In chronobiology, photoentrainment refers to the process by which an organism's biological clock, or circadian rhythm, synchronizes to daily cycles of light and dark in the environment. The mechanisms of photoentrainment differ from organism to organism. Photoentrainment plays a major role in maintaining proper timing of physiological processes and coordinating behavior within the natural environment. Studying organisms’ different photoentrainment mechanisms sheds light on how organisms may adapt to anthropogenic changes to the environment.
Background
24-hour physiological rhythms, known now as circadian rhythms, were first documented in 1729 by Jean Jacques d'Ortous de Mairan, a French astronomer who observed that mimosa plants (Mimosa pudica) would orient themselves to be toward the position of the sun despite being in a dark room. That observation spawned the field of chronobiology, which seeks to understand the mechanisms that underly endogenously expressed daily rhythms in organisms from cyanobacteria to mammals, which includes understanding and modeling the process of photoentrainment.
Two prominent 20th century chronobiologists, Jürgen Aschoff and Colin Pittendrigh, both worked throughout the 1960s to model of the process of photoentrainment, and despite examining the same subject, they arrived at different conclusions. Aschoff proposed a parametric model of entrainment, which assumed that organisms entrained to environmental timing cues (often referred to as zeitgebers, or "time givers" in German) gradually, changing their internal "circadian" period to be greater or less than 24 hours until it became aligned with the zeitgeber time. Conversely, Pittendreigh proposed a non-parametric model of entrainment, which assumed that organisms adjusted their internal clocks instantaneously when confronted with a light signal, or zeitgeber, that was out of sync with when their internal circadian time expected to see light. Pittendrigh developed his model based on the phase-response curve, which visualizes the effect of short light pulses on organisms that were free-running (not entrained to a zeitgeber). Pittendrigh determined that an organism’s response to light depended on when the signal was presented. It was determined that exposure to light in the organism’s early subjective night (the early portion of an organism’s “normal” dark period) produced a delay in onset of activity in the following day (phase delay). Additionally, light exposure in the late subjective night resulted in advanced activity in the following day (phase advance). The phase changes experienced by the organism could be represented by a phase-response curve consisting of portions including the advance zone, delay zone, and dead zone. This model became widely accepted over Aschoff's parametric model, but it is still unclear which model most effectively explains the process of photoentrainment.
Light intensity in conditions of constant light was found to also modulate an organism’s response. Exposure to higher-intensity light was found to either extend or shorten an organism’s period depending on species, dubbed Aschoff’s rule.
Mechanism
The molecular mechanism for photoentrainment in multicellular organisms such as in fungi and animals has been linked to the transcription-translation feedback loop (TTFL), where translated protein products influence gene transcription. The TTFL is composed of both a positive and negative arm, where the positive arm proteins promote transcription of negative arm genes while the negative arm proteins inhibit the activity of the positive arm. The TTFL has been found to be autonomous and have a period of roughly 24 hours. Components of the positive and negative arms differ by organism but in mammals positive arm components include CLOCK and BMAL1 while negative arm components include PER1, PER2, CRY1, and CRY2. In the case of many mammals, light signals detected by photoreceptors in the eye send signals to the mammalian master clock located in the suprachiasmatic nucleus (SCN) which then affects the timing of the various positive and negative arms. This results in changes in the expression of the various clock proteins is what allows the organism to undergo photoentrainment.
In single celled organisms, circadian rhythms are believed to be generated without the use of a TTFL, but rather with a 3 protein complex called the KaiABC Complex. The mechanism of entrainment in this system is known to be controlled by various proteins.