No abstract

Microorganisms are responsible for degrading the raw leachate generated in sanitary landfills, extracting the soluble fraction of the landfill waste and biotransforming organic matter and toxic residues. To increase our understanding of these highly contaminated ecosystems, we analyzed the microbial communities in the leachate produced by three landfill cells of different ages. Using high-throughput 454 pyrosequencing of the 16S rRNA gene, we describe the structure of the leachate communities and present their compositional characteristics. All three communities exhibited a high level of abundance but were undersampled, as indicated by the results of the rarefaction analysis. The distribution of the taxonomic operational units (OTUs) was highly skewed, suggesting a community structure with a few dominant members that are key for the degradation process and numerous rare microorganisms, which could act as a resilient microorganism seeder pool. Members of the phylum Firmicutes were dominant in all of the samples, accounting for up to 62% of the bacterial sequences, and their proportion increased with increasing landfill age. Other abundant phyla included Bacteroidetes, Proteobacteria, and Spirochaetes, which together with Firmicutes comprised 90% of the sequences. The data illustrate a microbial community that degrades organic matter in raw leachate in the early stages, before the methanogenic phase takes place. The genera found fit well into the classical pathways of anaerobic digestion processes.

This research shows for the first time the degradation of linear alkylbenzene sulfonates (LAS) under anaerobic conditions, together with the presence of metabolites and the identification of microorganisms involved in this process. This compound is the most widely used surfactant and its main environmental concern is related to its persistence in the absence of oxygen as LAS accumulates in anaerobic sediments and sewage sludges. Laboratory experiments performed with anoxic marine sediments spiked with 10-50 ppm of LAS demonstrated, however, that its degradation reached 79% in 165 days via the generation of sulfophenyl carboxylic acids (SPCs). Almost all of the added LAS (>99%) was found to be attached to the sediment while the less hydrophobic SPCs were predominant in solution, as their concentration increased progressively up to 3 ppm during the full course of the experiment. Average half-life for LAS has been estimated to be 90 days, although higher values should be expected when the LAS concentration exceeds 20 ppm, due to inhibition of the microbial community. Sulfate-reducing and methanogenic activities proved to be intense during the experiment. Several sulfate-reducing bacteria and firmicutes/clostridia have been identified as possible candidates for effecting this degradation. Our results imply that the persistence of LAS in anoxic compartments, such as marine sediments, should be reconsidered when evaluating its environmental risk.