Phototaxis of the dominant marine pico-eukaryote Micromonas sp.: from population to single cell.

Micromonas is a unicellular photosynthetic pico-eukaryote globally dominant in marine ecosystems. Although previously been described as strongly phototactic, its phototactic strategy and indeed its motility are currently poorly understood. It is also unclear how light is detected, given that the tiny cells do not possess the eyespot typical of larger unicellular green algae: the organism is essentially blind. Here we first perform population-scale phototactic experiments to show that this organism actively responds to a wide range of light wavelengths and intensities. These population responses follow a simple drift-diffusion framework displaying a all-or-none-type response to light. Single-cell tracking experiments detail thoroughly the way Micromonas sp. explore its environment. The extracted motility resembles the run-and-reverse styles of motion commonly observed in marine prokaryotes but with long stopping periods between runs and no specific pattern in the sequence of reversals. The associated peculiar microscopic changes upon photostimulation are finally described and integrating those into jump-diffusion simulations produces phototactic drifts that are quantitatively compatible with those obtained experimentally at the population level. These drifts match the natural sedimentation speed of cells, providing the cells with a mechanism to stay within the photic zone. We conclude with a perspective on the possible mechanism that the cells might utilize to recognise where the light is coming from.