Biodegradation of hydrocarbons in the environment.

Studies on the environmental fate of petroleum have demonstrated the nearly ubiquitous distribution of microorganisms that can metabolize hydrocarbons. The rates of degradation depend upon the concentrations of such microbes and upon the environmental characteristics of an oil-contaminated ecosystem. Given the appropriate environmental conditions, microorganisms effectively decontaminate, by their biodegradative metabolism, environments that have received petroleum pollutants. Higher-molecular-weight compounds, especially those with multiple condensed ring structures and with highly branched or substituted compounds, are relatively resistant to microbial attack. Despite the fact that a genetically engineered hydrocarbon degrader was the first organism ever patented and that seed cultures are produced by various commercial firms, enhanced biodegradation as a result of seeding generally has not been shown to be effective. Also, even though some anaerobes have now been demonstrated to be capable of hydrocarbon metabolism, hydrocarbons persist indefinitely in anoxic environments. Environmental modification, on the other hand, such as that achieved by aeration or fertilization with nitrogen and phosphorus, has been shown to enhance biodegradative removal of hydrocarbons. Having considered the various factors that influence the rates of hydrocarbon biodegradation, we are left with the question of what to do when environmental oil contamination occurs in order to minimize its persistence and thus its long-term effects. Clearly, treatment methods should enhance rather than inhibit the natural rates of oil biodegradation. In some cases, it is possible to modify environmental parameters to enhance rates of hydrocarbon biodegradation, but such methods are rarely undertaken. The translation of our scientific knowledge of hydrocarbon biodegradation into practical applications remains a major challenge. Specifically designed organisms are needed to degrade toxic aromatic components of refinery waste streams before environmental treatment. Specially designed reactors with specific microbial populations are also needed if oily sludges are to be degraded by biological means, either aerobically or anaerobically, in contained, environmentally safe reactors.

Response of microbial populations in arctic tundra soils to crude oil.

Experimental crude oil spillages of 5 and 12 litre/m2 were established on the four major topographically distinguished soils of Arctic coastal polygonized tundra. The response of microbial populations to contaminating oil was found to depend on soil type and depth. Increases in numbers of heterotrophs were initially restricted to the top 2 cm of the soils. Increase in oil-degrading populations were found in oil-treated soils. Increases in microbial populations in subsurface soils paralleled downward migration of the oil. Some of the observed population increases probably resulted from input of plant residues and products from oil biodegradation.

Response of microorganisms to an accidental gasoline spillage in an arctic freshwater ecosystem.

The response of microorganisms to an accidental spillage of 55,000 gallons of leaded gasoline into an Arctic freshwater lake was studied. Shifts in microbial populations were detected after the spillage, reflecting the migration pattern of the gasoline, enrichment for hydrocarbon utilizers, and selection for leaded-gasoline-tolerant microorganisms. Ratios of gasoline-tolerant/utilizing heterotrophs to "total" heterotrophs were found to be a sensitive indicator of the degree of hydrocarbon contamination. Respiration rates were elevated in the highly contaminated area, but did not reflect differences between moderately and lightly contaminated areas. Hydrocarbon biodegradation potential experiments showed that indigenous microorganisms could extensively convert hydrocarbons to CO(2). In situ measurement of gasoline degradation showed that, if untreated, sediment samples retained significant amounts of gasoline hydrocarbons including "volatile components" at the time the lake froze for the winter. Nutrient addition and bacterial inoculation resulted in enhanced biodegradative losses, significantly reducing the amount of residual hydrocarbons. Enhanced biodegradation, however, resulted in the appearance of compounds not detected in the gasoline. Since the contaminated lake serves as a drinking water supply, treatment to enhance microbial removal of much of the remaining gasoline still may be advisable.

Effects of temperature and crude oil composition on petroleum biodegradation.

The biodegradability of seven different crude oils was found to be highly dependent on their composition and on incubation temperature. At 20 C lighter oils had greater abiotic losses and were more susceptible to biodegradation than heavier oils. These light crude oils, however, possessed toxic volatile components which evaporated only slowly and inhibited microbial degradation of these oils at 10 C. No volatile toxic fraction was associated with the heavier oils tested. Rates of oil mineralization for the heavier oils were significantly lower at 20 C than for the lighter ones. Similar relative degradation rates were found with a mixed microbial community, using CO2 evolution as the measure, and with a Pseudomonas isolate from the Arctic, using O2 consumption as the measure. The paraffinic, aromatic, and asphaltic fractions were subject to biodegradation. Some preference was shown for paraffin degradation, especially at low temperatures. Branched paraffins, such as pristane, were degraded at both 10 and 20 C. At best, a 20% residue still remained after 42 days of incubation. Oil residues generally had a lower relative percentage of paraffins and higher percentage of asphaltics than fresh or weathered oil.