Phototaxis beyond turning: persistent accumulation and response acclimation of the microalga Chlamydomonas reinhardtii

Abstract

Phototaxis is an important reaction to light displayed by a wide range of motile microorganisms. Flagellated eukaryotic microalgae in particular, like the model organism Chlamydomonas reinhardtii, steer either towards or away from light by a rapid and precisely timed modulation of their flagellar activity. Cell steering, however, is only the beginning of a much longer process which ultimately allows cells to determine their light exposure history. This process is not well understood. Here we present a first quantitative study of the long timescale phototactic motility of Chlamydomonas at both single cell and population levels. Our results reveal that the phototactic strategy adopted by these microorganisms leads to an efficient exposure to light, and that the phototactic response is modulated over typical timescales of tens of seconds. The adaptation dynamics for phototaxis and chlorophyll fluorescence show a striking quantitative agreement, suggesting that photosynthesis controls quantitatively how cells navigate a light field.

An Adaptive Flagellar Photoresponse Determines the Dynamics of Accurate Phototactic Steering inChlamydomonas

“…This is evident from the fact that experiments exhibited a high level of difficulty regarding multiple measurements on the same cells of elicited positive photoresponse. This has to do with our lack of understanding of long-term adaptation to darkness or phototactic light for that matter, topics that only recently have begun to be addressed ( Arrieta et al, 2017).…”

Reorientation behavior in the helical motility of light-responsive spiral droplets

“…Although the evolutionary advantages of helical motion remain unclear, it appears that the directionality of helical motion is less sensitive to extracellular and intracellular random variations, compared with rectilinear motion 28 . Significantly, swimming in a screw-like fashion offers opportunity for efficient reorientation behaviors set to follow chemical and physical gradients 29,30 . Besides the well-known ‘run and tumble’ behavior of E. coli , these chirality-derived reorientation mechanisms often feature deterministic directional changes, such as in the ‘run, reverse, and flick’ of Vibrio alginolyticus , a marine bacterium that also moves along helical pathways, and for which the flick angle is pre-programmed, i.e., the re-direction does not occur randomly 31,32 .…”

Reentrant Efficiency of Phototaxis in Chlamydomonas reinhardtii Cells

“…Taxis, a transport phenomenon in which organisms undergo directed movement in response to a stimulus or a nutrient gradient, provides a particularly tractable context in which to explore collective behaviour. As a particular example, phototactic cells such as algae and cyanobacteria respond to light gradients [15–17]. Single celled eukaryotic algae Chlamydomonas reinhardtii (CR) is a model biological organism for studying phototaxis [18].…”