Titolo della tesi: Oxygen mediated algae-bacteria interactions and Polarotaxis in Chlamydomonas
Motile microorganisms rely on environmental cues to navigate and sustain metabolic activity.
Microswimmers such as the green alga Chlamydomonas reinhardtii and the bacterium Escherichia
coli provide powerful model systems for exploring how microorganisms sense, respond to, and
shape their microenvironment. Understanding how these single cells interact with external fields
such as light and chemical gradients, as well as how they interact with one another, is essential
for elucidating the fundamental principles of microbial navigation and ecological organization.
In this thesis, two distinct studies are presented that address different aspects of microswimmer
behaviour. Below, I provide a brief guide to the structure of the thesis
The first chapter introduces the physical constraints of life at the microscale and provides a
brief overview of microbial motility, focusing on how bacteria and unicellular algae navigate in
low-Reynolds-number environments.
The second chapter summarizes the cultivation methods for the organisms used in this thesis, outlining the essential procedures for maintaining Chlamydomonas, E. coli, and Chlorella under
controlled laboratory conditions.
The third chapter presents a study of algae–bacteria interactions at the cell level, examining
how a photosynthetic alga can modulate bacterial motility in aquatic environments. Combining experiments with a minimal theoretical framework, we investigated interactions between the
motile green alga Chlamydomonas reinhardtii, the non-motile alga Chlorella sorokiniana, and E. coli.
When photosynthesis is activated, the algae act as localized oxygen sources and directly influence the behaviour of nearby bacteria. At high bacterial densities, E. coli exhibits spatially varying motility patterns that reflect the local oxygen landscape, including the characteristic volcano
effect. At the single-cell level, bacteria adjust their run-and-tumble dynamics to climb oxygen
gradients, resulting in aerotactic trapping: prolonged circular orbits around the alga sustained
by a virtual confinement barrier. A temporal-sensing run-and-tumble model with fast and slow
oxygen-dependent filters, which modulate tumbling probability, quantitatively reproduces these
orbit-trapping trajectories and captures the essential features of the experimental observations.
The fourth chapter examines how Chlamydomonas reinhardtii responds to the polarization state
of light. Considering the multilayered structure of the eyespot and its specialized optical design,
which together grant the cell with a directional light-sensing capability, we investigated how polarization influences its swimming behaviour. Experiments carried out under spatially uniform
illumination with controlled polarization show that linearly polarized light reduces orientational
diffusion and induces a nematic alignment of swimming directions along the polarization axis—
an effect we refer to as polarotaxis. A stochastic angular-dynamics model, which includes an
alignment torque that increases with the degree of polarization, reproduces the observed orientation distributions and their dependence on light intensity