Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Dynamics of light-sensing microbial populations in microfluidic model habitats

Microalgae are photosynthetic microorganisms that are typically found in aqueous environments such as lakes or the ocean but also in temperate soil habitats. In soil environments, they constantly interact with surfaces where they can attach and form biofilms. We are interested in the motility of Chlamydomonas reinhardtii, a soil-dwelling unicellular microalga that performs phototaxis under blue and green light illumination to reach optimal light conditions for photosynthesis. Another characteristic is that it attaches to surfaces when exposed to blue light. While various investigations have been conducted analyzing the phototactic motion and interactions with solid surfaces separately, only little is known about how the interplay of both parameters affects the behavior of cells. Here, we analyze the effect of both by conducting experiments of cells moving through a quasi-two-dimensional model soil with controlled porosities in the presence of a light gradient. Our analysis includes the orientation dependent mean velocity, the diffusivity and the flight time of the cells. We then use a cell flow analysis to examine collective effects. Surprisingly, in the absence of phototactic effects, we find larger cell velocities in the direction along the light gradient. This indicates that a light sensing mechanism other than the one causing phototactic motion is responsible for this effect. For the flight time, we find an indication for an interplay of the porosity and the light intensity, as we observe an increase towards higher light intensities that is found only at low porosities of the model soil. By applying a cell flow analysis, we identify positive phototaxis and, under blue light, cell adhesion for light intensities above   41018 ph m-2 s -1. This means that in their natural habitat cells tend to ac-cumulate at regions with this intensity, which is a probable survival mechanism of the cells to maximize their photosynthetic efficiency.