Circadian clocks, molecular timekeepers that can synchronize to 24-hour day/night cycles and thus allow cells to adapt to daily rhythms, have been characterized and studied in multicellular organisms for centuries. Their existence in single-celled organisms, on the other hand, has been questioned. In the 1980s and 90s, circadian clocks were found to regulate gene expression in photosynthetic bacteria. But, what about in bacteria who don’t directly depend on the sun for food?

This question was answered in a recent study published in Science Advances. It identified a circadian clock in the non-photosynthetic bacterium Bacillus subtilis, which is often found in the human gut and in soil. The authors observed that biofilm-forming cultures of these bacteria could synchronize their gene expression activities to 24-hour light or temperature cycles. 

A biofilm is a collection of microorganisms that are held together by a sticky extracellular matrix. Different different parts of the biofilm can take on specialized roles; in this way, bacteria in a biofilm act like cells in a tissue, displaying behavior similar to multicellular development. It is plausible that adaptation to daily rhythms is tied to biofilm formation or maintenance, but the exact function of this newly-discovered clock remain unclear. Time and further research will tell whether circadian clocks also play roles in bacteria that aren’t inclined to live in biofilms.