Functional photoreceptors regulating light-switchable adhesion of Chlamydomonas to surfaces

by Rodrigo Esteban Catalán López
Doctoral thesis
Date of Examination:2023-09-14
Date of issue:2024-04-08
Advisor:Dr. Oliver Bäumchen
Referee:Prof. Dr. Jörg Enderlein
Referee:Prof. Dr. Marcus Müller
Referee:Prof. Dr. Timo Betz
Referee:Prof. Dr. Stefan Klumpp
Referee:Dr. David Zwicker
Persistent Address: http://dx.doi.org/10.53846/goediss-10432

 

 

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Abstract

English

The harnessing of light is one of the most remarkable evolutionary features of microalgae, allowing them to adapt and thrive in highly dynamic environments. In order to respond to environmental changes, these photoactive microorganisms have developed an intricate network of light-sensitive molecules, known as photoreceptors, that enables the cells to locate optimal areas for growth and photosynthetic performance. Under favorable conditions, microalgae colonize surfaces and form biofilms, which have substantial technological and medical implications. Notably, the adhesion of microalgae to surfaces can be triggered by light. However, the underlying mechanisms controlling the adhesion response to a light stimulus remain poorly understood. This thesis aims to identify the photoreceptor mediating light-switchable flagellar adhesion of the microalga Chlamydomonas reinhardtii to surfaces. Using wild-type and genetically modified strains, I investigated the surface colonization kinetics of a population of motile Chlamydomonas cells in response to light. Complementary single-cell micropipette force spectroscopy measurements were conducted to quantify the flagellar adhesion forces. Using a time-delayed Langmuir-type kinetics model, I found that cell adsorption occurs significantly faster than desorption for all the strains exhibiting the phenotype. This difference is attributed to the protein-mediated adhesion mechanism of the cells. Furthermore, the adsorption kinetics are not affected by phototactic responses or by the disruption of most of the photoreceptors studied. Micropipette force measurements revealed similar adhesion forces between wild-type and mutant cells, allowing to rule out the most prominent photoreceptors: channelrhodopsins and phototropin. Further characterization of the action spectrum of flagellar adhesion forces suggests that the photoreceptor mediating adhesion contains a flavin-based chromophore, which motivated the deletion of cryptochrome photoreceptors. Adsorption and force spectroscopy experiments showed that the deletion of both plant- and animal-cryptochromes is necessary to completely disrupt the adhesion phenotype of the wild-type strain. Interestingly, cryptochromes are known to be located in the nucleus of Chlamydomonas cells, where they function as transcription factors. I found that the time scale associated to light-switchable flagellar adhesion is about 15 s, which is significantly faster than that of gene transcription. This suggests that a so-far-unconsidered flavin-based photoreceptor exists in the flagella of Chlamydomonas and is the one directly mediating the switching of the adhesion, while the cryptochromes regulate the transcription of the unknown photoreceptor.
Keywords: Chlamydomonas; Photoreceptors; Cell adhesion; Cryptochromes; Force spectroscopy
 
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