Differential timing for glucose assimilation in Prochlorococcus and coexistent microbial populations at the North Pacific Subtropical Gyre

The marine cyanobacterium Prochlorococcus can utilize glucose as a source of carbon. However, the relative importance of inorganic and organic carbon assimilation and the timing of glucose assimilation are still poorly understood in these numerically dominant cyanobacteria. Here we investigated whole microbial community and group-specific primary production and glucose assimilation, using incubations with radioisotopes combined with flow cytometry cell sorting. We also studied changes in the microbial community structure in response to glucose enrichments and analyzed the transcription of Prochlorocccus genes involved in carbon metabolism and photosynthesis. Our results showed a circadian rhythm for glucose assimilation in Prochlorococcus, with maximum assimilation during the midday and minimum at midnight, which was different compared with that of the total microbial community. This suggests that rhythms in glucose assimilation have been adapted in Prochlorococcus to couple the active transport to photosynthetic light reactions producing energy, and possibly to avoid competition from the rest of the microbial community. High-light Prochlorococcus strains showed most transcriptional changes upon glucose enrichment. Pathways involved in glucose metabolism as the pentose phosphate, the Entner-Dudoroff, glycolysis, respiration and glucose transport showed an increase in the transcript level. A few genes of the low-light strains showed opposite changes, suggesting that glucose assimilation has been subjected to diversification along the Prochlorococcus evolution.