Bacterial circadian rhythms

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. 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. The conclusion that only eukaryotes have circadian oscillators seemed reasonable, because it was assumed that an endogenous timekeeper with a period close to 24 hours would not be useful to prokaryotic organisms that often divide more rapidly than once every 24 hours. The assumption might be stated as, 'why have a timer for a cycle that is longer than your lifetime?' While intuitive, the conclusion was flawed. It was based on the assumption that a bacterial cell is equivalent to a sexually reproducing multicellular organism. However, a bacterial culture is more like a mass of protoplasm that grows larger and larger and incidentally subdivides. From this perspective, it is reasonable that a 24-hour temporal program could be adaptive to a rapidly dividing protoplasm if the fitness of that protoplasm changes as a function of daily alterations in the environment (light intensity, temperature, etc.). In 1985–86, 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, who were the first to demonstrate temperature compensation of a daily rhythm in the marine cyanobacterium, Synechococcus WH7803. 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. The luminescence rhythms expressed by these transformed S. elongatus fulfilled all three key criteria of circadian rhythms: persistence of a 24-hour oscillation in constant conditions, temperature compensation, and entrainment. Thus, the work with various Synechococcus species firmly established that prokaryotic bacteria are capable of circadian rhythmicity, displacing the prior 'no circadian clocks in prokaryotes' dogma. Nevertheless, persuasive evidence for circadian programs in bacteria other than the cyanobacteria is still lacking. Despite predictions that circadian clocks would not be expressed by cells that are doubling faster than once per 24 hours, the cyanobacterial rhythms continue in cultures that are growing with doubling times as rapid as one division every 5–6 hours. Apparently cyanobacteria are able to simultaneously and accurately keep track of two timing processes that express significantly different periods. Do circadian timekeepers enhance the fitness of organisms growing under natural conditions? Despite the expectation that circadian clocks are usually assumed to enhance the fitness of organisms by improving their ability to adapt to daily cycles in environmental factors, there have been few rigorous tests of that proposition in any organism. Cyanobacteria are one of the few organisms in which such a test has been performed. The adaptive fitness test was done by mixing cyanobacterial strains that express different circadian properties (i.e., rhythmicity vs. arhythmicity, different periods, etc.) and growing them in competition under different environmental conditions. The idea was to determine if having an appropriately functional clock system enhances fitness under competitive conditions. The result was that strains with a functioning biological clock out-compete arhythmic strains in environments that have a rhythmic light/dark cycle (e.g., 12 hours of light alternating with 12 hours of darkness), whereas in 'constant' environments (e.g., constant illumination) rhythmic and arhythmic strains grow at comparable rates. Among rhythmic strains with different periods, the strains whose endogenous period most closely matches the period of the environmental cycle is able to out-compete strains whose period does not match that of the environment. Therefore, in rhythmic environments, the fitness of cyanobacteria is improved when the clock is operational and when its circadian period is similar to the period of the environmental cycle. These were among the first rigorous demonstrations in any organism of a fitness advantage conferred by a circadian system.