One of the best examples demonstrating the importance of circadian clocks for coping with an environment that has a daily structure, is the unicellular alga <i>Gonyaulax polyedra</i>. It belongs to the phylum of dinoflagellates. And the oldest fossil records of dinoflagellates go back as much as 400 million years. This single cell, with a diameter of approximately 13 micrometers, uses a complex circadian program to solve a problem. During the day <i>Gonyaulax</i> swims at the surface of the ocean, harvesting energy and fixating carbon from light with the help of photosynthesis. But cells need more than energy and carbohydrates to survive and proliferate. And that creates a dilemma for <i>Gonyaulax</i>. While surface waters are the best place to do photosynthesis, they are almost void of nutrients, such as nitrate, phosphate, sulphur or other nutrients that are necessary for metabolism. The source of these nutrients are decaying organisms. And dead organisms sink. That's why these nutrients can be found in higher concentrations much deeper down in the ocean at thermoclines. Which are horizontal temperature barriers, or at the bottom. For this reason <i>Gonyaulax</i> and many other plankton migrate huge distances to deeper nutrient rich waters. And travel back to the surface to photosynthesize. Although <i>Gonyaulax</i> has flagella to swim, it could never actively swim these large distances. That's why it changes its density, so that it can either sink down or float upwards. For <i>Gonyaulax</i>, energy and nutrients are separated in different spatial niches, surface water, and depth. In addition, their energy source, sunlight is only available during a temporal niche, the day. The circadian program of these algae combines these spatial and temporal structures of the environment into an intelligent program. At the end of the day, the cells increase their density and begin their long journey towards deeper waters and nutrients. At the same time, they start to synthesize a biochemical machinery that allows them to emit light, so called bioluminescence. The blue green light emitted by dinoflagellate gives rise to the most beautiful nightly scenes. During the night, <i>Gonyaulax</i> gathers nutrients deep down in the ocean. But before the sun rises, they produce small gas bubbles trapped inside their cells and start their journey back upwards. Where they form dense clouds of many aggregating cells. One can record both bioluminescence, and the formation of aggregations in the laboratory. Two phenomena that alternate and thus form two rhythms out of phase. These rhythms are true circadian rhythms in that they continue in constant conditions in the lab. But they are not the only circadian rhythms within this single cell. The synthesis and degradation of the entire bioluminescence machinery. The changes of cell density. Many of the components needed for photosynthesis are under circadian control. But also sensory functions like phototaxis, the ability to orient towards or away from a light source and gravitaxis the ability to orient up and down. Are controlled by this temporal program. As we will see later in this course, it is essential that circadian rhythms can be perturbed by special factors. This is the basis of keeping them in synchrony with their environment. The circadian system of <i>Gonyaulax</i> can be changed by both light and nutrients. The two important resources that make them travel large distances every day. It is essential that these trips up and down the water column are accurately timed and that <i>Gonyaulax</i> is able to anticipate when the sun will set and rise, long before it actually happens. All aspects of this complex daily behavior need to be planned ahead. And this is only possible with an internal clock, or endogenous temporal program. We now know the ecological importance for <i>Gonyaulax</i> to combine spatial and temporal niches. But we still don't know why they give off light at night. As a matter of fact, there are several hypotheses. One is the startling hypothesis that claims that the predators of <i>Gonyaulax</i> are startled when they collide with these flashing cells and move on. The other is the burglar alarm hypothesis, that claims the algae's predators have predators of their own. Which can easily identify their prey by the light, so that it is better for them to feed on lightless algae. My favorite hypothesis is the stay together hypothesis. <i>Gonyaulax</i> cells cannot afford to be diluted in the vast ocean. One of the reasons is that they come together for sexual reproduction once a year. During the day, when the cells swim towards the surface, they form dense clouds that stay together for physical reasons called convection. But when they sink to greater depth, these physical reasons cease. Interestingly, <i>Gonyaulax</i> becomes strongly attracted to light at night. In the lab, this attraction can be so strong that the algae kill themselves by swimming too near to a bright light source. The stay gather hypothesis claims that <i>Gonyaulax</i> produces light and swims towards this light. Thereby making sure that the swarms stay together also during the night. [SOUND]
