Doctorant en biophysique (M/F)

Job Description

Thesis Subject

Title: Coupling between photosynthesis and gliding motility in the microalga Chlamydomonas reinhardtii: experimental study and modeling of bistable dynamics

Scientific context: Photosynthetic microorganisms play a major role in biogeochemical cycles and serve as ideal models for understanding the mechanisms by which cells adapt to their environment. Among them, the single-celled microalga Chlamydomonas reinhardtii is considered as a model organism in biophysics due to its ability to combine active motility, light perception, and photosynthesis [1].
While swimming and phototaxis have been extensively studied, one particular mode of displacement remains poorly understood: gliding motility [2]. When in contact with a surface, the cells can move along their long axis through the interaction between their flagella and the substrate. One the one side, preliminary laboratory results indicate that gliding cells do not move continuously but alternate between two preferred spatial positions. This dynamic suggests bistable behaviour, the physical and biological origins of which remain unknown [3]. On the other side, several studies suggest that photosynthesis [4] may directly influence a cell's motor responses, but no quantitative link has yet been established between the bioenergetic state of an individual cell and its sliding dynamics. This thesis aims to address this gap by simultaneously studying motility and photosynthetic activity at the single-cell level.


Scientific questions:
- Which light parameters control the emergence of bistable behaviors during gliding?
- Is there a correlation between this bistability and the cell's photosynthetic state?
- How can bioenergetic processes be integrated into a quantitative physical description of motility?

Thesis Objectives:
1) Quantitatively characterize the bistability of sliding cells. The doctoral student will study the trajectories of sliding cells under different light conditions in order to (i) determine the conditions under which bistability occurs; (ii) measure the residence times in each state; (iii) quantify the velocity and position distributions; (iv) construct phase diagrams as a function of light parameters.
2) Develop a method for measuring photosynthesis at the single-cell level This thesis will contribute to the development of an experimental platform combining microscopy, automated cell tracking, and PAM (Pulse Amplitude Modulation) fluorometry. This instrumentation will enable the real-time measurement of photosynthetic parameters such as the effective photosynthetic rate.
3) Develop a model integrating bioenergetics and motility Based on experimental data, the doctoral student will develop models describing transitions between sliding states, the influence of light on cellular dynamics, and the role of the photosynthetic state as an internal variable governing motility.

Methodological approach: - Microalgae culture - Sample preparation for microscopy - Optical microscopy - Automatic tracking of cell trajectories - Image processing - Statistical analysis of trajectories and extraction of quantitative parameters - Development of a PAM fluorometry system adapted for mobile cells - Stochastic processes, Langevin equations, model inference from experimental data.

Qualifications: Master's degree or equivalent in Physics, Biophysics, Statistical Physics, or Applied Physics. An experience on cell culture and on the description of dynamical systems is required. Skills in scientific programming (Python, MATLAB, or equivalent), data analysis, and experimental instrumentation are preferred. A strong interest in interdisciplinary approaches at the interface between physics and biology is required.

Required documents: CV, motivation letter and one to two recommendation letters.

References:
[1] Jeanneret, R. et al. Eur. Phys. J. Spec. Top. 225, 2141–2156 (2016)
[2] Till, S. et al. Phys. Rev. Res. 4, L042046 (2022)
[3] Brückner, D. B. et al. Nat. Phys. 15, 595–601 (2019)
[4] Baker, N. R. Annu. Rev. Plant Biol. 59, 89–113 (2008)

Your Work Environment

The Aquitaine Laboratory of Waves and Matter (LOMA) is affiliated with the CNRS and the University of Bordeaux. Its research activities aim to explore and characterize the physical properties of matter from fundamental and applied, theoretical and experimental perspectives. It has an international reputation in Soft Matter and Condensed Matter Physics, Photonics, Materials Science, and Biophysics.
This range of research topics places it at the crossroads of physics, chemistry, biology, and mathematics, thereby forging multidisciplinary and interdisciplinary links between various fields, from basic research to technology transfer. LOMA's core research areas are organized around three major teams: Soft Matter and Biophysics (MMB), Photonics and Materials (PM), and Condensed Matter Theory (TMC). The doctoral student will join the MMB team.

Constraints and risks

N/A

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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