Juan José Peña Cabriales

Isolation and characterization of hexavalent chromium-reducing rhizospheric bacteria from a wetland.

Scirpus americanus Pers. occurs naturally in “San Germán,” a pond that serves as a receptor of industrial wastewater in Guanajuato, México. This plant accumulates metals mainly in the root: concentrations (mg/kg) of Cr, As, Cd and Se were 970, 49, 41, and 85 respectively. Analysis of rhizosphere samples indicated bacterial population of 108 cfu g−1 in media with 0.2 mM Cr(VI) and 10 mM sodium gluconate. Thirteen isolates were obtained and phylogenetic analyses (16S rRNA) indicated they corresponded to genera of Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas and Pseudomonas. Cr(VI) reduction was evaluated using the diphenyl carbazide method. The isolates accomplished 5–40% (20 μM) of reduction in assays of resting cell and tolerated 0.5–5.0 mM Cr(VI). Eight strains used nitrate and thirteen used iron and chromium as electron acceptors to grow under anaerobic conditions. Cr(VI) reduction by five strains occurred at pH values (7–9) and NaCl concentrations (0.5–1.0 M) in basal medium. A mixed culture of strains (S17 and S28) reached a chromium removal of 100% at 0.2 mM Cr(VI) initial concentration. Aerobically, this consortium was capable of 93.8% Cr(VI) reduction of 81 μgL−1 Cr(VI) of the industrial effluent, indicating their possible use in environmental cleanup.

Spatial Variability of Heavy Metals in Soils and Sediments of “La Zacatecana” Lagoon, Mexico

Anthropogenic activities have greatly increased heavy metal pollution worldwide. Due to inadequate waste management, mining is one of the chief causes. One particularly affected area in Mexico is the “La Zacatecana” Lagoon, in the municipality of Guadalupe, Zacatecas. From colonial times until the mid-nineteenth century, about 20 million tons of mine tailings were deposited at this site. Here, we catalogue the heavy metal content and their distribution in soils and sediments of La Zacatecana. The mobility of lead in soils was also assayed by sequential extraction. Concentrations of the different metals analysed were as follows: Pb > Cr > As > Ni > Hg > Cd. Site VIII accumulated the highest amount of Pb (3070 mg·kg−1) sevenfold more than the limit established by the Mexican standards for agricultural soils (i.e., 400 mg·kg−1). On the other hand, the contents of Cd, Cr, and Ni were within the levels accepted by the above normativity, set at 37, 280, and 1600 mg·kg−1, respectively. Concentrations of Hg and Pb were highest in the north-northwest zone of the lagoon and decreased towards the southeast. Except for Site VIII where 30% of the Pb was in an interchangeable form or bound to carbonates, most Pb in La Zacatecana soils was present in an unavailable form, associated with Fe-Mn oxides.

Microorganisms role in the bioremediation of contaminated soils with heavy metals

A nivel mundial, la contaminacion por metales pesados es un problema que afecta la salud humana. Por su toxicidad y abundancia, los metales mas estudiados son el mercurio, el arsenico y el plomo. La toxicidad de los metales pesados en suelos esta asociada con la fraccion disponible que es asimilada por los seres vivos, la cual esta determinada por las caracteris ticas fisicoquimicas del suelo (principalmente pH, potencial redox y materia organica). Las tecnicas para la remediacion de suelos se basan en metodos fisicoquimicos y biologicos; estas ultimas llamadas biorremediacion , ya que aprovecha el potencial metabolico de organismos vivos (bacterias y hongos) para la descontaminacion. Los procesos mas usados en la biorremediacion son la sorcion, precipitacion, lixiviacion y volatilizacion de metales pesados. Sin embargo, son necesarios mas estudios sobre la diversidad microbiana de los sitios contaminados con metales pesados para encontrar cepas mejor adaptadas y con mayor capacidad de biorremediacion de estos contaminantes.