Induction of collective behavior by β-1,3-glucans in microalgae

β-1,3-glucans are multifaceted molecules, playing a key role in aquatic ecosystems where they store atmospheric CO 2 , as well as in human health, where they help fight pathogens and have anticancer properties. Until now, no role other than sugar storage had been attributed to β-1,3-glucans in algae. We show here that in Chlamydomonas reinhardtii , β-1,3-glucans control aggregation, a collective behavior in response to stress. We have identified the enzymes specifically involved in β-1,3-glucan synthesis and degradation during aggregation. The transcriptome of β-1,3-glucan–induced aggregation revealed an early effect of this elicitor. A comparative genomic analysis allowed us to envisage a potential role for β-1,3-glucans in the transition to multicellularity in Volvocales. The finding of the function of β-1,3-glucans in the stress response of the unicellular model organism Chlamydomonas sheds light into their mode of action. This is particularly relevant given their potential roles combatting pathogenic fungi in plants and animals.