Responses of the marine bacterium Pseudomonas fluorescens to an excess of heavy metals: Physiological and biochemical aspects

A Pseudomonas fluorescens strain was isolated from oxic marine sediments obtained from the strand zone of the St Anne Bay (a moderately metal-contaminated site to the west of Cherbourgharbour).Thestrain,which exhibitedahigh tolerance tometal contaminationwhen cultivated(minimal inhibitory concentration=950 μM[62mgL−1] forZn, 660 μM[42mgL−1] forCu, and 505 μM [57mg L−1] for Cd), was further characterized by its physiological and biochemical responses tometal additions to the culturemedium. Bacterial growthwas significantly disturbed by 380 μM Zn (25mg L−1), 315 μM Cu (20mg L−1) and 90 μM Cd (10mg L−1). The Zn-containing alkaline phosphatase was studied as an intoxication biomarker. Its activitywas stimulated (+9%) by an excess of Zn, but inhibited by Cd (−55%) and Cu (−10%), these two elements could displace the native Zn or/and disturb the enzyme 3D-structure. BacterialO2 consumptionwas recorded as a global physiological response tometal stress. This parameter dropped with increasing Cd and Cu contamination (−49% and −45%, respectively, at 20 mg L−1). By contrast, Zn increased O2 consumption (approximately +40% for the different tested concentrations). The proteomes of bacteria grown in the presence or absence of 20 mg metal L−1 were characterized by 2D-gel electrophoresis. The number of spots exhibiting a difference in intensity between the contaminated sample and the control was 65, 68, and 103, for Zn, Cu and Cd, respectively. Among them, 45, 61 and 82 spots respectively appeared de novo or increased in intensity, indicative ofmetal-stimulated synthesis, particularly forCuandCd. Insummary,whereasCdand Cu treatments both stressed cells and slowed down primary metabolismto differing extents, Zn has a stimulating action on several physiological and biochemical parameters.