, indicating that most FL-IN were released from MPB biomass. The relationships between chlorophyll a and FL-IN in winter and summer were significantly different, suggesting that nutrient accumulation by MPB changes seasonally and might affect nutrient cycling in intertidal sediments.

Nitrate addition stimulated sulfide oxidation by increasing the activity of nitrate-reducing sulfide-oxidizing bacteria (NR-SOB), decreasing the concentration of dissolved H 2 S in the water phase and, consequently, its release to the atmosphere of a pilot-scale anaerobic bioreactor. The effect of four different concentrations of nitrate (0.12, 0.24, 0.50, and 1.00 mM) was investigated for a period of 3 days in relation to sulfide concentration in two bioreactors set up at Guadalete wastewater treatment plant (Jerez de la Frontera, Spain). Physicochemical variables were measured in water and air, and the activity of bacteria implicated in the sulfur and nitrogen cycles was analyzed in the biofilms and in the water phase of the bioreactors. Biofilms were a net source of sulfide for the water and gas phases (7.22±5.3 μmol s −1) in the absence of nitrate dosing. Addition of nitrate resulted in a quick (within 3 h) decrease of sulfide both in the water and atmospheric phases. Sulfide elimination efficiency in the water phase increased with nitrate concentrations following the Michaelis-Menten kinetics (K s =0.63 mM NO 3 −). The end of nitrate addition resulted in a recovery or increase of initial net sulfide production in about 3 h. Addition of nitrate increased the activity of NR-SOB and decreased the activity of sulfate-reducing bacteria. Results confirmed the role of NR-SOB on hydrogen sulfide consumption coupled with nitrate reduction and sulfate recycling, revealing Sulfurimonas denitrificans and Paracoccus denitrificans as NR-SOB of great importance in this process.

Complex microbial communities exhibit a large diversity, hampering differentiation by DNA fingerprinting. Herein, differential display-denaturing gradient gel electrophoresis is proposed. By adding a nucleotide to the 3 ends of PCR primers, 16 primer pairs and fingerprints were generated per community. Complexity reduction in each partial fingerprint facilitates sample comparison.

Nitrate decreases sulfide release in wastewater treatment plants (WWTP), but little is known on how it affects the microzonation and kinetics of related microbial processes within the biofilm. The effect of nitrate addition on these properties for sulfate reduction, sulfide oxidation, and oxygen respiration were studied with the use of microelectrodes in microaerophilic wastewater biofilms. Mass balance calaculations and community composition analysis were also performed. At basal WWTP conditions, the biofilm presented a double-layer system. The upper microaerophilic layer (~300 μm) showed low sulfide production (0.31 μmol cm-3 h-1) and oxygen consumption rates (0.01 μmol cm-3 h-1). The anoxic lower layer showed high sulfide production (2.7 μmol cm-3 h-1). Nitrate addition decreased net sulfide production rates, caused by an increase in sulfide oxidation rates (SOR) in the upper layer, rather than an inhibition of sulfate reducing bacteria (SRB). This suggests that the indigenous nitrate reducing-sulfide oxidizing bacteria (NR-SOB) were immediately activated by nitrate. The functional vertical structure of the biofilm changed to a triple-layer system, where the previously upper sulfide-producing layer in the absence of nitrate split into two new layers: 1) an upper sulfide-consuming layer, whose thickness is probably determined by the nitrate penetration depth within the biofilm, and 2) a middle layer producing sulfide at an even higher rate than in the absence of nitrate in some cases. Below these layers, the lower net sulfide-producing layer remained unaffected. Net SOR varied from 0.05 to 0.72 μmol cm-3 h-1 depending on nitrate and sulfate availability. Addition of low nitrate concentrations likely increased sulfate availability within the biofilm and resulted in an increase of both net sulfate reduction and net sulfide oxidation by overcoming sulfate diffusional limitation from the water phase and the strong coupling between SRB and NR-SOB syntrophic relationship.

ResumenEn este trabajo se ha estudiado la influencia de las biopelículas en los procesos de corrosión de diferentes aleaciones de acero inoxidable, situadas durante 4 años en dos puntos de una estación depuradora de aguas residuales. Se caracterizó el microambiente físico-químico en el interior de las biopelículas mediante microelectrodos de O 2 , H 2 S y pH, y se cuantificaron las tasas de corrosión a partir del número, diámetro y profundidad de picadura. Los resultados obtenidos muestran un desarrollo más notable de las biopelículas y un número de picaduras significativamente mayor en el canal de salida de desbastes que en el canal de recirculación de fangos. Con base en las características del agua sobrenadante y en las medidas realizadas con microelectrodos, se sugiere que la biopelícula induce la corrosión a través de tres posibles mecanismos: creación de celdas de aireación diferencial, zonas con diferente pH y zonas con elevada producción de sulfuro capaz de reaccionar con iones metálicos. Palabras claveBiopelículas; Biocorrosión; Acero inoxidable; Microelectrodos; Sulfuro. Effect of biofilm in the corrosion of austenitic stainless steels in wastewater treatment plants AbstractIn this work, the influence of the biofilms in the corrosion process of different alloys of stainless steel was studied in two sampling points in a wastewater treatment plant during 4 years. The physicochemical microenvironment within the biofilms was characterized through O 2 , H 2 S and pH microelectrodes. Corrosion rates were quantified from the number, diameter and depth of pits. The results show a remarkable development of the biofilm and a significantly greater number of pits in the grit removal channel than in the sludge recirculation channel. Based on the characteristics of the water phase and microelectrode measurements, our results suggest that biofilms induced corrosion throughout 3 mechanisms: creation of differential aeration cells, areas with different pH and areas having high sulphide production which may react with metal ions.