Abstract

The unicellular algae Chlamydomonas exhibits two flagella whose beating enable it to swim in a stroke and move towards light. Similarly, the multicellular algae Volvox, made of thousands of Chlamydomonas-like cells, shows a coherent collective behaviour, also swimming towards light. Here, we study the phototactic behaviour of Gonium, a colony made of 16 Chlamydomonas-like cells, arranged in a square plate shape. The very specific organisation of the cells within the colony makes the algae an interesting intermediate step in the transition from unicellular to complex multicellular organisms. It shows a strong phototactic behaviour, as it is able to alter its swimming direction within a couple of seconds to face light. To understand the efficiency of this reaction, we follow individual flagella responses after a change in the light environment. Variations in flagella frequency and shape, as well as flow observation with micro-PIV, offer an insight into the underlying mechanisms of this efficient adaptation to light.