Abstract
In this investigation, the phototactic behavior of the swimmer alga Chlamydomonas reinhardtii was studied in oscillatory flows. Two light-emitting diodes (LED) were placed next to one of the lateral walls of a long PDMS microchannel of rectangular cross-section (width = 500 μm, height = 85 μm) which was enclosed in a dark box. Oscillating flow of algal suspension was generated using a pressure-driven control system. Both low and high concentrations of the cells were considered. The short- and long-term effects of light exposure on the cells swimming, and the influence of amplitude and frequency of flow oscillation in the presence of light stimulus were determined.
It was shown that if the period of flow oscillation is shorter than the response time of the cells to a light stimulus (about 1 second), the cells cannot react immediately to the light and thus, the increase in the percentage of cells population showing a negative phototaxis (Nup) occurs later compared to the cases where the oscillation period is long enough (low frequencies). Moreover, it was shown that a smaller oscillation amplitude results in a sharper and earlier increase in Nup since the cells are less affected by the flow advection and respond more efficiently to the light stimulus.
Regarding the long-term effects of light exposure, it was shown that the phototactic behavior of the cells is a function of time, in both low and high concentrated suspensions; however, this evolution with time depends on the concentration. Over a cycle of light exposure on the dilute suspensions, Nup increases in the beginning of the cycle until reaching a peak; then it decreases to the initial value indicating that the effect of light is no longer perceptible by the cells.
