Phototaxis of the unicellular red alga Cyanidioschyzon merolae is mediated by novel actin-driven tentacles

Phototaxis - which is the ability to move towards or away from a light source autonomously - is a common mechanism of unicellular algae. It evolved multiple times independently in different plant lineages1. As of yet, algal phototaxis has been mainly linked to the presence of cilia, the only known locomotive organelle in unicellular algae. Consequently, phototaxis was believed to be largely absent in red algae (Rhodophyta) that lack cilia in all stages of their life cycle1. Remarkably, the unicellular red alga Cyanidioschyzon merolae (C. merolae) is able to move towards the light. However, it has remained unclear how C. merolae can achieve movement, and the presence of a completely new mechanism was suggested2. Here we show that the basis of this movement are novel retractable projections that were termed tentacles due to their distinct morphology. The tentacles could be reproducibly induced within 20 minutes by increasing the salt concentration of the culture medium. Electron microscopy revealed filamentous structures inside the tentacles that we identified to be actin filaments. This is surprising as C. merolaes single actin gene was previously published to not be expressed3,4. Based on our findings, we propose a model for C. merolaes actin-driven but myosin-independent motility. To our knowledge, the described tentacles represent a novel motility mechanism.