Biological activities are rhythmic and repeated at regular time intervals. The various types of biological activities of eukaryotes are respiration, eating, sleeping, defecation, reproduction, flowering, nesting, migration, hibernation and the list is endless. These activities are also dependent on environmental factors like light, temperature and humidity while as some activities are instinctive (borne behavior). Rhythmic biological activities or biological rhythms are generally of three types: Circadian, Circannual and Breeding cycles. Biorhythms are present in both animals and plants. Circadian stands for biological activities related to cycles of day and night (a cycle of day and night makes one day of 24h). This rhythmic periodicity roughly accounts for 24 hours. This periodic response is also called as internal biological clock or circadian clock. The principle criteria of circadian rhythm are: the persistent 24h oscillation in constant conditions of growth meaning that they remain for 24h period provided the conditions are constant; temperature compensation that is the periodicity of rhythm is stable with different temperature ranges and the entrainment or resetting of circadian rhythms by environmental signals.
About three decades back it was unknown that microbes also possess biorhythms and circadian rhythms in particular. Based on three criteria and their evidence, now it has been proved that prokaryotes like bacteria, fungi, cyanobacteria and yeasts show circadian rhythms. Their rhythmicity is mainly the response to environmental changes like temperature and light particularly. This also reflects adaptive capacity of microbes. Circadian rhythm is still not proved in viruses or bacteriophages but the scientists are hopeful.
Cyanobacteria and photosynthetic bacteria: In cyanobacteria (Synechococcus, Gloeobacter and Pseudoanabaena) photosynthesis and nitrogen fixation are controlled by circadian rhythm. They show rhythmic nitrogen fixation cycles. Both the processes do not occur at the same time as the enzyme nitrogenase of nitrogen fixation process is highly sensitive to oxygen. The oxygen is released during photosynthesis and it may inhibit nitrogen fixation if both the processes would occur simultaneously. Photosynthetic bacteria like purple S (Chromatium, Thiocystis, and Thiospirillum) and non S (Rhodospirillum, Rhodomicrobium etc) bacteria are sensitive to light and darkness of day and night respectively. They consist of a protein assembly which functions as central oscillator or circadian clock. It control light dependent phosphorylation and dephosphorylation of photosynthesis and respiration in these bacteria.
Chemotrophic nonphotosynthetic bacteria: This is best exemplified with root associated bacteria. The interactions between plant and rhizobacteria are enriched because of root exudates. Rhizobacteria are dependent on root exudates for carbon and energy source. There is variation in root exudation of different plants with respect to day and night cycles, temperature and soil type. The composition of root exudate secreted in day is different from that exuded at night. Depending upon the changes in exudates, rhizobacteria also respond to this circadian periodicity pattern of exudation and subsequent interactions with the host plant. The rhythmic periodicity has also been observed in bacterial cell division and growth cycle.
Fungi: Circadian clock systems have elucidated in Neurospora, Ascobolus, Podospora and Nectria. These fungi have shown periodicity with respect to changes in growth media or hyphal growth, sporulation or metabolic activities.
Yeasts: In budding yeasts expression of cellular and metabolic activities are coordinated and shown to follow high periodicity even in nutrient stress conditions. Saccharomyces cerevisiae and its genetic system have been investigated as molecular model to study circadian clocks in organisms.
Viruses: There has been no evidence of circadian rhythms in viruses. But simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) are responsible for disrupting biological clock and cause circadian clock related disorders in infected humans and other mammals.
Thus periodicity seems to be very important in functioning of both prokaryotic and eukaryotic organisms; even bacteria are concerned about time. The examples of microbial circadian rhythms are scanty and very complex to prove scientifically as difference between temporal responses or adaptations or quorum sensing or circadian rhythms is hard to distinguish. This field of study has great research potential and therefore warrants further investigations.
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