Molecular assessment of phosphorus and iron physiology in Trichodesmium populations from the western Central and western South Atlantic

Abstract

Trichodesmium is capable of responding to low phosphorus (P) and low iron (Fe) concentrations through the induction of alkaline phosphatase (AP) activity (an enzyme that hydrolyzes inorganic phosphate from phosphomonoesters) and the IdiA protein (an iron deficiency protein that putatively binds iron), respectively. We monitored AP activity and IdiA expression as molecular diagnostics of P and Fe stress in Trichodesmium field samples on a transect from the western central to the western South Atlantic and compared the expression of these markers with physical, chemical, and N2 fixation measurements. Trichodesmium AP activity was detected at many stations, whereas IdiA expression was not. Incubation additions of inorganic P at one station resulted in a loss of AP activity and an increase in N2 fixation. Our data suggest that P, and not Fe, is the constraining factor limiting Trichodesmium N2 fixation along most of the transect. These results support previous modeling efforts predicting that diazotrophs in this region are P‐stressed and further validate the use of molecular diagnostics of nutritional physiology, in concert with more traditional approaches, for identifying constraints on marine N2 fixation.