Abstract
Microbial communities form the base of food webs in freshwater ecosystems, yet the interactions within these diverse assemblages are poorly understood. Based on evidence showing that primary production and respiration follow diurnal trends in lakes, we hypothesized that gene expression in freshwater microbes would have similar diel cycles. We used three 2‐d time series of metatranscriptomes to test this hypothesis in a eutrophic lake, an oligotrophic lake, and a humic lake. We identified prominent diel cycles in all three lakes, particularly in genes related to photosynthesis, sugar transport, and carbon fixation. The maximal time of expression for sugar transport genes tended to trail that of photosynthesis genes by several hours, indicating possible metabolic exchange between co‐occurring microbial lineages. These results provide an initial step in understanding sophisticated multispecies transcriptional organization within freshwater microbial communities.