Response of heterotrophic bacteria in a mesoscale iron enrichment in the northeast subarctic Pacific Ocean

The response of heterotrophic bacteria in a mesoscale iron (Fe) enrichment was measured in the northeast subarctic Pacific Ocean in July 2002. Addition of FeSO4 increased the dissolved Fe concentration in the fertilized patch to 2-3 nmol L-1 and triggered an increase in concentration of Fe(III)-binding ligands that complexed all of the dissolved Fe. Two to three days later, leucine incorporation rate and specific growth rate of bacteria doubled. Physiological markers of bacterial Fe nutritional state varied during the experiment as the microbial community assimilated the added Fe. Cellular uptake rate of an iron-siderophore complex, Fe-55-ferrioxamine B (FB), increased twofold to fourfold over background values and then declined by day 4.5. The fastest rate of Fe-FB uptake on day 2.5 coincided roughly with a transient increase in outer-membrane, Fe-55-FB-binding protein(s) in bacteria and with the peak in ligand concentration. Maximum potential uptake rate of inorganic Fe (V-max) was 8 zmol Fe bacterium(-1) h(-1) prior to Fe enrichment and then decreased by a factor of four within 2.5 d of fertilizing the patch as bacteria became Fe sufficient. V-max gradually increased by day 6.5 as the bacterial community re-entered iron deficiency. Similar changes in growth and Fe uptake kinetics were observed after a second Fe addition. Heterotrophic bacteria in the subarctic Pacific were Fe-deficient and responded directly to Fe addition by up-regulating pathways for Fe-siderophore acquisition and assimilating complexed Fe. The observation that increases in Fe uptake pathways and production were synchronous is consistent with the hypothesis that bacterial growth was directly limited by Fe.