Microscopic pacemakers--also known as circadian clocks--are found in everything from pond scum to human beings and appear to help organize a dizzying array of biochemical processes. Despite the important role that they play, scientists are just beginning to understand the benefits that these internal pacemakers provide when they work and the problems they cause when they malfunction.
A study performed by researchers at Vanderbilt University and published in the Aug. 24 issue of the journal Current Biology sheds new light on this issue. Using blue-green algae--the simplest organism known to possess these mechanisms--the researchers report that the benefits of biological clocks are directly linked to environments with regular day/night cycle and totally disappear in conditions of constant illumination.
"Circadian clocks are so widespread that we think they must enhance the fitness of organisms by improving their ability to adapt to environmental influences, specifically daily changes in light, temperature and humidity," says Carl H. Johnson, professor of biological sciences and Kennedy Center investigator who directed the study. "Some people have even suggested that, once invented, these clocks are such a powerful organizational tool that their benefits go beyond responding to external cycles. However, there have been practically no rigorous tests of either proposition."
To test these ideas directly, Johnson's research team used genetic engineering techniques to completely disrupt the biological clocks in one group of algae and to damp the frequency of the clocks in a second group. The researchers were careful to employ "point" mutations in the clock genes that didn't stunt the growth of the microscopic plants.
They then mixed the algae with disrupted clocks with algae with normally functioning clocks. When the mixture was placed in an environment with a 24-hour day/night cycle, the normal algae grew dramatically faster than those that lacked functional internal timers. The normal algae also outperformed the algae with the damped clocks, but by a smaller margin.
The result was presaged by a series of experiments that Johnson conducted in 1998 with Susan S. Golden from Texas A&M University and Takao Kondo from Nagoya University. In the previous experiments, the researchers created two new algae strains with clocks of 22 hours and 30 hours. (The frequency of the biological clocks in normal blue-green algae is 25 hours.) They created mixed colonies by combining the strains in pairs: wild type and 22 hour; wild type and 30 hour; 22 hour and 30 hour. Then they put these mixed cultures into incubators with three different light-dark cycles--22 hours, 24 hours and 30 hours--and monitored them for about a month.
When they pulled the cultures out, the researchers found that the strain whose internal clock most closely matched the light-dark cycle invariably outgrew the competing strain. In fact, they found that the selective advantage of having the correctly tuned biological clock was surprisingly strong: The strains with matching frequencies grew 20 to 30 percent faster than the out-of-synch strains.
The second part of the current experiment was designed to test whether the biological clocks also provide an intrinsic advantage, a hypothesis advanced by the late Colin Pittendrigh of Stanford. He suggested that circadian clocks might be beneficial even in an unchanging environment. There was some indirect support for this proposition. In one experiment, for example, populations of the fruit fly, Drosophila melanogaster, were raised in constant illumination for hundreds of generations. Nevertheless, their biological clocks continued to function, suggesting that they continue to have adaptive value.
When the algae strains were placed in a chamber with constant light, however, the researchers were surprised to discover that the shoe was on the other foot: The algae with the disrupted internal clock divided and grew at a slightly faster rate than their clockwatching cousins, both those with natural biological clocks and those whose clocks were damped.
"This was the most surprising result of our study," says Johnson. "Under constant conditions, the circadian clock system is of no benefit and, in fact, might even be bad for the algae."
The scientist doesn't know for certain why this happens, but he has some ideas. The microscopic plants use their biological clocks to turn their photosynthesis system on and off. In a normal 14-hour day/night cycle, this allows the microscopic plant to maximize the amount of chemical energy it can extract during daylight.
"In constant illumination, however, the biological clocks may keep shutting down photosynthesis in expectation of the darkness that never comes," says Johnson.
Co-authors on the study are post-doctoral fellows Mark A. Woelfle and Yan Ouyang and graduate student Kittiporn Phanvijhitsiri. The research was supported by the National Institutes of Health.
For more news about Vanderbilt research, visit Exploration, Vanderbilt's online research magazine at www.exploration.vanderbilt.edu
Journal
Current Biology