FIGURE 7.5 Some representative data obtained during the bioremediation of the Exxon Valdez spill in Alaska in 1990. (A) Representative radiorespirometry data (88). (B) Representative data using hopane as a conserved internal analyte (22) from the same beach. Error bars represent estimates of standard errors, and again we note that these data demonstrate the variability typically seen in the field and that the simplistic statistical treatment here was verified with more rigorous approaches in the cited references. GC, gas chromatography.
lated the biodégradation of hydrocarbons on this beach.
3. Does the chemical composition of residual oil on the beach indicate that it has changed because of biodégradation? It has long been recognized that microorganisms show a preference for biodegrading some compounds in crude oil before others. For example, «-alkanes are degraded before the wo-alkanes (100), so a decrease in the ratio of ra-heptadec-
ane to pristane is an early indicator that biodégradation is under way. Pristane itself is readily degraded, however, so it soon fails as a conserved analyte within the oil. Hopanes (120, 127) (Fig. 7.1) are much more resistant to biodegradation, so they serve as excellent conserved reference analytes within the oil. Changes in the relative concentrations of other analytes to an individual hopane can then be attributed to biodégradation (or in some cases to photooxidation ), since physical movement of oil should not lead to any change in the ratios of analytes to hopane. A representative data set from the same shoreline as for Fig. 7.5A is shown in Fig. 7.5B. The ratio of the total gas chromatography-detectable hydrocarbons to hopane on fertilized and unfertilized portions of the beach is plotted. The ratio did not change significantly for samples from the unfertilized area, but there was a statistically significant decrease in the ratio on the fertilized part (see reference 22 for a detailed statistical analysis). The data are also suggestive that the ratio began to drop for the unfertilized portion of the beach after fertilizer was added on day 70 (see above). Similar results (22) were seen in the ratio of the »-alkanes to hopane and total polycyclic aromatic hydrocarbons on the U.S. EPA priority pollutant list (76), indicating that the bioremediation strategy was indeed leading to the biodégradation of all components of the spilled oil.
In cases where the hopanes are unavailable, such as for diesel, we have used alkylated phen-anthrenes as conserved internal analytes, since they are among the most biodegradation-resistant molecules in diesel fuels (40). These also proved useful in the Spitsbergen experiment (130, 131). Since these molecules are known to be biodegradable, it is likely that using them as conserved analytes will actually underestimate the extent of biodégradation, but it seems better to err on the conservative side in assessing field data.
Use of this tiered approach, and comparison of data from fertilized and unfertilized control plots, can give confidence that a bioremediation protocol is indeed effective.
WHEN SHOULD BIOREMEDIATION BE USED FOR TREATING OIL-CONTAMINATED MARINE WATERS AND SEDIMENTS?
Although bioremediation by addition of fertilizers will speed the biodégradation of an oil spill and thereby diminish its environmental impact, it is important to bear in mind that careless application of fertilizers may have un wanted negative impacts on the environment, and they should be used with care. Coastal regions throughout the world are in danger of eutrophication from a wide range of anthropogenic sources (49, 108), and the total amount of nitrogen used in a bioremediation operation should be minimized. Excess inorganic nitrogen is acutely toxic to amphipods and fishes, and it can stimulate unwanted algal and planktonic growth. Ammonia releases into coastal zones from agricultural runoff are sometimes blamed for fish kills. Careful applications, aiming at achieving 100 to 200 |xM nitrogen in the interstitial water of an oiled shoreline, should minimize the possibility of these effects, and where they have been monitored there have been no adverse findings (92, 126, 129, 155, 171). Other potential concerns include the possibility that enhanced microbial activity following bioremediation treatment might increase the bioavailability of oil components to other organisms, perhaps by solvation through surfactant release or by the release of partially oxidized intermediates that might be toxic or mutagenic. In fact, no evidence for such effects has been reported (34, 155), and, to the contrary, experiments have shown that bioremediation by nutrient addition leads to a reduction in toxicity (102). Another potential hazard is that increased biodégradation of spilled oil might cause anaerobiosis in previously aerobic sediments. Of course this is a possibility, especially in sediments that are close to anoxic before bioremediation treatment. Nevertheless, it has not been seen in several situations where it might conceivably have occurred (129—131).
As discussed above, there is little evidence to suggest that the asphaltene (polar) fraction of crude oils and heavy fuels is biodegradable, so it is likely to remain even after a successful bioremediation treatment has removed the majority of hydrocarbons from a shoreline. Since the asphaltene fraction is responsible for the color centers in oils, one might expect the black coloration associated with an oil spill to persist. Yet this is not generally the case; in fact, asphaltenes, in the absence of hydrocarbons, lack the oiliness and stickiness of crude oil and no longer adhere to rocks and gravel. Being almost neutrally buoyant, they wash away and become tiny particles, subject to abrasion and dispersion, that are hard to distinguish from humic and fulvic acid residues of more recent biomass.
From the foregoing discussion it is clear that bioremediation through biostimulation of indigenous microbial populations by adding fertilizers can dramatically stimulate the rate of oil biodégradation when the target oil is a relatively thin film of biodegradable material. It is thus quite appropriate that bioremediation should be an important part of the oil spill response "tool kit" for such situations (e.g., see references 29 and 114). While bioremediation clearly works most effectively on highly biodegradable oils and in situations where there are only thin layers of oil on sediment particles, a case can be made for using it more widely during a spill response, even if physical collection of the oil will eventually be achieved.
The principal reason for employing bioremediation is to reduce the ecological impact of a spill and in particular the time during which the ecosystem may be impacted by hydrocarbons. Stimulation of the removal of such compounds by microorganisms before they come in contact with multicellular organisms is bound to decrease the environmental impact of the spill.
Thus, bioremediation should also be considered as part of a cleanup operation even for heavy fuel oils, such as those from the Erika (113) and Prestige (16), or for Orimulsion (a dispersant-stabilized dispersion of a Cerro Negro bitumen in water) (122, 132), in which only a small fraction of the oil is biodegradable. Removal of the bioavailable fraction more rapidly with a bioremediation treatment should help to minimize the environmental impact of a spill.
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