Nuoto di alghe girotattiche in vorticità uniforme

The goal of this research is to investigate the dynamics of green gyrotactic algae Chlamydomonas augustae in presence of a quasi two-dimensional flow charaterized by uniform vorticity. Green algae are unicellular phytoplanktonic microorganisms extremely interesting for different reasons; they contribute to the carbon cycle and produce oxygen, they form the primary food source in marine ecosystems and can also provide a biofuel with an elevant energetic content. In this dissertation we consider motile algae explaining the different mechanisms that propel the cells. We also treat the reasons why algae swim in a particular direction, focusing on the ability to orientate themselves upward against gravity, the gyrotaxis. Chlamydomonas augustae are gyrotactic algae able to propel themselves at the speed of about 50−70 μm/s and to reorient themselves in a time whose order of magnitude is few seconds; the swimming speed vs and the gyrotactic reorientation time B are biological quantities of great interest. We start by considering the mathematical model presented by Kessler, adapting it to the rotating system we consider. We proceed analyzing the system in a totally deterministic way, for then investigating the stochastic and more realistic case. In the latter instance, we introduce the randomness in algae's motion by means of rotational diffusion coefficient Dr that is a biological parameter typical of the algae. In the end we approach the system experimentally, thanks to an experimental setup built purposely. In this way we try to verify the goodness of model's results and estimate the important biological quantities vs, B and Dr.