The effect of iron limitation on cyanobacteria major nutrient and trace element stoichiometry

Phytoplankton elemental stoichiometry provides a window into the interactions between marine nutrient distributions and phytoplankton growth. Here, we report the extended elemental stoichiometry of two non-diazotrophic cyanobacteria strains, Synechococcus WH7803 and Prochlorococcus MED4, in both Fe-replete and Fe-deplete media. Fe concentrations were reduced by two orders of magnitude in the Fe-deplete media, causing growth rates to decline by 38% and 24%, respectively. The average elemental composition of Fe-replete cells was (C76.5N19P1)1000Fe52.5Mn1.90Zn0.86Cu0.40Ni0.40Co0.05Cd0.0020, while Fe-limited cells averaged (C121N30P1)1000Fe12.2Mn3.22Zn1.70Cu0.41Ni0.36Co0.11Cd0.0038 The trace-metal stoichiometries measured here are similar to the ranges previously measured for a strain of Synechococcus and for many species of large eukaryotic phytoplankton. In contrast to large eukaryotic phytoplankton, which have previously exhibited either unchanged or decreased N : P under Fe-limited conditions, Synechococcus and Prochlorococcus increased N : P in Fe-deplete media by 58% and 67%, respectively. Previous studies have examined the direct role of Fe in limiting nitrogen fixation by diazotrophs, but this study suggests a second mechanism by which Fe may impact nutrient cycling, by influencing the N : P uptake ratio of non-diazotrophic cyanobacteria. A model of cyanobacteria distribution and Fe limitation is combined with the N : P stoichiometries measured here to suggest that up to 40% of the particulate organic nitrogen in the surface ocean might be associated with Fe-limited waters, with the strongest effect observed in the Prochlorococcus dominated subtropical Pacific. Thus, if the effect on N : P we observe in culture is widespread in the oceans it would mean that Fe-limitation could play a major role in global nitrogen and carbon cycling.