Behavior of magnetic microswimmers : simulations for natural swimmers and synthetic propellers Microswimmers i.e. swimmers of micron size experiencing low Reynolds numbers have received a great deal of attention in the last years since many applications are envisioned in medicine and bioremediation. A promising field is the one of magnetic swimmers since magnetism is biocom patible and could be used to direct or actuate the swimmers. This thesis studies two examples of magnetic microswimmers from a physics point of view. The first system to be studied are magnetic cells which can be magnetic biohybrids (a swimming cell coupled with a magnetic synthetic component) or magnetotactic bacteria (naturally occurring bacteria that produce an intracellular chain of magnetic crystals). A magnetic cell can passively interact with external magnetic fields which can be used for direction. The aim of the thesis is to understand how magnetic cells couple this magnetic interaction to their swimming strategies mainly how they combine it with chemotaxis (the ability to sense external gradient of chemical species and to bias their walk on…