author: Kirkegaard, Julius Bier
language: en
keywords: biophysics, choanoflagellates, applied mathematics, navigation, stochastic processes, biological fluid mechanics
license: https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/07db04ef-5a8d-4a1e-bf79-daf4e0eef52f/download
license: https://creativecommons.org/licenses/by-nc-sa/4.0/
date published: 2018-08-30
date modified: 2026-07-22T22:24:03Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.24863