MEPS 531:105-120 (2015)  -  DOI: https://doi.org/10.3354/meps11338

ABSTRACT: The kinetics of growth and siderophore secretion by Pseudoalteromonas haloplanktis, a γ-proteobacterium from Fe-deficient waters of the subarctic Pacific Ocean, were examined under Fe-limiting conditions. Hydroxamate siderophore concentration was about 4-fold higher in batch cultures grown in low than in high Fe medium. Addition of Fe to the Fe-limited cultures increased cell growth and electron transport system (ETS) activity and repressed siderophore secretion. In continuous culture, hydroxamate concentration increased by 4-fold and the net cellular hydroxamate siderophore secretion rate (NSSR) decreased by 20% as Fe was made more limiting to growth. An instantaneous decrease in chemostat dilution rate caused a reduction in ETS activity but had no effect on NSSR. However, pulsed addition of Fe to the chemostats significantly increased the hydroxamate concentration to a maximum within 6 to 12 h, which then subsequently declined as the cultures returned to steady state. The cellular NSSR doubled immediately after the perturbation and then dropped off to a minimum when ETS rates were fastest. Continuous addition of low levels of Fe to the chemostats increased the hydroxamate concentration in direct proportion to the quantity of Fe added, suggesting bacteria maintained tight control over Fe availability in the medium. The results show an unexpected increase in hydroxamate concentration in Fe-limited bacterial cultures following Fe enrichment and are consistent with the hypothesis that some of the Fe-binding ligands produced in iron-fertilized ocean waters are microbial siderophores.

KEY WORDS: Iron limitation · Heterotrophic bacteria · Siderophore secretion