Phototaxis in the Unicellular Red Algae Cyanidioschyzon merolae and Cyanidium caldarium

Abstract

Summary Phototaxis of 2 cyanidiaceae, Cyanidioschyzon merolae and Cyanidium caldarium, was studied by population experiments. We found that cells of both C. merolae and C. caldarium moved towards light in liquid medium, but that the degree of migration was quite different. When laterally illuminated, most of the C. merolae cells moved towards light at a velocity of 0.27 mm/h. In contrast, only a small proportion of C. caldarium cells showed migration towards light and most of the cells remained dispersed. The exterior cell surface of C. merolae was observed by scanning electron microscopy. It appeared thick and flexible enough to enable crawling movement.

Phototaxis of the Unicellular Red Alga Cyanidioschyzon merolae Is Mediated by Novel Actin-Driven Tentacles

“…We found that sedimented cells of a liquid C. merolae culture condensed at a focused light spot within 18 h ( Figure 1 a,b). Similar behaviour was previously described for C. merolae and Cyanidium caldarium [ 9 ] as well as for Porphyridium cruentum [ 6 ]. Upon moving the light, the condensed cells immediately followed the spot with a velocity of approximately 2.5 µm⸱min −1 ( Figure 1 c, Supplementary Movie S1 ).…”

“…Nevertheless, no actin cDNA clones could be obtained [ 13 ] nor was it detected by fluorescence microscopy with FITC-phalloidin, which specifically binds actin filaments, or by transmission electron microscopy in dividing cells [ 15 ]. Consequently, Ohnuma et al proposed a “non-conventional system” to enable C. merolae ’s movement [ 9 ]. Looking at single moving cells, we found that this “system” consists of single or few long protrusions.…”

“…Remarkably, 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. Even in the acidophilic, thermophilic unicellular Cyanidales, a sister group to all other red algae, phototaxis was observed in Cyanidioschyzon merolae ( C. merolae ) and Cyanidium caldarium [ 9 ]. Surprisingly, no external appendages were observed to be involved in cell movement.…”

“…Surprisingly, no external appendages were observed to be involved in cell movement. Consequently, the presence of a completely new motility mechanism was suggested [ 9 ]. We combined molecular biological, biochemical, and imaging techniques to analyse the structural basis of C. merolae ’s phototactic movement.…”

Origins of eukaryotic excitability

“…Gliding by cilia is present in many species, and relies on a surface-mechanism mediated by interactions between transmembrane proteins and the substrate [38]. Red algae glide on surfaces and can move towards light by directional steering [39,40]. Since red algae generally lack cilia and helical swimming, their phototactic movement may rely on spatial sensing (shading or focusing by lipid droplets), and directional growth of cellular protrusions [41].…”