Abstract

Microscopic green algae are commonly found in mud, puddles or lakes, and show great diversity in structural complexity. One encounters the unicellular Chlamydomonas, exhibiting two flagella whose beating enables it to swim in a breast stroke towards light, a nowadays widely studied behaviour favouring photosynthesis. One also finds Gonium, an algae made of 16 Chlamydomonas-like cells arranged in two concentric squares, with all flagella on one side of the plate. It already shows cell differentiation, and is one of the first multicellular algae: Gonium is a key organism to understand the evolution towards multicellularity. In the absence of central nervous system, how can each cell adapt its individual photoresponse to efficiently reorient the whole algae? In this talk, I will present our experiments investigating the phototactic swimming of Gonium, from trajectory tracking under various light conditions to micro-pipette experiments offering access to the flagella description. I will explain our model linking the flagella response to the colony trajectory, upgrading the response to light of Chlamydomonas to a multicellular dynamics. This eventually emphasises the importance of biological noise to succeed swimming towards light.