Many animals base their reproductive behavior on the phases of the moon. But how they distinguish its light from other natural light sources has only been partially clarified so far. Using a marine bristleworm as an example, researchers have now identified a molecule that acts as a light sensor. Its reaction, which differs depending on the type of light, allows the animals to distinguish moonlight and sunlight and thus the different clocks.
The moon not only controls the tides of the seas and causes sleeplessness on full moon nights, but the celestial body also influences the reproduction of many marine organisms. For example, brown algae, fish, corals, turtles, and bristleworms synchronize their behavior and reproduction with the lunar cycle. In some species, such as the marine bristleworm Platynereis dumerilii, laboratory experiments have shown that moonlight serves as a clock for an internal monthly calendar in the bristleworm's body. All organs, cells and bodily functions then work in rhythm with this lunar calendar and are thus controlled by the moonlight.
moon or sun?
But in natural habitats, the light conditions fluctuate considerably - even if weather conditions such as cloud cover and storms are neglected, the regular interaction of the sun and moon creates highly complex patterns. How can the animals tell the difference? So far, this was unclear, but it seems likely that, similar to the human internal clock, certain genes and the molecules they produce play a role. "Several reports link the so-called cryptochromeres (CRYs) with the phases of the moon and suggest that these genes are involved in the measurement of circalunar time," explain Birgit Poehn from the University of Vienna and her colleagues. The scientists have therefore examined the genetics and biochemistry of a variant of these cryptochromeres using the example of the bristleworm Platynereis dumerilii
It was shown that the L-Cry molecule in particular reacts differently depending on the light stimulus: "We have now discovered that a light-sensitive molecule, L-Cry, is able to distinguish between different light values," reports Poehn. Since the moon and the sun radiate with very different light intensities, the Cryptochrom L-Cry can measure the intensity and duration of the incident light as a light sensor. This allows him to distinguish between the light sources and helps the animals choose the "right" light to time their internal timing system monthly.
Intensity matters
The cryptochrome L-Cry can therefore distinguish between different light sources based on intensity, but how exactly does this reaction work? As Poehn and her colleagues discovered, the molecule assumes different molecular states in response to exposure to light. It contains special co-factors called flavin adenine dinucleotides (FAD), which are normally adapted to darkness and which change to a different biochemical state when exposed to light. This is achieved in different ways depending on the light intensity and duration of the lighting. For example, under the influence of the comparatively weak moonlight, L-Cry proteins collect a small number of photons over several hours, with only about half of all FAD molecules changing into another state.
The situation is completely different with the significantly more intense sunlight, which has a more than 10,000-fold higher number of photons: As a result, all FAD molecules are biochemically changed within minutes. The researchers suspect that the L-Cry proteins take on different structural and biochemical properties depending on the state of the FADs. Through this interaction, the bristleworm can perceive and distinguish a wide range of natural light intensities. The scientists were also able to show that L-Cry changes its position in the cell depending on the light it is exposed to. How this different localization leads to different signaling pathways, which in turn control the behavior and physiology of the bristleworms, remains an open question.
Artificial light disrupts the lunar calendar
According to the research team, however, the basic mechanism of this molecular light sensor could also be present in creatures other than bristleworms: "It could be a more general mechanism that helps the organisms to interpret natural light sources," explains senior author Kristin Tessmar-Raible von the University of Vienna. “This is of key ecological relevance for any organism that controls its physiology and behavior through light. In addition, moonlight is not just a weaker version of sunlight, but has a completely different temporal and ecological significance for organisms.”
However, the sensitive response to different light sources also means that disturbances from artificial nocturnal light sources pose a serious threat to many natural ecosystems and also to human health. A better understanding of how moonlight is perceived and processed can also help to assess and limit the negative effects of artificial light.
Source: University of Vienna, specialist article: Nature Communications; doi: 10.1038/s41467-022-32562-z