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.
Highlights
Phototaxis is an important reaction to light displayed by a wide range of motile microorganisms
Studying the accumulation dynamics around a localised source, we show that cells use tight circulation around the maximum light intensity as a strategy to maximise their overall light exposure before spontaneously leaving the illuminated region
The quantitative modulation of phototactic response tracks the dynamics of chlorophyll fluorescence, used here as a proxy for the photosynthetic activity of the cells
Summary
Phototaxis is an important reaction to light displayed by a wide range of motile microorganisms. Chemotaxis features (almost) perfect adaptation to persistent stimuli over intermediate timescales (~10–100 s)[8, 9] and can stimulate/inhibit gene expression through a variety of chemosensory pathways[10] This paradigmatic sensory system highlights the important crosstalk happening between responses acting across a wide spectrum of time intervals, and exemplifies the need for a consistent cross-timescale framework to understand motility regulation in microorganisms. At the single cell level, phototaxis will modulate cell irradiance and can be expected to impact both cell metabolism -through chloroplast stimulation- and light-sensitive gene expression[30] Studies of these links are currently limited to qualitative accounts of red-light[31] or redox state[32] control of phototactic sign, and switch from negative to positive phototaxis after prolonged illumination[33]. The quantitative modulation of phototactic response tracks the dynamics of chlorophyll fluorescence, used here as a proxy for the photosynthetic activity of the cells
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