Indeed, scientists recently discovered that cyanobacteria can anticipate seasonal change by sensing the extension and contraction of daylight hours, or photoperiods, from summer to winter. These changes occur over extended periods of time, in contrast to daily fluctuations in temperature and light, and rapidly dividing bacterial cells may not experience an entire photoperiod during their brief lifespans. Nevertheless, it is advantageous for organisms of all sizes to sense seasonal changes in their environment and adapt accordingly.
Circadian Rhythms: Nature's Daily Clock
Sensing photoperiodic change requires a circadian clock, a molecular mechanism that organisms from large multicellular mammals to plants, fungi and bacteria use to keep time internally. This molecular clock helps organisms structure and synchronize vast networks of biological processes, including hormone production, core body temperature and cell regeneration in humans over the 24-hour day. This circadian rhythm promotes appropriate responses to environmental cues, including changes in light and temperature.
The discovery of how circadian rhythms are controlled was so significant that in 2017, 3 pioneers in the field, Jeffrey C. Hall, Michael Rosbash and Michael W. Young, were awarded a Nobel Prize for their work.
Until the late 1980s, only eukaryotes were thought to have circadian rhythms, as bacteria were considered too simple on a molecular level to have, or need, such controls. However, this view was challenged when cyanobacteria, bacteria that are capable of photosynthesis, were discovered to have internal rhythms that allow them to restrict certain biological processes to specific times of day. For example, they tend to activate photosynthesis pathways at dawn and deactivate the same pathways in favor of respiratory pathways at dusk.
The cyanobacterial circadian clock is usually composed of 3 main proteins, KaiA, KaiB and KaiC. Together, these proteins form the circadian oscillator, which receives information about the environment from signaling molecules that are sensitive to the cell's internal conditions and regulates the timing of cellular processes like replication and expression of certain genes.
From Daily Rhythms to Seasonal Changes
Though it was accepted nearly 3 decades ago that bacteria can maintain a circadian rhythm, it was not known until recently that they can use this mechanism to sense seasonal changes over longer time periods. To investigate this, researchers grew a unicellular cyanobacterium, Synechococcus elongatus PCC 7942, under different photoperiods: more light than dark (long day), more dark than light (short day), or equal light and dark periods (equinox day). After growing under these conditions, the cells were plunged in an ice bath to assess their level of cold adaptation. The researchers found that cyanobacteria that had been exposed to short days survived the cold treatment better than those exposed to equinox or long days, suggesting that the short-day cells had used photoperiodic information to adapt to wintry conditions.
What's more, when the researchers deleted the kaiABC genes and repeated the experiment, the cells were equally susceptible to the cold, regardless of the day length they had experienced before. These findings demonstrated that a functional circadian clock, and the internal rhythm it creates, are necessary for cyanobacteria to adequately sense environmental changes over longer periods of time.
Yet, one uncharacteristically chilly day in early Fall is usually not enough to convince us that winter is here to stay-and cyanobacteria wouldn't be fooled, either. Scientists further demonstrated that the cold adaptation response required around 4 short days to fully develop, suggesting that the cells store information about the conditions they experienced the day before. However, it appears that the adaptation is also rapidly reversed-after just 24 hours in continuous light, short-day cells lost their cold resistance and survival advantage over long-day and equinox cells.
The Molecular Basis of Cold Adaptation
Looking more closely at the short-day cells, scientists identified changes in cell membrane composition. Namely, the cyanobacterial cells that had been grown in short-day conditions possessed desaturated lipids in their cell membranes, meaning they introduced carbon-carbon double bonds into the lipid chains. This is an important (known) mechanism of cold adaptation in bacteria, as it helps to produce kinks in the lipid chain and keep their cell membranes fluid even when temperatures drop. In addition, the short-day cells upregulated production of glycogen, which is a major energy storage compound that can help cyanobacteria to survive when they cannot perform photosynthesis.
It may seem puzzling that cyanobacteria should be able to sense and respond to seasons, when a single cell may not live long enough to experience a full seasonal shift. However, natural selection operates not on single bacterial cells, but the entire population-in this sense, the advantages of seasonal adaptation become clearer. A population that can sense change and adapt accordingly is more likely to survive conditions such as colder temperatures than populations that do not adapt, as the researchers showed in their ice bath experiment.
Research continues to show that bacteria are anything but simple organisms, Bacteria harbor many molecular mechanisms that are important in prokaryotes and eukaryotes alike, and this list now includes photoperiodic sensing and seasonal adaptation. Such discoveries highlight how ancient these mechanisms could really be and suggest that traits previously thought to be unique to eukaryotes may have evolved much longer ago.
Want to learn more about bacterial circadian rhythms in a variety of environments, including the human gut? Check out our next article!
