It sounds like the beginning of a joke: "How do bacteria know what time it is?" The surprising answer, reported today in Nature, is that some of these single-celled entities actually have internal clocks.

In a living organism, changes in gene expression, physiology and behavior that follow the cycle of day and night are called circadian rhythms. This oscillation is well known to occur in various mammals, insects, plants and fungi. But in recent years researchers have discovered that some single-celled organisms, too, display circadian rhythms. Irina Mihalcescu of the University Joseph Fourier in Saint Martin d'Heres, France, and her colleagues studied cyanobacteria, or blue-green algae, that had been engineered to light up when a particular regulatory gene turned on. By observing the pattern of illumination under a microscope, the scientists could measure the internal clockwork of individual bacteria.

Previous research had found that these photosynthetic bacteria keep a daily schedule even without stimuli from the outside world. Mihalcescu and her co-workers found that, under such constant conditions, the bacteria maintained a specific rhythm even following cell division--that is, the clock-setting was passed on to subsequent generations. To determine whether intercellular interactions influence this synchronicity, the team grew two cell colonies shifted in time by three hours--like New Yorkers and Californians. When the scientists combined the bacteria, individuals resisted changing their clocks even when butting up against those from a different time zone.

Such resiliency in internal timing is not seen in more complex single-celled organisms, nor in mammalian nerve cells. Instead, isolated cells of these nonbacterial organisms tend to "have rather 'sloppy' daily oscillations," writes Carl Hirschie Johnson of Vanderbilt University in an accompanying commentary. "Cyanobacterial clocks can dance alone with high precision." --Michael Schirber