Recent studies in microbial degradation of petroleum hydrocarbons in hypersaline environments.

Babu Z Fathepure

doi:10.3389/fmicb.2014.00173

Abstract

Many hypersaline environments are often contaminated with petroleum compounds. Among these, oil and natural gas production sites all over the world and hundreds of kilometers of coastlines in the more arid regions of Gulf countries are of major concern due to the extent and magnitude of contamination. Because conventional microbiological processes do not function well at elevated salinities, bioremediation of hypersaline environments can only be accomplished using high salt-tolerant microorganisms capable of degrading petroleum compounds. In the last two decades, there have been many reports on the biodegradation of hydrocarbons in moderate to high salinity environments. Numerous microorganisms belonging to the domain Bacteria and Archaea have been isolated and their phylogeny and metabolic capacity to degrade a variety of aliphatic and aromatic hydrocarbons in varying salinities have been demonstrated. This article focuses on our growing understanding of bacteria and archaea responsible for the degradation of hydrocarbons under aerobic conditions in moderate to high salinity conditions. Even though organisms belonging to various genera have been shown to degrade hydrocarbons, members of the genera Halomonas Alcanivorax, Marinobacter, Haloferax, Haloarcula, and Halobacterium dominate the published literature. Despite rapid advances in understanding microbial taxa that degrade hydrocarbons under aerobic conditions, not much is known about organisms that carry out similar processes in anaerobic conditions. Also, information on molecular mechanisms and pathways of hydrocarbon degradation in high salinity is scarce and only recently there have been a few reports describing genes, enzymes and breakdown steps for some hydrocarbons. These limited studies have clearly revealed that degradation of oxygenated and non-oxygenated hydrocarbons by halophilic and halotolerant microorganisms occur by pathways similar to those found in non-halophiles.

Highlights

Many hypersaline environments including natural saline lakes, salt flats, saline industrial effluents, oil fields, and salt marshes are often contaminated with high levels of petroleum hydrocarbons
The goal of this review is to provide an overview of our current knowledge of the biodegradation of non-oxygenated and oxygenated hydrocarbons by bacteria and archaea in wide ranging salinities (6–30% NaCl) and to highlight recent discoveries in molecular mechanisms of degradation by halophilic and halotolerant organisms
A few recent studies have shown that the degradation of hydrocarbons at high salinity occurs using enzymes described for many non-halophiles

Summary

BACKGROUND

Many hypersaline environments including natural saline lakes, salt flats, saline industrial effluents, oil fields, and salt marshes are often contaminated with high levels of petroleum hydrocarbons. Al-Mailem et al (2010) have isolated extremely halophilic archaeal strains of Haloferax, Halobacterium, and Halococcus from a hypersaline coastal area of the Arabian Gulf in a mineral salt medium with crude oil vapor as the source of carbon in the presence of >26% NaCl and at 40– 45◦C These organisms metabolized various aliphatic and aromatic hydrocarbons as the sole sources of carbon and energy at high salinity. Three extremely halophilic archaeal strains, Haloferax, Halobacterium and Halococcus isolated on the basis of crude oil utilization degraded n-alkanes and mono and polyaromatic compounds as the sole sources of carbon and energy in the presence of 26% NaCl (Al-Mailem et al, 2010). Studies have reported the degradation PAHs by nonhalophiles and in marine habitats, little is known about the fate

C24 H50 C25 H52 C10 –C30 C10 –C34 C10 –C34 C9 –C40

Findings

CONCLUSIONS