María T. Del Panno

Photodegradation of Soil Organic Matter and its Effect on Gram-negative Bacterial Growth

To learn more about the role of the reactive oxygen species (ROS) in the production of bioavailable products of the dissolved organic matter, we investigate here the effect of the photolysis (λexc > 320 nm) of a soil extract (SE) on the growth of bacteria isolated from the same soil as used for obtaining the extract. Comparative experiments with Aldrich humic acid (AHA) as substrate were performed. The photodegradation of the SE was evaluated with different techniques—UV–visible absorption spectroscopy, fluorescence excitation emission matrices (EEM) and Fourier transform infrared spectroscopy (FTIR). Known ROS scavengers were employed to study the effect of photochemically produced ROS on the photodegradation of the substrates. To evaluate the effect of irradiation on the bioavailability of the SE and AHA, photolyzed and nonphotolyzed substrates were added to different culture media and the growth of Pseudomonas sp. isolated from the soil and a strain of Escherichia coli were studied. The different results obtained were assigned to the dissimilar metabolisms of both bacteria.

Remediation of a soil chronically contaminated with hydrocarbons through persulfate oxidation and bioremediation

The impact of remediation combining chemical oxidation followed by biological treatment on soil matrix and microbial community was studied, of a chronically hydrocarbon contaminated soil sourced from a landfarming treatment. Oxidation by ammonium persulfate produced a significant elimination of polycyclic aromatic hydrocarbons (PAHs) and an increase in PAH bioavailability. Organic-matter oxidation mobilized nutrients from the soil matrix. The bacterial populations were affected negatively, with a marked diminution in the diversity indices. In this combined treatment with oxidation and bioremediation working in tandem, the aliphatic-hydrocarbon fractions were largely eliminated along with additional PAHs. The chemical and spectroscopic analyses indicated a change in soil nutrients. In spite of the high residual-sulfate concentration, a rapid recovery of the cultivable bacterial population and the establishment of a diverse and equitable microbial community were obtained. Pyrosequencing analysis demonstrated a marked succession throughout this twofold intervention in accordance with the chemical and biologic shifts observed. These remediation steps produced different effects on the soil physiology. Spectroscopic analysis became a useful tool for following and comparing those treatments, which involved acute changes in a matrix of such chronically hydrocarbon-contaminated soil. The combined treatment increased the elimination efficiency of both the aliphatic hydrocarbons and the PAHs at the expense of the mobilized organic matter, thus sustaining the recovery of the resilient populations throughout the treatment. The high-throughput-DNA-sequencing techniques enabled the identification of the predominant populations that were associated with the changes observed during the treatments.

Assessment of the Responsiveness to Different Stresses of the Microbial Community from Long-Term Hydrocarbon-Contaminated Soils

Soils exposed to long-term contamination with hydrocarbons may present extreme challenges to maintain the biological resilience to the stress. To elucidate the relationships between the initial event of contamination and the responsiveness to the stress, we investigated the extent of the microbial resilience of biological functions from two contaminated soils sampled from a petrochemical area (S1, underwent diffuse hydrocarbon contamination, and S2, from a land farming unit where an alkaline petrochemical sludge was treated) after the Cd, saline, and acid stresses. Both contaminated soils were characterized by low organic matter content compared with a pristine soil. Although similar Shannon diversity index and heterotrophic bacterial count were observed, different bacterial community structures (PCR-DGGE) and less enzymatic activities characterized the contaminated soils. Particularly, functional diversity determined by Biolog EcoPlates™ was not detected in S2 soil. Only the S1 soil showed resilience of the enzymatic activities and functional diversity, suggesting the presence of a well-adapted microbial community able to face with the stresses. The S2 was the most disturbed and less responsive soil. However, an increase in the functional diversity was evidenced after acidification, and it is possible to correlate this responsiveness with the sludge properties treated in the land farming unit. In addition, if the selected stress can reverse the soil condition provoked for the first disturbance, responsiveness could be expected.

Bioremediation of PAH-Contaminated Soil by Fungi

Polycyclic aromatic hydrocarbons (PAHs) are by-products of the incomplete combustion of organic materials. They are considered to be priority pollutants in the environment due to their recalcitrance and mutagenic properties. The principal PAH loss process from soil is through microbial degradation; therefore, the bioremediation is considered as an efficient, financially affordable, and adaptable alternative for the recuperation of PAH-contaminated soil. Several microorganisms, such as bacteria, yeasts, and filamentous fungi, are capable of degrading different types of PAHs. The ability of the fungi to degrade the high-molecular-weight PAHs, together with their physiological versatility, converts the fungal remediation in a promising technique for the cleanup of PAH-contaminated soil. This chapter summarizes the recent information on the metabolic pathway of the fungal transformation of PAHs and provides a critical review of previous work about fungal bioremediation of PAH-contaminated soil. Also, some of the most recently used fungal technology to enhance PAHs bioremediation processes is discussed.