Into the darkness of the microbial dark matter activities through expression profiles of populations.

form a highly diverse and widespread superphylum of uncultured microorganisms representing a third of the global microbial diversity. Most of our knowledge on putative physiology relies on metagenomic mining and metagenome-assembled genomes, but the activities and the ecophysiology of these microorganisms have been rarely explored, leaving the role of in ecosystems elusive. Using a genome-centric metatranscriptomic approach, we analyzed the diel and seasonal gene transcription profiles of 18 populations in brackish microbial mats to test whether our understanding of metabolism allows the extrapolation of their activities. Although our results revealed a circadian cycle in activities, a strong streamlined genetic expression characterized the populations. This result has a major consequence for the extrapolation of their physiology and environmental function since most transcribed genes were uncharacterized, indicating that the ecophysiology of cannot be yet reliably predicted from genomic data.