Light-switchable adhesion of microalgae to surfaces

Microalgae are promising candidates for biotechnological applications like the production of raw materials, such as oil, proteins and starch. Microalgae can be typically found both in seawater and freshwater, where they exist individually or colonize interfaces. The photoactive microalgae Chlamydomonas reinhardtii lives in soil and has two modes of locomotion: freely swimming and gliding on a surface. The surface-based gliding motility bases on adhesive contacts between the flagella and the surface. Here, we present the results on adhesion forces generated by C. reinhardtii from in vivo force spectroscopy measurements. Micropipette experiments reveal that the adhesion forces are typically in the range of 1 −6 nN. Repeated force-distance curves show a variability in adhesion force of several nanonewton. To explain this observation we study the flagella configuration during the adhesive contact on the substrate and find that the measured adhesion forces result from a 180 orientation of straight flagella on the substrate. Force-distance experiments with varying flagella length on the substrate suggests that the variability of the measured adhesion forces bases on the sections of the flagella in contact with the substrate. Recently, it has been discovered, that the flagella adhesiveness can be reversibly switched on and off by changing the illumination from white to red light. We show that this effect is a more generic trait of photoactive microalgae, by performing experiments in different light conditions with further organisms that are closely related to C. reinhardtii. In addition to force-distance curves, we perform so-called auto-adhesion experiments to mimic the transition between the planktonic and the surface-associated state of a cell. During this process the flagella move on the substrate, as seen in the gliding motility, until the cell is in complete contact with the substrate. We describe this process with a minimal model to estimate the cooperative effort of the IFT trains. We compute total IFT forces in the range of 200 − 1200 pN and compare the result to recent studies on the dynamics of single IFT trains