Differential Timing for Glucose Assimilation in and Coexistent Microbial Populations in the North Pacific Subtropical Gyre.

The marine cyanobacterium 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 genes involved in carbon metabolism and photosynthesis. Our results showed a diel variation for glucose assimilation in , with maximum assimilation at midday and minimum at midnight (~2-fold change), which was different from that of the total microbial community. This suggests that the timing in glucose assimilation in is coupled to photosynthetic light reactions producing energy, it being more convenient for to show maximum glucose uptake precisely when the rest of microbial populations have their minimum glucose uptake. Many transcriptional responses to glucose enrichment occurred after 12- and 24-h periods, but community composition did not change. High-light strains were the most impacted by glucose addition, with transcript-level increases observed for genes in pathways for glucose metabolism, such as the pentose phosphate pathway, the Entner-Doudoroff pathway, glycolysis, respiration, and glucose transport. While C assimilation from glucose represented less than 0.1% of the bacterium's photosynthetic C fixation, increased assimilation during the day and gene upregulation upon glucose enrichment indicate an important role of mixotrophic C assimilation by natural populations of Several studies have demonstrated that , the most abundant photosynthetic organism on Earth, can assimilate organic molecules, such as amino acids, amino sugars, ATP, phosphonates, and dimethylsulfoniopropionate. This autotroph can also assimilate small amounts of glucose, supporting the hypothesis that is mixotrophic. Our results show, for the first time, a diel variability in glucose assimilation by natural populations of with maximum assimilation during midday. Based on our previous results, this indicates that could maximize glucose uptake by using ATP made during the light reactions of photosynthesis. Furthermore, showed a different timing of glucose assimilation from the total population, which may offer considerable fitness advantages over competitors "temporal niches." Finally, we observed transcriptional changes in some of the genes involved in carbon metabolism, suggesting that can use both pathways previously proposed in cyanobacteria to metabolize glucose.