Ultradian and circadian rhythms of phototaxis in chlamydomonas reinhardii

We investigated phototactic and circadian rhythmicity in Chlamydomonas reinhardii under photoautotrophic (HSM) and photoorganotrophic (HSA) conditions to explore the influence of metabolic context on rhythmic behavior. Cultures were grown under controlled conditions (LD 12:12, 28 °C). Phototaxis was monitored using an automated multi-channel system that continuously recorded light transmission changes associated with photoaccumulation. Cell density and chlorophyll <i>a</i> were also quantified. Organisms incubated in continuous dim light showed a robust free-running circadian rhythm (∼24 h) with phase-dependent responses to prior light-dark entrainment, with the maximum rate of phototaxis occurring during the subjective day. The phototaxis rhythm could also be initiated by a decrease in light intensity, and showed temperature compensation for organisms in photoorganotrophic and phototoautotrophic growth media respectively over a temperature range 17°- 28 °C. Time series of phototactic activity were analysed using the GaMoSEC framework-a multiscale wavelet-based approach for identifying periodicities and quantifying rhythmic phase and power. The rhythm persisted longer under photoautotrophic conditions and was temperature-compensated (Q10 of 1.01 for HSM- and 1.07 for HSA-grown organisms). Wavelet analyses also revealed faster oscillations (60-80 min) superimposed on the circadian component, particularly under photoorganotrophic growth. These findings demonstrate that phototactic rhythmicity in <i>C. reinhardii</i> is influenced by metabolic conditions and light history and that GaMoSEC analysis effectively captures multiscale temporal organization in biological time series.