Microbes can't tell the time from a clock face, of course, but some bacteria have an internal timekeeping system that is synchronised with their environment. Similar to how our internal clock regulates sleep and wakefulness, some bacteria track time to switch between activities. It can be important for bacteria to know whether it is day or night and when it gets dark. Synchronising certain tasks with day and night allows them to make better use of the energy invested, to separate interfering processes in time or to be prepared for special conditions, such as harmful radiation from the sun.
We know the most about time measurement from photosynthetic bacteria.
Cyanobacteria, for example, obtain energy from
sunlight during the day through photosynthesis, similar to plants. At night, they feed on the sugar they have produced during the day. To determine the time of day, cyanobacteria have an internal clock.
How does this clock work? Cyanobacteria have a specific set of molecules that slowly bind to and release each other. This sequence lasts 24 hours and thus determines the tempo of the biological clock. Thanks to their internal clock, cyanobacteria can, for example, produce certain proteins that they need for photosynthesis in the morning before sunrise.
Just as we adapt to the new rhythm after a while when travelling to other time zones, cyanobacteria can readjust their internal clock, for example when we expose them to a longer period of darkness in the laboratory. In addition to photosynthesis, the internal clock of cyanobacteria also controls nitrogen fixation and cell division. This ensures that the right conditions are provided for the respective processes.
Some of our intestinal bacteria can also adapt to fresh food thanks to their internal clock. This gives these bacteria an advantage over competitors without a clock. Interestingly, melatonin, the hormone that maintains our day-night rhythm, also helps bacteria to tell the time.
The bacterium Aliivibrio fischeri, which lives in the specialised organs of Hawaiian dwarf squid, also has an internal clock. The bacteria emit light, which camouflages the squid when it hunts in the moonlit shallow waters of the ocean. Accordingly, the Aliivibrio bacteria glow in a diurnal rhythm, i.e. more intensely at night and less intensely during the day.
Read and watch more:
Cohen SE., Golden SS. (2015) Circadian Rhythms in Cyanobacteria. MicrobiolMolBiol Rev. 79(4):373-85
Video: Cyanobacterial Circadian Oscillator Animation
Johnson CH, Zhao C, Xu Y, Mori T. (2017) Timing the day: what makes bacterial clocks tick?. Nat Rev Microbiol. 15(4):232–242. doi:10.1038/nrmicro.2016.196
Wier, A. M. et al. Transcriptional patterns in both host and bacterium underlie a daily rhythm of anatomical and metabolic change in a beneficial symbiosis. Proc. Natl. Acad. Sci. 107, 2259–2264 (2010).
© Text: Christin Köbler and Nina Scheurer (AK Annegret Wilde) annegret.wilde[at]biologie.uni-freiburg.de
Ramya Ganesan und Laura Flórez, raganesa[at]uni-mainz.de / laflorez[at]uni-mainz.de
Figure: Ramya Ganesan; used according to CC4.0
