Dr. Joe E Lepo

Biosurfactants play an important role in facilitating the natural biodegradation of petroleum hydrocarbons. In order to characterize the biosurfactant-producing potential of environments at risk of anthropogenic oil contamination, we collected a number of distinct marine bacteria that emulsified crude oil from relatively pristine and from polluted areas. The bacteria belonged to diverse taxonomic groups and varied widely in oil-emulsifying capability, surface tension reduction of culture media, and hydrophobicity. We used laboratory assays that directly evaluated biosurfactant preparations from these and other strains for use as cleaning agents for oiled surfaces. The subjects of the study were the oil emulsification capabilities and the surface-active characteristics of the biosurfactants, which were compared to those of selected synthetic surfactants. We also examined the ability of different biosurfactants and synthetic surfactants to wash oil from porcelain tiles. To assess the efficacy of biosurfactants as enhancers of oil biodegradation, we applied preparations of biosurfactants along with our standard mix of hydrocarbon-degrading bacteria in either shake-flask experiments or in sandy-beach microcosms. In general, surfactants of either class (biosurfactants or synthetic surfactants) performed similarly in all of these experiments. Although some biosurfactants show merit as cleaning agents, the current production technology may not make their use cost-effective.

Safety issues relating to biosurfactants were addressed by examining their toxicity and their biodegradability (and thus persistence in the environment) relative to selected synthetic surfactants. Standard toxicology bioassays were used: (1) the mysid, 7-day chronic estimator, and (2) the inland silverside 7-day chronic estimator. The biodegradability of biosurfactants and synthetic surfactants was compared by measuring the increased biochemical oxygen demand generated by the surfactants in raw seawater. Some biosurfactants were indeed highly biodegradable; however, we found no tight association of biodegradability to either class of surfactants. These results suggest that the environmental safety of surfactants is best evaluated on the basis of toxicity and degradability data for specific cases.

A field evaluation of the use of bioremediation to treat oiled fine sand in the intertidal zone of Stert Flats (Somerset, UK) was conducted and the use of in situ respirometry to monitor bioremediation success was evaluated. Previous experimental studies had shown that superficial oil is rapidly removed from Stert Flats with tidal action removing or depositing 0.05-0.10 m of fine sand in single tidal cycle. Thus, only the oil found at depth, as a result of penetration or burial by sediment deposition is persistent. To evaluate the feasibility of bioremediation to treat this stranded sub-surface oil, a field trial was conducted using inorganic sources of nitrogen and phosphate. Arabian light crude oil (weathered and emulsified with 25% seawater) was added to selected plots a coverage of 4 l.m (super -2) . Regular addition of nutrients (sodium nitrate and potassium dihydrogen orthophosphate) were made throughout the three month experiment, beginning 1 week after oil application. The application rate was determined by a separate laboratory study. The success of the bioremediation strategy was determined by chemical analysis of the residual hydrocarbons and monitoring CO (sub 2) evolution in situ. The results suggest that inorganic fertiliser does stimulate the biodegradation and mineralisation of oil buried in the aerobic zone of fine sediments.