Variability in the production of organic ligands, by Synechococcus PCC 7002, under different iron scenarios

Several Fe-uptake mechanisms suggest the importance of the presence of certain organic ligands in phytoplankton exudates. Here, it has been studied how Synechococcus (strain PCC 7002) acclimates to Fe-bioavailability, comparing the growth and organic exudation under two different Fe regimes. These cyanobacteria were incubated in UV-treated seawater supplemented only with major nutrients and two different iron scenarios (Low-Fe and High-Fe), without chelating agents, in order to analyze the organic ligands production. The levels of dissolved organic carbon (DOC) and two natural ligands (hydroxamic and phenolic moieties) were monitored. The responses in the organic extracellular release rates (ER), normalized per cell, were statistically analyzed considering Fe scenarios and different development stages. Growth of Synechococcus was significantly slower under Low-Fe treatment, suggesting that these cultures were iron limited compared to those flourished with higher levels of iron in the medium. Although the concentration of DOC increased to 127.13 +/- 8.38 and 150.51 +/- 8.59 mu mol C L-1 under Low-Fe and High-Fe conditions, respectively, no-significant variations were found in the DOCER, among growth phases and iron bioavailability scenarios. Under High-Fe conditions, the production of hydroxamic ligands was inhibited, while the extracellular release rate of phenolic compounds decreased, regarding to Low-Fe conditions. Growth phases of Synechococcus also modified the extracellular release rates both of hydroxamic and phenolic moieties. The present study, therefore, demonstrates that iron availability and growth stages might be key parameters in regulating the release performance of extracellular Fe-specific organic ligands by cyanobacteria.