Freshwater lakes play a critical role in the global carbon budget. Given that microbes are at the center of the carbon cycle in freshwater, understanding how their metabolic processes are affected by their environment is of interest. In surface freshwater, sunlight is both an environmental cue and source of energy for primary producers and other bacteria. Within freshwater bacterial communities, the Actinobacteria are highly abundant. Multiple species within the Microbacteriaceae family of the Actinobacteria phylum have putative proteins that capture light energy, and while light mediated gene expression of phototrophs has been well characterized, in heterotrophs it is not as well understood. Here, we characterize transcription throughout a light/dark cycle of R. lacicola and Aurantimicrobium sp. strain MWH-Mo1, which are Microbacteriaceae strains that grow faster in light than in dark. Although both strains share a phenotype, their gene expression changes through a light/dark cycle are quite different. R. lacicola has a cyclical response, where transcripts of genes involved in mediating oxidative damage, iron-sulfur assembly, and transcription are downregulated in the light and upregulated in the dark, while transcripts of growth-related genes involved in gluconeogenesis, DNA replication, and amino acid uptake are upregulated in the light and down regulated in the dark. In Aurantimicrobium sp. strain MWH-Mo1, porphyrin synthesis is upregulated in the light and downregulated in the dark. Additionally, dark triggers downregulation of molecular chaperone genes. This indicates the light-enhanced growth phenotype and the ability to respond to light/dark may be widespread in freshwater Actinobacteria, but the genetic networks controlled by light/dark may vary significantly.
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