CONICET | Buscador de Institutos y Recursos Humanos

Polycyclic aromatic hydrocarbons (PAH) are pollutants that persist in the environment due to their low solubility in water and their sequestration by soil and sediments. The exploitation of microbes for cleaning contaminated sites is still one of the big challenges to environmental microbiologists. Although several PAH degrading bacterial species have been isolated, it is not expected that a single isolate would exhibit the ability to degrade completely all PAHs. Bacterial consortia are of special concern since syntrophic interactions are an extremely interesting example of fitness support, especially in the biodegradation of aromatic compounds where the biochemical steps are shared among community members in order to achieve its complete mineralization. In the present work the effects of the bioaugmentation with an undefined (natural) and a defined consortium were compared. The natural PAH-degrading consortium (PHE-CON) was previously isolated from a chronically contaminated soil and characterized in terms of structure, diversity and functionality. In liquid phenanthrene-saturated mineral medium (PhMM) the PHE-CON showed the capacity of degrading the 58 % of the supplied phenanthrene, after 7 days of incubation. The composition of the PHE-CON was studied by 16S rDNA clone libraries; and some of the genera found were Ochrobacterium sp., Alcaligenes sp., Inquilinus sp., Achromobacter sp. and Sphingomonadaceae family. There have also been isolated five strains from the PHE-CON, identified as Sphingobium sp. (AM), Enterobacter sp. (B and B1) and Pseudomonas sp. (T and Bc), but only the AM isolate showed phenanthrene degradation. To study the cooperative metabolic activities of the isolated strains, a defined consortium was formed with isolates AM plus Bc. This defined consortium showed the ability to degrade the 97% of phenanthrene after 7 days of incubation in PhMM, these values was significantly higher than the ones reached by PHE-CON and the AM strain at the same time of incubation. The bioaugmentation studies were performed in soil microcosms artificially contaminated with 2000 mg of phenanthrene/Kg of dry soil and prepared in triplicate. The microcosms were inoculated with 1.4x108 of total cfu/g of dry soil, and incubated at 20±2°C and 21% humidity for 63 days. One non-inoculated was used as control. At different time during incubation period, the concentration of residual phenanthrene (HPLC), the numbers of cultivable heterotrophic and PAH degrading bacteria, the soil biological activity (dehydrogenase activity) and the genetic diversity of the bacterial soil community (PCR-DGGE and clone libraries) were analyzed . Both microcosms inoculated with the PHE-CON and the defined consortia (AM+Bc) showed higher values of PAH degrading bacteria and dehydrogenase activity than the control microcosm; in concordance a significant increase in the phenanthrene elimination rate was observed. Both inoculations provoked strong changes on the dynamics and the diversity of the bacterial soil community. In the DNA clone library, it was evidenced that in the PHE-CON microcosms the predominant clones were affiliated with Sphingomonadaceae family as happened in the clone libraries of PHE-CON. Also there were found some of the genera present in the PHE-CON as Inquilinus sp., Alcaligenes sp. and Achromobacter sp. By the other hand, in the microcosm inoculated with the defined consortium, the predominant clones were affiliated with Sphingobium sp. (coincident with the strain AM) but the strain Bc was not found neither in the DGGE nor in the cloning assays. This study proved that defined and undefined consortia are able to compete and settle into the microbial soil community, enhancing the phenanthrene elimination. Since the utilization of defined consortia presents several advantages as operational management in field bioremediation processes, this type of consortia could have a great potential in bioaugmentation procedures.