Monitoring of (bio)available labile metal fraction in a drinking water treatment plant by diffusive gradients in thin films.

A performance study of diffusive gradients in thin films (DGT) and inductively coupled plasma optical emission spectrometry (ICP-OES) was applied for the monitoring of the labile fraction of metals Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn, in Sant Joan Despí Drinking Water Treatment Plant located in the South of Barcelona's Metropolitan Area (Spain). The DWTP monitoring protocol was optimized by working for 1 day of deployment (24 h) with the DGT device in contact with both treated and river water matrixes. Additionally, it was demonstrated that an increase in the deployment time of 1 week did not decrease the evaluated concentrations of the studied metals. The quality parameters of the DGT device and ICP-OES determination, such as limit of quantification, accuracy expressed as relative error (%) and reproducibility expressed as relative standard deviation, were evaluated. Good results were obtained for all the metals in ultrapure water; limits of quantification ranged from 1.5 µg L( - 1) for cadmium to 28 µg L( - 1) for zinc when deployment time of 24 h was used and from 0.2 µg L( - 1) for cadmium to 4 µg L( - 1) for zinc when this time was increased by 1 week. Accuracy and precisions lower than or equal to 10% were obtained at a parametric concentration value of the metals regulated in the European Drinking Water Guidelines (98/83/EC). DGT deployment was tested in river and treated water, and good results were obtained for Cd, Ni, Co and Zn, whereas for the other metals, a continuous control of their metallic labile fractions was monitored. Therefore, DGT device allows the continuous monitoring of the labile metal species in a drinking water treatment plant.

Importance of trehalose biosynthesis for Sinorhizobium meliloti Osmotolerance and nodulation of Alfalfa roots.

The disaccharide trehalose is a well-known osmoprotectant, and trehalose accumulation through de novo biosynthesis is a common response of bacteria to abiotic stress. In this study, we have investigated the role of endogenous trehalose synthesis in the osmotolerance of Sinorhizobium meliloti. Genes coding for three possible trehalose synthesis pathways are present in the genome of S. meliloti 1021: OtsA, TreYZ, and TreS. Among these, OtsA has a major role in trehalose accumulation under all of the conditions tested and is the main system involved in osmoadaptation. Nevertheless, the other two systems are also important for growth in hyperosmotic medium. Genes for the three pathways are transcriptionally responsive to osmotic stress. The presence of at least one functional trehalose biosynthesis pathway is required for optimal competitiveness of S. meliloti to nodulate alfalfa roots.

An experimental and modelling exploration of the host-sanction hypothesis in legume-rhizobia mutualism.

Despite the importance of mutualism as a key ecological process, its persistence in nature is difficult to explain since the existence of exploitative, "cheating" partners that could erode the interaction is common. By analogy with the proposed policing strategy stabilizing intraspecific cooperation, host sanctions against non-N(2) fixing, cheating symbionts have been proposed as a force stabilizing mutualism in legume-Rhizobium symbiosis. Following this proposal, penalizations would include decreased nodular rhizobial viability and/or early nodule senescence in nodules occupied by cheating rhizobia. In this work, we analyse the stability of Rhizobium-legume symbiosis when non-fixing, cheating strains are present, using an experimental and modelling approach. We used split-root experiments with soybean plants inoculated with two rhizobial strains, a cooperative, normal N(2) fixing strain and an isogenic non-fixing, "perfect" cheating mutant derivative that lacks nitrogenase activity but has the same nodulation abilities inoculated to split-root plants. We found no experimental evidence of functioning plant host sanctions to cheater rhizobia based on nodular rhizobia viability and nodule senescence and maturity molecular markers. Based on these experiments, we developed a population dynamic model with and without the inclusion of plant host sanctions. We show that plant populations persist in spite of the presence of cheating rhizobia without the need of incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N(2) from the effectively mutualistic rhizobia occupying some nodules. Inclusion of plant sanctions leads to the unrealistic effect of ultimate extinction of cheater strains in soil. Our simulation results are in agreement with increasing experimental evidence and theoretical work showing that mutualisms can persist in presence of cheating partners.

Transcriptome profiling reveals the importance of plasmid pSymB for osmoadaptation of Sinorhizobium meliloti.

In this work, DNA microarrays were used to investigate genome-wide transcriptional responses of Sinorhizobium meliloti to a sudden increase in external osmolarity elicited by addition of either NaCl or sucrose to exponentially growing cultures. A time course of the response within the first 4 h after the osmotic shock was established. We found that there was a general redundancy in the differentially expressed genes after NaCl or sucrose addition. Both kinds of stress resulted in induction of a large number of genes having unknown functions and in repression of many genes coding for proteins with known functions. There was a strong replicon bias in the pattern of the osmotic stress response; whereas 64% of the upregulated genes had a plasmid localization, 85% of the downregulated genes were chromosomal. Among the pSymB osmoresponsive genes, 83% were upregulated, suggesting the importance of this plasmid for S. meliloti osmoadaptation. Indeed, we identified a 200-kb region in pSymB needed for adaptation to saline shock which has a high density of osmoregulated genes.