Abstract
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 lineages. As of yet, algal phototaxis has been linked mainly to the presence of cilia, the only known locomotive organelle in unicellular algae. Red algae (Rhodophyta), however, lack cilia in all stages of their life cycle. Remarkably, multiple unicellular red algae like the extremophile Cyanidioschyzon merolae (C. merolae) can move towards light. Remarkably, it has remained unclear how C. merolae achieves movement, and the presence of a completely new mechanism has been suggested. Here we show that the basis of this movement are novel retractable projections, termed tentacles due to their distinct morphology. These tentacles could be reproducibly induced within 20 min 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. merolae’s single actin gene was previously published to not be expressed. Based on our findings, we propose a model for C. merolae’s actin-driven but myosin-independent motility. To our knowledge, the described tentacles represent a novel motility mechanism.
Highlights
Phototaxis is the ability of an organism to autonomously move towards or away from a light source
We found that sedimented cells of a liquid C. merolae culture condensed at a focused light spot within 18 h (Figure 1a,b)
Similar behaviour was previously described for C. merolae and Cyanidium caldarium [9] as well as for Porphyridium cruentum [6]
Summary
Phototaxis is the ability of an organism to autonomously move towards or away from a light source. It has evolved multiple times independently in different plant lineages and is a widespread acquirement of unicellular algae [1]. Phototactic unicellular and colonial green and brown algae use flagella or cilia to swim in helical trajectories, like most phototactic eukaryotic organisms [2,3]. The common molecular basis of flagella and cilia is the so-called axoneme which consists of a characteristic arrangement of nine microtubule doublets. Positive phototaxis by the unicellular Rhodophyta Porphyridium cruentum [6], Porphyridium purpureum, Rhodella maculata, Dixoniella grisea [7], Timspurckia oligopyrenoides, and Erythrolobus madagascarensis [8] was observed when exposed to unilateral light. Biochemical, and imaging techniques to analyse the structural basis of C. merolae’s phototactic movement
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