Role of extracellular compounds in Cd-sequestration relative to Cd uptake by bacterium Sinorhizobium meliloti.

The role of bacterially derived compounds in Cd(II) complexation and uptake by bacterium Sinorhizobium meliloti wild type (WT) and genetically modified ExoY-mutant, deficient in exopolysaccharide production, was explored combining chemical speciation measurements and assays with living bacteria. Obtained results demonstrated that WT- and ExoY-strains excreted siderophores in comparable amounts, while WT-strain produced much higher amount of exopolysaccharides and less exoproteins. An evaluation of Cd(II) distribution in bacterial suspensions under short term exposure conditions, showed that most of the Cd is bound to bacterial surface envelope, including Cd bound to the cell wall and to the attached extracellular polymeric substances. However, the amount of Cd bound to the dissolved extracellular compounds increases at high Cd(II) concentrations. The implications of these findings to more general understanding of the Cd(II) fate and cycling in the environment is discussed.

Effect of competing ions and complexing organic substances on the cadmium uptake by the soil bacterium Sinorhizobium meliloti.

In an effort to improve the understanding and prediction of Cd uptake by soil bacteria, adsorbed and intracellular Cd were determined in unpolluted and highly polluted model soil solutions within a concentration range spanning from 10(-9) to 5 x 10(-5) M Cd. In parallel, the free Cd ion concentrations ([Cd(2+)]) were measured by a hollow fiber permeation liquid membrane. Obtained results demonstrated that Cd uptake by bacterium Sinorhizobium meliloti was related to [Cd(2+)] in the solution. Addition of different complexing organic substances reduced [Cd(2+)] in the bacterial medium and decreased both adsorbed and intracellular Cd. The adsorbed Cd was considerably reduced in the presence of 10(-4) to 5 x 10(-2) M [Ca] or [Mg]. No effect on Cd adsorption was observed in the presence of Zn or Mn, even at 100-fold excess. Intracellular Cd decreased in the presence of a high excess of Ca and Zn, while no significant effect was observed in the presence of Mg. An increase of dissolved Mn from 10(-9) to 10(-8) M resulted in a twofold decrease of the intracellular Cd, but no clear trend was observed in the presence of 10(-7) to 10(-6) M Mn. Based on complexation and competition studies, the stability constants, necessary for quantitative description of Cd uptake by S. meliloti and the development of the bacterial biotic ligand model for Cd, were derived and validated in the solutions containing mixtures of Cd, Zn, Ca, and Mg. Further numerical simulations of Cd uptake by S. meliloti exposed to soil pore waters demonstrated the importance of Zn competition and the insignificant influence of Ca and H on Cd uptake.

Nutrient dynamics at the sediment-water interface in a Mediterranean lagoon (Thau, France): influence of biodeposition by shellfish farming activities.

The Thau Lagoon, a French Mediterranean shallow lagoon, is a site where extensive shellfish farming occurs. The aim of the present work is to evaluate the role of this activity on nutrient exchange at the sediment-water interface in relation to organic matter (OM) sedimentation and degradation. Two stations inside (C5) and outside (C4) of the shellfish farming areas were sampled at three seasons. Porewater chemistry surveys and calculated diffusive fluxes were used to evaluate the trophic status of the Thau lagoon. Quantitative (Particulate Organic Carbon) as well as qualitative OM (Hydrogen Index, Carbohydrates) analyses were performed on sediments to assess OM characteristics. Results emphasized that surficial sediments at C5 are always more enriched in OM. Porewater nutrient concentrations are 10-20 times higher at C5 than at C4. In June 2003, the porewater profiles exhibit a sharp gradient at the bottom waters, indicating a hypereutrophic status, leading to an anoxic crisis.