Metatranscriptome sequence analysis reveals diel periodicity of microbial community gene expression in the ocean’s interior

Previous work has revealed diel periodicity of gene expression in key metabolic pathways in both autotrophic and heterotrophic microbes in the surface ocean. In this study, we investigated patterns of diel periodicity of gene expression in depth profiles (25, 75, 125 and 250 meters). We postulated that microbial diel transcriptional signals would be increasingly dampened with depth, and that the timing of peak expression of specific transcripts would be shifted in time between depths, in accordance with depth-dependent diel light variability. Bacterioplankton were sampled from four depths every four hours at station ALOHA (22° 45’ N 158° W) over 2 days. RNA was extracted from cells preserved on filters, converted to cDNA, and sequenced on the Illumina platform. Surprisingly, harmonic regression analysis revealed an increasing proportion of genes with diel periodic expression patterns with increasing depth between 25- 125 meters. At 250 meters, the proportion of genes exhibiting diel expression patterns decreased an order of magnitude compared to the photic zone. Community composition, functional gene categories, and diel patterns of gene expression were significantly different between the photic zone and 250 meter samples. The signals driving diel periodic gene expression in microbes at 250 meters is under further investigation. These data are now beginning provide a better understanding of the tempo and mode of microbial dynamics among specific taxa, throughout the ocean’s interior.