Light Control of the Diffusion Coefficient of Active Fluids

Abstract

Active fluids refer to the fluids that contain self-propelled particles such as bacteria or micro-algae, whose properties differ fundamentally from the passive fluids. Such particles often exhibit an intermittent motion, with high-motility "run" periods broken by low-motility "tumble" periods. The average motion can be modified with external stresses, such as nutrient or light gradients, leading to a directed movement called chemotaxis and phototaxis, respectively. Using cyanobacterium Synechocystis sp.PCC 6803, a model micro-organism to study photosynthesis, we track the bacterial response to light stimuli, under isotropic and non-isotropic (directed) conditions. In particular, we investigate how the intermittent motility is influenced by illumination. We find that just after a rise in light intensity, the probability to be in the run state increases. This feature vanishes after a typical characteristic time of about 1 hour, when initial probability is recovered. Our results are well described by a mathematical model based on the linear response theory. When the perturbation is anisotropic, we observe a collective motion toward the light source (phototaxis). We show that the bias emerges due to more frequent runs in the direction of the light, whereas the run durations are longer whatever the direction.

Cyanobacteria: Model Microorganisms and Beyond

“…This feature, however, vanishes after a typical characteristic time of about 1 h, when the initial probability is recovered. These results were well described by a mathematical model based on the linear response theory proposed by Vourc'h et al [150].…”

“…Red to green wavelengths result in positive phototaxis, while blue wavelength leads to negative phototaxis [159]. It was identified that the motility of cyanobacteria is partially controlled by light and several genes that are important for phototaxis [139,150]. The mutant strain of Synechocystis sp.…”

“…Single Synechocystis cells display an intermittent motion in cell suspensions with two phases; a high-motility "run" and a low-motility "tumble" (Figure 10) [150]. The two phases can be modified under various external stressors.…”

Rheological and Biophysical Properties of Living Fluids Under Shear: Active Suspensions ofSynechocystissp. CPCC 534

“…Although photosynthetic micro-organisms suspensions have attracted a large amount of attention in recent years as sources of high-value chemical products and biofuels, their physical properties, especially under flow conditions that occur in practical bioreactors, have not been sufficiently studied. Recent studies show that some fundamental physical properties of active suspensions, such as the diffusion coefficient, exhibit unexpected behavior and have therefore prompted extensive research [22][23][24]. Because individual bacterial motions have a large random component, it seems plausible to search for the origin of these unexpected physical properties of active fluids in bacterial locomotion and signal transduction.…”

“…Vourc'h et al [22] proposed a mathematical model for the decrease in diffusion coefficient based on the secretion of EPS by the micro-organisms. It has also been shown that surface hardness [23] as well as light intensity and direction [24] affect the diffusion dynamics of Synechocystis.…”