Nitrogen isotope ratio of cyanobacterial chlorophyll: Chemostat vs. batch culture

Nitrogen isotope records of chloropigments can be used to reconstruct photic zone processes. However, the 15N offset between chlorophyll and biomass (εpor=δ15Nbiomass−δ15Nchlorophyll) of cyanobacteria and eukaryotes grown in laboratory batch cultures differs significantly and the cause of the difference remains unknown. Here, in four experiments with Synechocystis sp. PCC 6803, values of εpor were statistically invariant when cultures were maintained at pH 6.5–9.5 under constant growth conditions in a chemostat. The results suggest that the negative values of εpor observed for freshwater cyanobacteria are not artifacts of batch culturing.