Characterization of ferredoxin and flavodoxin as molecular indicators of iron limitation in marine eukaryotic phytoplankton

Abstract

Abstract : Expression and regulation of the proteins ferredoxin and flavodoxin were investigated to assess their usefulness as biomarkers of iron limitation in marine phytoplankton. A phylogenetic survey of seventeen species found replacement of ferredoxin by flavodoxin to be a common response to iron stress. A minority of organisms examined never expressed flavodoxin, a condition associated with, but not characteristic of, neritic habitats. Flavodoxin induction responded specifically to iron limitation but not to nitrogen, phosphorous, silicate, zinc or light deficiency. Under conditions of iron stress, the relative cellular ferredoxin and flavodoxin content varied with iron-limited growth rate and not affected by growth on nitrate or ammonium as a sole nitrogen source. HPLC measurements of ferredoxin and flavodoxin were successfully used to monitor phytoplankton response to iron addition during the IronExII mesoscale enrichment experiment. A persistent flavodoxin signal observed in the absence of ferredoxin resynthesis suggests that Fe addition alleviated iron starvation but was insufficient to completely relieve physiological iron limitation. Results of a laboratory iron addition experiment investigating differences between changes in cellular ferredoxin/flavodoxin content and photosynthetic efficiency were combined with data from IronExII to construct a model that describes a complementary relationship between the two measures.