The influence of the slowing of Earth's rotation: A hypothesis to explain cell division synchrony under different day duration in earlier and later evolved unicellular algae

Every year the Earth's rotation period is reduced, mainly due to the tidal drag of the moon. The length of day increases continuously by about 1 h every 200 million years. The period of rotation around the Sun remains constant; hence, the length of the year remains constant, so years acquire progressively fewer days. Many unicellular algae show rhythmicity in their cell division cycle. If primitive algae evolved under a shorter day duration, then it is possible that the early-evolved algae had to synchronize their cell division cycle to shorter lengths of day than did later-evolved algae. We tested this hypothesis by growing Cyanobacteria, Dinophyceae, Prasinophyceae, Bacillariophyceae and Conjugatophyceae (evolutionary appearance probably in this order) at 8∶8 h light-dark cycles (LD), 10∶10 LD, and 12∶12 LD, at 20 or 27°C. Cyanobacteria synchronized their cell division cycles optimally at 8∶8 h LD, Dinophyceae and Prasinophyceae at 10∶10 h LD, and Conjugatophyceae and Bacillariophyceae at 12∶12 h LD. The synchrony of cell division was scarcely affected by temperature. Results suggested that the early evolved unicellular autotrophic organisms such as the Cyanobacteria synchronized their cell division cycle under a shorter day duration than later-evolved unicellular algae, and these traits may have been conserved by quiescent genes up to the present day.