I’ve spent most of my July posts on oil (hydrocarbon) break-down and bioremediation in the sea environment. When dealing with an open-ocean oil spill, we have a few things on our side with regard to oil biodegradation, including the natural mixing of the ocean currents, which not only dilutes the toxins in the oil, but also refreshes the oxygen levels vital to break-down processes. But eventually we have to face the inevitable movement of oil onto the shores and into the more terrestrial environs of the coastal communities, which then makes this a soil and sediment contamination problem.
So, to explore this issue of oil biodegradation in soils, I have a good old-fashioned compare and contrast between two very interesting research articles today. The first (Wang, et al.) uses a laboratory experiment to test how well two different soil microbial communities break down a variety of the persistent organic pollutant (POPs) in oil, while the authors of the second article (Short, et al.) returned to Prince Williams Sound 16 years after the Exxon Valdez spill to conduct a field-study to check the beaches for remaining oil.
First, the good news.
Generally speaking, natural soil communities which have a history of prior exposure to a specific toxin will be better adapted to rapidly break it down than those soil communities that haven’t been exposed before. For instance, let’s say we have two corn fields of the exact same soil type right next to one another. On one field the grower applied Roundup (a.k.a. glyphosate) for weed control – we’ll call this farmer “Pete”. His neighbor (farmer “John”) understands the consequences of pesticide use, so he utilized non-toxic weed-prevention techniques and didn’t apply Roundup to his fields.
Now, let’s just say I came out to each of these fields in the middle of the summer and took soil samples. Back in the lab I would find that the microbial communities of Pete’s fields would be able to break down glyphosate, while the microbial communities of farmer John’s fields, which had never received an application of Roundup, could not. This has to do with rapid rates of adaptation and microbial gene transfer (and is also the basis for antibiotic resistance – which I’ll get to in another post later on). In general, we’ve assumed this rule of thumb to be true with most contaminants, but the article by Wang, et al., asserts that this may not be the case for hydrocarbon pollutants such as crude oil.
They set up a series of “bioreactors” (fancy way to say 250 mL flasks) to which they added a mixture of the same toxins that are found in crude oil and added soil as a source of microbes (a.k.a. inoculum). They used two different soils to test the concept I mentioned above: 1.) a “pristine” agricultural soil which had never been exposed to hydrocarbons before; or 2.) a soil which had been continuously contaminated with oil for many years. They placed these flasks on a shaker and tracked the number of bacteria and fungi, as well as the break-down of the toxins over the course of 180 days.
Not surprisingly, the numbers of bacteria and fungi correspondingly increased as the toxins were rapidly degraded, and the ole rule of thumb may require some modification; the soils with a history of contamination degraded the toxins more rapidly than the other soils (in fact, by the end of their 180 day study, the previously contaminated soil microbiota had broken down the oil toxins almost 100%!) But, what was more interesting, was how quickly the “pristine” soils were also capable of breaking down the hydrocarbon toxins. This suggests that the ability to degrade oil may be widespread in soils. Meaning, regardless of the soil’s history, the microbes within it may be capable of rapidly adapting to oil contamination and act to break it down. Unless, of course, their “pristine” soils weren’t as untainted as the researchers thought. But either way, good news in the Gulf, for sure.
The not so good news.
But here we reach the age-old debate over the utility of small-sale laboratory incubations in predicting what will happen in an ecosystem. The authors of the article by Short, et al., set out to rebuff all the lab studies that predict the natural soil communities should be great at breaking down oil by revisiting the site of a real-life oil spill and see where things stand 16 years later.
The problems with the “microcosm” and “bioreactor” type studies have a lot to do with oxygen and exposure (although there are other issues, but not enough time to get into all of them). With a soil/oil/microbe mixture on a shaker in the lab, oxygen is constantly allowed to diffuse in where it’s needed. But, out in the real world, once the oil percolates into the porous beaches and down into the soil, the oxygen which is absolutely crucial to microbial and chemical break-down processes becomes much more limited, if not down-right sparse.
Apparently the oil loss rate (break-down rate) in Prince William Sound (PWS) in the first 3 years after the spill was around 68% per year, which made everybody think all the oil would be gone in the next few years. But as the oil percolated deeper into the sand, soil, and sediments (between 1992 and 2001), the loss rate dropped to around 20% per year. After 2001 it dropped even further to under 5% per year.
However, the authors don’t actually attribute the slow-down entirely to low oxygen levels, but also to low nitrogen levels in the water and soils of the area, as well as low temperatures, and emulsification. Basically, the oil was whipped into a nasty “mousse” in some places which increased the viscosity of the oil (it was whipped into a gummy mess). This decreased the surface area where microbes could attack, and some of the oil in that state worked it’s way down into the soil where it sits to this day. The results of the study suggest that subsurface oil may persist for decades, even where there is oxygen available. Bad news for the Gulf.
So, what’s the take-home message?
The microbes of the sandy beaches, sediments and soils of the Gulf have had prior exposure to oil, albeit at much lower levels. This means that those organisms should be “primed” and able to break down the hydrocarbons and toxins in the oil. But it’s anybody’s ballgame as to how quickly we’ll see it happen.
The limiting factors to biodegradation will be: 1.) oxygen; 2.) nutrients; and 3.) surface area of the oil. The surface area of the oil was taken care of (at least in part) with millions of gallons of dispersant, resulting in less emulsification that what was seen in PWS. So, speeding the break-down of the oil on the surface before it can leach down into the soil profile or sediments where oxygen is depleted will be very important.
Biostimulation (fertilization) with nutrients such as nitrogen could theoretically achieve a more rapid degradation by providing limiting nutrients to the hydrocarbon-degrading microbes. I feel like I’ve said that before… weird.
Wang, C., Wang, F., Wang, T., Bian, Y., Yang, X., and X. Jiang. (2010). PAHs biodegradation potential of indigenous consortia from agricultural soil and contaminated soil in two-liquid-phase bioreactor (TLPB). Journal of Hazardous Materials, 176: 41-47. doi:10.1016/j.jhazmat.2009.10.123
Short, J.W. et al. (2007). Slightly weathered Exxon Valdez oil persists in Gulf of Alaska beach sediments after 16 years. Environmental Science & Technology, 41: 1245-1250. doi:10.1021/es0620033