Contribution to the Themed Section: Scaling from individual lankton to marine ecosystems Diel variability in the elemental composition of the marine cyanobacterium Synechococcus

The ratio of elements such as carbon: nitrogen: phosphorus (C:N:P) in phytoplankton is known to vary substantially within single isolates and across environmental gradients. In addition, C: N: P is known to vary throughout the day due to diel patterns in nutrient acquisition and storage. It has been hypothesized that small phytoplankton such as marine cyanobacteria have relatively invariable elemental ratios during a 24 h period, whereas larger phytoplankton have a greater capacity to store elements and thus a wider diel range of C: N: P. To test this hypothesis, we examined diel variability in cellular C: N: P, using a chemostat culturing system, for one of the most abundant marine cyanobacteria, Synechococcus (WH8102) during two 24 h periods. The cellular C quota nearly doubled during the 14 h light period and was subsequently reduced during the dark period. The cellular N quota also varied considerably, whereas the P quota remained relatively stable. These daily changes in elemental quotas led to highly variable C: Ncell and C: Pcell. Furthermore, the magnitude of variability in cellular elemental stoichiometry of Synechococcus was positively related to the growth rate. We constructed a model to test the extent to which variation in C: Ncell and C: Pcell is related to reserve carbon accumulation and depletion over each light-dark cycle. Results imply that, in addition to growth-related respiratory losses, Synechococcus also purges excess C during the dark period in order to maintain a nutritive balance within cells. Our data suggest that diel variation in C: Ncell and C: Pcell of Synechococcus is of the same order of magnitude as stoichiometric variation within plankton communities between major ocean environments.