When the freshwater microalga Chlorella sorokiniana and the plant growth-promoting bacterium Azospirillum brasilense were deployed as free suspensions in unsterile, municipal wastewater for tertiary wastewater treatment, their population was significantly lower compared with their populations in sterile wastewater. At the same time, the numbers of natural microfauna and wastewater bacteria increased. Immobilization of C. sorokiniana and A. brasilense in small (2-4 mm in diameter), polymer Ca-alginate beads significantly enhanced their populations when these beads were suspended in normal wastewater. All microbial populations within and on the surface of the beads were evaluated by quantitative fluorescence in situ hybridization combined with scanning electron microscopy and direct measurements. Submerging immobilizing beads in wastewater created the following sequence of events: (a) a biofilm composed of wastewater bacteria and A. brasilense was created on the surface of the beads, (b) the bead inhibited penetration of outside organisms into the beads, (c) the bead inhibited liberation of the immobilized microorganisms into the wastewater, and (d) permitted an uninterrupted reduction of ammonium and phosphorus from the wastewater. This study demonstrated that wastewater microbial populations are responsible for decreasing populations of biological agents used for wastewater treatment and immobilization in alginate beads provided a protective environment for these agents to carry out uninterrupted tertiary wastewater treatment.

Palabras clave: remediación, microorganismos, suelos RESUMEN La contaminación por metales pesados en México es un problema que va en aumento debido a la actividad antrópica, específicamente la minería. Los metales contaminantes más importantes en México, dada su toxicidad y abundancia son: mercurio, arsénico, plomo y cromo. Entre los sitios más afectados por las altas concentraciones de metales pesados en suelos se encuentran los estados de Zacatecas, Querétaro, Hidalgo y San Luis Potosí. Una alternativa para contribuir a la solución de este problema, es el uso de especies vegetales para la remoción de metales pesados del suelo o "fitorremediación". Al respecto, en México se han caracterizado especies con capacidad de acumulación de metales, como Scirpus americanus, Typha latifolia, Jatropha dioica, Eichhornia crassipes y Amaranthus hybridus. Una estrategia para mejorar el proceso de fitoextracción de metales es a través de la inoculación de microorganismos del suelo. Bacterias de los géneros Rhizobium, Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas y Pseudomonas, han mostrado resultados promisorios al ser empleados en estos tratamientos, así como el uso de hongos micorrízicos arbusculares específicamente del género Glomus. Sin embargo, son necesarios más estudios para optimizar los sistemas de tratamiento basados en la selección de las mejores plantas y bacterias endémicas de los sitios contaminados.Key words: remediation, microorganisms, soils ABSTRACT Heavy metal pollution is a growing problem due to anthropic activities such as mining. The most hazardous metals in Mexico are mercury, arsenic, lead, and chromium. States like Zacatecas, Querétaro, Hidalgo, and San Luis Potosi have been affected by the pollution of soils with heavy metals. In this sense, phytoremediation has emerged as an alternative to solve this problem. In Mexico, plant species with metal accumulative capacity have been identified. These include Scirpus americanus, Typha latifolia, Jatropha dioica, Eichhornia crassipes and Amaranthus hybridus. Metal phytoextraction could be improved by coupling bioaugmentation of soil microorganism. Bacteria from the genus Rhizobium, Agrobacterium, Arthrobacter, Microbacterium, Curtobacterium, Rhodococcus, Xanthomonas and Pseudomonas, have shown promising results enhancing Rev. Int. Contam. Ambie. 33 (Especial Biotecnología e ingeniería ambiental) 7-21, 2017

40El papel de los microorganismos en la biorremediación de suelos contaminados con metales pesados Microorganisms role in the bioremediation of contaminated soils with heavy metals RESUMENA nivel mundial, la contaminación por metales pesados es un problema que afecta la salud humana. Por su toxicidad y abundancia, los metales más estudiados son el mercurio, el arsénico y el plomo. La toxicidad de los metales pesados en suelos está asociada con la fracción disponible que es asimilada por los seres vivos, la cual está determinada por las caracterís-ticas fisicoquímicas del suelo (principalmente pH, potencial redox y materia orgánica). Las técnicas para la remediación de suelos se basan en métodos fisicoquímicos y biológicos; estas últimas llamadas biorremediación, ya que aprovecha el potencial metabólico de organismos vivos (bacterias y hongos) para la descontaminación. Los procesos más usados en la biorremediación son la sorción, precipitación, lixiviación y volatilización de metales pesados. Sin embargo, son necesarios más estudios sobre la diversidad microbiana de los sitios contaminados con metales pesados para encontrar cepas mejor adaptadas y con mayor capacidad de biorremediación de estos contaminantes. ABSTRACTHeavy metal pollution is a worldwide problem that affects human health. Metals are nondegradable elements that move through the biosphere by changes in its redox state. Considering its toxicity and abundance, the most studied heavy metals are mercury, arsenic and lead. In soils heavy metal toxicity is due to fraction that is available to organisms. This fraction is determined by the physicochemical characteristics of the soil (pH, redox potential and organic matter). Soil remediation techniques are based on physicochemical as well as biological methods or bioremediation. Bioremediation exploit the metabolic potential of live microorganisms (bacteria and fungi) for decontamination. Some of the most prominent biological methods for heavy metal remediation are sorption, precipitation, leaching and volatilization. However, more studies are needed on microbial diversity of sites contaminated with heavy metals to find the best-adapted strains with enhanced capabilities for bioremediation.

Fungi represent an essential component of ecosystems, functioning as decomposers and biotrophs, and they are one of the most diverse groups of Eukarya. In the tropics, many species are unknown. In this work, high-throughput DNA sequencing was used to discover the biodiversity of soil fungi in the Aguarongo forest reserve, one of the richest biodiversity hotspots in Ecuador. The rDNA metabarcoding analysis revealed the presence of seven phyla: Ascomycota, Basidiomycota, Mortierellomycota, Mucoromycota, Glomeromycota, Chytridiomycota, and Monoblepharomycota. A total of 440 identified species were recorded. They mainly belonged to Ascomycota (263) and Basidiomycota (127). In Mortierellomycota, 12 species were recorded, among which Podila verticillata is extremely frequent and represents the dominant species in the entire mycobiota of Aguarongo. The present research provides the first account of the entire soil mycobiota in the Aguarongo forest, where many fungal species exist that have strong application potential in agriculture, bioremediation, chemical, and the food industry. The Aguarongo forest hides a huge number of unknown fungal species that could be assessed, and its protection is of the utmost importance.

Consistent with their reported abundance in soils, several Burkholderia sensu lato strains were isolated from the rhizosphere of maize plants cultivated at different sites in central México. Comparative analysis of their 16S rRNA gene sequences permitted their separation into three distinctive clades, which were further subdivided into six other clusters by their close resemblance to (1) Trinickia dinghuensis; (2) Paraburkholderia kirstenboschensis, P. graminis, P. dilworthii and P. rhynchosiae; (3) B. gladioli; (4) B. arboris; (5) B. contaminans, or (6) B. metallica representative species. Direct confrontation assays revealed that these strains inhibited the growth of pathogenic Fusarium oxysporum f. sp. radicis-lycopersici, and F. verticillioides within a roughly 3–55% inhibition range. The use of a DIESI-based non-targeted mass spectroscopy experimental strategy further indicated that this method is an option for rapid determination of the pathogen inhibitory capacity of Burkholderia sensu lato strains based solely on the analysis of their exometabolome. Furthermore, it showed that the highest anti-fungal activity observed in B. contaminans and B. arboris was associated with a distinctive abundance of certain m/z ions, some of which were identified as components of the ornbactin and pyochelin siderophores. These results highlight the chemical diversity of Burkholderia sensu lato bacteria and suggest that their capacity to inhibit the Fusarium-related infection of maize in suppressive soils is associated with siderophore synthesis.