Polycyclic aromatic hydrocarbons (PAHs) include a group of organic priority pollutants of critical environmental and public health concern due to their toxic, genotoxic, mutagenic and/or carcinogenic properties and their ubiquitous occurrence as well as recalcitrance. The increased awareness of their various adverse effects on ecosystem and human health has led to a dramatic increase in research aimed toward removing PAHs from the environment. PAHs may undergo adsorption, volatilization, photolysis, and chemical oxidation, although transformation by microorganisms is the major neutralization process of PAH-contaminated sites in an ecologically accepted manner. Microbial degradation of PAHs depends on various environmental conditions, such as nutrients, number and kind of the microorganisms, nature as well as chemical property of the PAH being degraded. A wide variety of bacterial, fungal and algal species have the potential to degrade/transform PAHs, among which bacteria and fungi mediated degradation has been studied most extensively. In last few decades microbial community analysis, biochemical pathway for PAHs degradation, gene organization, enzyme system, genetic regulation for PAH degradation have been explored in great detail. Although, xenobiotic-degrading microorganisms have incredible potential to restore contaminated environments inexpensively yet effectively, but new advancements are required to make such microbes effective and more powerful in removing those compounds, which were once thought to be recalcitrant. Recent analytical chemistry and genetic engineering tools might help to improve the efficiency of degradation of PAHs by microorganisms, and minimize uncertainties of successful bioremediation. However, appropriate implementation of the potential of naturally occurring microorganisms for field bioremediation could be considerably enhanced by optimizing certain factors such as bioavailability, adsorption and mass transfer of PAHs. The main purpose of this review is to provide an overview of current knowledge of bacteria, halophilic archaea, fungi and algae mediated degradation/transformation of PAHs. In addition, factors affecting PAHs degradation in the environment, recent advancement in genetic, genomic, proteomic and metabolomic techniques are also highlighted with an aim to facilitate the development of a new insight into the bioremediation of PAH in the environment.

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The purpose of this study was to evaluate the effect of pH and inorganic nutrient supplementations for anaerobic hydrolysis and acidogenesis of particulate organic materials at both mesophilic (35 degrees C) and thermophilic (55 degrees C) temperatures. Hydrolysis and acidogenesis of a synthetic sludge was observed in batch operation for the evaluation of the pH effect. pH was uncontrolled in one reactor and controlled at 4.5, 5.5, and 6.5 in the other three reactors at both temperatures. The greatest degree of hydrolysis and acidogenesis occurred when the pH was controlled at 6.5. The pH of the uncontrolled reactor dropped to 3.4 at both temperatures severely retarding hydrolysis and acidogenesis. Concentrations of acetic and n-butyric acids predominated with lower concentrations of propionic acid at both temperatures in all reactors. Lactic acid was produced as the earliest intermediate but as the reaction proceeded, short chain VFAs were produced as final end products with a decrease in lactic acid. The higher the pH, the earlier this trend was observed. For the controlled reactors at pH 6.5, the soluble COD production and the VSS reduction peaked in 4 days at 55 degrees C whereas it took about 11 days at 35 degrees C to obtain the same result. During the linear SCOD production period at a pH of 6.5 the hydrolysis rate of the thermophilic reactor was greater than that for mesophilic. Thermophilic conditions appeared to be more sensitive to pH than mesophilic ones for both hydrolysis and acidogenesis. Additional experiments were conducted to establish the effect of inorganic nutrient (Ca, Fe, Co, and Ni) supplementation on hydrolysis and acidogenesis at both temperatures. It has, prior to this, been assumed that only methanogenesis benefited from trace metal supplementation. However, the results demonstrated the importance of inorganic nutrient supplementation to optimize hydrolysis and acidogenesis at both temperatures.

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