  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.
