Phototaxis Assay for Chlamydomonas reinhardtii

Structure and function of FAP47 in the central pair apparatus of Chlamydomonas flagella

Motile cilia have a so-called "9 + 2" structure, which consists of nine doublet microtubules and a central pair apparatus. The central pair apparatus (CA) is thought to interact mechanically with radial spokes and to control the flagellar beating. Recently, the components of the CA have been identified by proteomic and genomic analyses. Still, the mechanism of how the CA contributes to ciliary motility has much to be revealed. Here, we focused on one CA component with a large molecular weight: FAP47, and its relationship with two other CA components with large molecular weight: HYDIN, and CPC1. The analyses of motility of the Chlamydomonas mutants revealed that in contrast to cpc1 or hydin, which swam more slowly than the wild type, fap47 cells displayed wild-type swimming velocity and flagellar beat frequency, yet interestingly, fap47 cells have phototaxis defects and swim straighter than the wild-type cells. Furthermore, the double mutant fap47cpc1 and fap47hydin showed significantly slower swimming than cpc1 and hydin cells, and the motility defect of fap47cpc1 was rescued to the cpc1 level with GFP-tagged FAP47, indicating that the lack of FAP47 makes the motility defect of cpc1 worse. Cryo-electron tomography demonstrated that the fap47 lacks a part of the C1-C2 bridge of CA. Taken together, these observations indicate that FAP47 maintains the structural stiffness of the CA, which is important for flagellar regulation.

The four-celled Volvocales green alga Tetrabaena socialis exhibits weak photobehavior and high-photoprotection ability

Photo-induced behavioral responses (photobehaviors) are crucial to the survival of motile phototrophic organisms in changing light conditions. Volvocine green algae are excellent model organisms for studying the regulatory mechanisms of photobehavior. We recently reported that unicellular Chlamydomonas reinhardtii and multicellular Volvox rousseletii exhibit similar photobehaviors, such as phototactic and photoshock responses, via different ciliary regulations. To clarify how the regulatory systems have changed during the evolution of multicellularity, we investigated the photobehaviors of four-celled Tetrabaena socialis. Surprisingly, unlike C. reinhardtii and V. rousseletii, T. socialis did not exhibit immediate photobehaviors after light illumination. Electrophysiological analysis revealed that the T. socialis eyespot does not function as a photoreceptor. Instead, T. socialis exhibited slow accumulation toward the light source in a photosynthesis-dependent manner. Our assessment of photosynthetic activities showed that T. socialis chloroplasts possess higher photoprotection abilities against strong light than C. reinhardtii. These data suggest that C. reinhardtii and T. socialis employ different strategies to avoid high-light stress (moving away rapidly and gaining photoprotection, respectively) despite their close phylogenetic relationship.

Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams

Unicellular Chlamydomonas reinhardtii micro-algae cells were inserted into a quasi-2D Hele-Shaw chamber filled with saponin foam. The movement of the algae along the bubble borders was then manipulated and tracked. These self-propelled particles generate flow and stresses in their surrounding matter. In addition, the algae possess the capability of exerting forces that alter bubble boundaries while maintaining an imminent phototactic movement. We find that by controlling the gas fraction of the foam we can change the interaction of the algae and bubbles. Specifically, our data expose three distinct swimming regimes for the algae with respect to the level of confinement due to the Plateau border cross-section: unlimited bulk, transition, and overdamped regimes. At the transition regime we find the speed of the algae to be modeled by a simple force balance equation emerging from the shear inside the Plateau border. Thus, we have shown that it is possible to create an algae-friendly foam while controlling the algae motion. This opens doors to multiple applications where the flow of nutrients, oxygen and recirculation of living organisms is essential.