Dispersants are said to "accelerate the biodegradation" of hydrocarbons released into the Gulf by breaking the oil up into smaller particles. While this may be true, there has been little evidence presented to unequivocally verify these claims. Contrary evidence also exists (as listed in the studies below) which shows that dispersants inhibit the microbial biodegradation of certain lesser soluble hydrocarbons, while also accelerating the absorption of hydrocarbons into plant and animal life.
1) Dispersants like the Corexit 9500 used in the Gulf are composed of chemicals which have been shown to have significant toxicity to a wide range of animal, plant and microbial life. The question is: "how does the dispersant effect the populations of naturally occuring oil-consuming bacteria?"
In a report published in 1999 by the Alaskan Department of Environmental Conservation entitled: "Biodegradation of Dispersed Oil Using COREXIT 9500" the dispersant was shown to inhibit the biodegradation of less soluble hydrocarons by marine bacteria, while accelerating the breakdown of the more soluble hydrocarbons. The authors state:
"Thus, while adding dispersant may increase microbial oil degradation activity as a whole, this increase may be restricted to only some components of the crude oil, resulting in selective enrichment of other components in the residual oil. This could result in either an increase or a decrease in the toxicity of the residual oil." (pg. 2)
"Radiorespirometry data suggest that addition of dispersant may inhibit mineralization of relatively insoluble substrates (e.g.,
hexadecane and phenanthrene; see Figures 1, 2 and 3), perhaps due to microbial preference for
the more bioavailable substrates." (pg. 24)
"We found that dispersant addition to oil increased total CO2 evolution from our defined consortium, but not as much as dispersant alone (Figure 5A), and hydrocarbon degraders were elevated much more than hexadecane
degraders or phenanthrene degraders (Figure 7)." (pg. 24)
"Our data suggest that addition of dispersant to oil increases total carbon mineralized, and numbers of hydrocarbon degraders. This suggests that dispersant increases oil biodegradation, but total C [carbon] mineralization data and hydrocarbon degrader data are together insufficient to evaluate whether this observed increase in oil degradation is consistent across chemical classes of compounds contained in the oil. Our data indicate that dispersant may inhibit biodegradation of some components of the crude oil. At this point no data currently exist allowing evaluation of the effects of Corexit 9500 on biodegradation of the more acutely and chronically toxic components of crude oil. Following dispersant use, if the residual oil is selectively enriched in components of greater toxicity than those components biodegraded, the toxicity of the resulting oil residue (on an oil mass basis) may be increased." (pg. 25)