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 nondiazotrophic 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.