  Nearly all living organisms, including some bacterial species, exhibit
biological processes with a period of about 24 h called circadian (from the
Latin circa, about, dies, day) rhythms. These rhythms allow living organisms to
anticipate the daily alternation of light and darkness. Experiments carried out
in cyanobacteria have shown the adaptive value of circadian clocks. In these
experiments a wild type cyanobacterial strain (with a 24 h circadian rhythm)
and a mutant strain (with a longer or shorter period) grow in competition. In
different experiments the external light dark cycle was changed in order to
match the circadian period of the different strains, revealing that the strain
whose circadian period matches the light-dark has a larger fitness. As a
consequence the initial population of one strain grows while the other decays.
  These experiments were made under fixed light and dark intervals. However, in
Nature this relationship changes according to the season. Therefore, seasonal
changes in light could affect the results of the competition. Using a
theoretical model we analyze how modulation of light can change the survival of
the different cyanobacterial strains. Our results show that there is a clear
shift in the competition due to the modulation of light, which could be
verified experimentally.
