Figure 1.

Light/dark sampling regime and analysis of the photophobic response. (a) Light/dark conditions. A C. reinhardtii preculture was grown under a 15/9 h light/dark cycle for 2–3 d. This culture was then inoculated into several bottles and grown for 1–2 d more (total 3–4 d). On the day before the experiment, half of the bottles were kept under the regular illumination regime, and the other half were wrapped with aluminum foil and kept completely in the dark. Experiments were initiated ∼6 h after the beginning of the light phase. We prepared samples and/or observed cell behavior for each treatment group, and then switched the light conditions; i.e., light-adapted cells were placed in total darkness and vice-versa. Samples were taken 30 and 60 min after the light switch (arrows); for assessment of the in vivo redox state (Fig. 6), additional samples were taken at 1 and 120 min. For chloroplast mutants, precultures were grown in complete darkness for 6–7 d. 1 h before the experiment, half of the culture was transferred to the light. (b) The PPR. When C. reinhardtii cells perceive a sudden change in light intensity, they exhibit the PPR. Cells alter their flagellar waveform from an asymmetric (or ciliary) beat to a symmetric (or flagellar) wave (Phase I) and swim backward or simply stop. After a short period, the cell body rotates to randomize swimming direction (Phase II), and, subsequently, the cell returns to forward swimming. We measured the duration of Phase I, II, and total PPR of single cells by video microscopy.