Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: (a) in constant conditions (i.e. constant temperature and either constant light {LL} or constant darkness {DD}) they oscillate with a period that is close to, but not exactly, 24 hours in duration, (b) this "free-running" rhythm is temperature compensated, and (c) the rhythm will entrain to an appropriate environmental cycle.
Until the mid-1980s, it was thought that only eukaryotic cells had circadian rhythms. It is now known that cyanobacteria (a phylum of photosynthetic eubacteria) have well-documented circadian rhythms that meet all the criteria of bona fide circadian rhythms. In these bacteria, three key proteins whose structures have been determined can form a molecular clockwork that orchestrates global gene expression. This system enhances the fitness of cyanobacteria in rhythmic environments.
Before the mid-1980s, it was believed that only eukaryotes had circadian systems.
In 1985–6, several research groups discovered that cyanobacteria display daily rhythms of nitrogen fixation in both light/dark (LD) cycles and in constant light. The group of Huang and co-workers was the first to recognize clearly that the cyanobacterium Synechococcus sp. RF-1 was exhibiting circadian rhythms, and in a series of publications beginning in 1986 demonstrated all three of the salient characteristics of circadian rhythms described above in the same organism, the unicellular freshwater Synechococcus sp. RF-1. Another ground-breaking study was that of Sweeney and Borgese.
Inspired by the research of the aforementioned pioneers, the cyanobacterium Synechococcus elongatus was genetically transformed with a luciferase reporter that allowed rhythmic gene expression to be assayed non-invasively as rhythmically "glowing" cells. This system allowed an exquisitely precise circadian rhythm of luminescence to be measured from cell populations and even from single cyanobacterial cells.
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A circadian rhythm (sərˈkeɪdiən), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrained by the environment). Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual.
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