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

Abstract

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 Cherbourg harbour). The strain, which exhibited a high tolerance to metal contamination when cultivated (minimal inhibitory concentration = 950 μM [62 mg L − 1 ] for Zn, 660 μM [42 mg L − 1 ] for Cu, and 505 μM [57 mg L − 1 ] for Cd), was further characterized by its physiological and biochemical responses to metal additions to the culture medium. Bacterial growth was significantly disturbed by 380 μM Zn (25 mg L − 1 ), 315 μM Cu (20 mg L − 1 ) and 90 μM Cd (10 mg L − 1 ). The Zn-containing alkaline phosphatase was studied as an intoxication biomarker. Its activity was 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. Bacterial O 2 consumption was recorded as a global physiological response to metal stress. This parameter dropped with increasing Cd and Cu contamination (− 49% and − 45%, respectively, at 20 mg L − 1 ). By contrast, Zn increased O 2 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 of metal-stimulated synthesis, particularly for Cu and Cd. In summary, whereas Cd and Cu treatments both stressed cells and slowed down primary metabolism to differing extents, Zn has a stimulating action on several physiological and biochemical parameters.