Adaptation of Chlamydomonas phototaxis: I. A light‐scattering apparatus for measuring the phototactic rate of microorganisms with high time resolution

Phototaxis of the unicellular alga Chlamydomonas was studied with subsecond time resolution by using a newly developed taxigraph. The taxigraph determines the cell density in a particular volume element of a cuvette by measuring the amount of scattered light originating from the cells in this region. When the cell density is kept below 106/ml, a linear relationship exists between the scattered photon irradiance and the number of scattering particles. Time‐dependent scattering changes can be used to determine direction and extent of phototactic activity as well as the time course of various adaptation processes. This communication describes design and performance of the taxigraph in detail and compares results obtained from Chlamydomonas cell populations with those obtained from single‐cell analysis by using a computer‐aided motion analysis system. The high time resolution of the taxigraph permits the study of rapid adaptational processes. Chlamydomonas strain 806 cells, which have been reported to show exclusively negative phototaxis, were found to turn transiently towards the light upon a rapid change in irradiance, before eventually moving away from it. The duration of the initial positive phototactic response was critically dependent on the magnitude of the irradiance increment. Adaptation to a step‐up stimulus was consistently faster than to a step‐down stimulation. The statistical nature of the switch from positive to negative phototaxis is demonstrated by single‐cell observation. Different adaptation levels were characterised by stimulus‐response curves, either in the presence of a constant background or following a defined delay after long previous irradiation. To describe the observed behaviour the existence of two adaptation processes, occurring on a vastly different time scale, must be anticipated: a rapid (seconds) background adaptation and a slow desensitisation in steady light which is completed in 30‐40 min.