Kinetics of siderophore production by a marine bacterium, Pseudoalteromonas haloplanktis

• Main Author: Sijerčić, Ada.
• Format: Thesis
• Language: English
• Published: McGill University 2008
• Subjects: Marine bacteria; Siderophores; Pseudoalteromonas; Pacific; Subarctic
• Online Access: http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=116077

Siderophores are secreted by marine bacteria to increase Fe uptake when Fe is limiting but are not produced when sufficient Fe is present to saturate growth. These results are well established in laboratory batch cultures of a number of isolates obtained from the open sea. Little is known, however, regarding the kinetics of siderophore secretion by heterotrophic bacteria in response to transients in Fe deprivation and resupply. We examined growth, hydroxamate siderophore concentration, and electron transport chain activity (a biochemical measure of Fe nutritional state) of Pseudoalteromonas haloplanktis, a representative gamma-proteobacterium from the Fe deficient region of the subarctic Pacific Ocean. Hydroxamate concentration was roughly 5-fold higher in batch cultures grown in low than in high Fe medium. Iron injection to the low Fe cultures repressed hydroxamic acid production and increased growth and ETC activity. Steady-state hydroxamate concentration in the chemostat increased 5-fold as Fe-limited growth rate declined from 9.8 to 2.8 d -1. This increase compounded to a 2.8-fold change in hydroxamates cell-1 reflecting the greater costs of growth at low Fe. Three types of Fe perturbation were made to Fe-limited chemostat cultures: 1) A switch perturbation that decreased the dilution rate of the chemostat-by ∼3-fold caused a transient increase in cell density that subsequently declined to a new steady state level. Hydroxamate concentration increased linearly over the same time. 2) A transient addition of dissolved Fe increased the total hydroxamate concentration in the chemostat within 1-3 hours which was followed by a decrease and then subsequent increase as the cells re-entered Fe-limitation. Dilution rate affected the response. Normalized to bacteria density, hydroxamate concentration remained constant for the first 2 hours after the Fe addition and then declined and returned to pre-infusion levels. Thus, Fe addition stimulated siderophore production by increasing the density of bacteria, which continued to secrete hydroxamates at a Fe-limited rate. 3) A continuous addition of low levels of dissolved Fe increased bacteria density and siderophore concentration. The net secretion rate of siderophores was proportional to the increase in Fe supply rate to the chemostat. At high Fe concentration, hydroxamate concentration declined to undetectable levels as the bacteria became Fe-sufficient and C-limited. Siderophore secretion by Fe-limited P. haloplanktis was repressed after 2 hours (corresponding roughly to 1-2 cell generations) following Fe re-supply.