LAUSR

Cyanobacteria are major primary producers in coastal microbial mats and provide biochemical energy, organic carbon, and bound nitrogen to the mat community through oxygenic photosynthesis and dinitrogen fixation. In order to anticipate the specific requirements to optimize their metabolism and growth during a day-and-night cycle, Cyanobacteria possess a unique molecular timing mechanism known as the circadian clock that is well-studied under laboratory conditions but little is known about its function in a natural complex community. Here, we investigated daily rhythmicity of gene expression in a coastal microbial mat community sampled at 6 time points during a 24-h period. In order to identify diel expressed genes, meta-transcriptome data was fitted to periodic functions. Out of 24,035 conserved gene transcript clusters, approximately 7% revealed a significant rhythmic expression pattern. These rhythmic genes were assigned to phototrophic micro-eukaryotes, Cyanobacteria but also to Proteobacteria and Bacteroidetes. Analysis of MG-RAST annotated genes and mRNA recruitment analysis of two cyanobacterial and three proteobacterial microbial mat members confirmed that homologs of the cyanobacterial circadian clock genes were also found in other bacterial members of the microbial mat community. These results suggest that various microbial mat members other than Cyanobacteria have their own molecular clock, which can be entrained by a cocktail of Zeitgebers such as light, temperature or metabolites from neighboring species. Hence, microbial mats can be compared to a complex organism consisting of multiple sub-systems that have to be entrained in a cooperative way such that the corpus functions optimally.Circadian rhythms: molecular clocks in microbial matsMixed populations in microbial mats can have cooperative cycles of gene expression like daily circadian rhythms in individual organisms. Henk Bolhuis and colleagues at the Royal Netherlands Institute for Sea Research in the Netherlands studied variations in gene expression in a coastal microbial mat by sampling every six hours in a 24-h period. More than 1000 genes from a variety of microorganisms displayed circadian-like variation, indicative of coherent “molecular clock” processes governed by the 24-h day. The results suggest that microbial mats can be compared to more complex organisms that have different systems working in a cooperative and synchronized manner for the overall advantage of the constituent parts. This extends knowledge of microbial molecular clocks beyond the cyanobacteria and some algae, which until now were the only microorganisms known to display such activity.