When an oil spill occurs, chemical dispersants are routinely applied to the surface of the oil-contaminated seawater or into deeper water regions. By breaking down the oil into small droplets, the dispersants prevent slicks from spreading and reaching coastal areas kilometres away. The oil that remains suspended in the water has a greater surface area than before dispersion and is thus more easily accessible to oil-degrading microorganisms. This accelerates the process of degradation. Dr. Sara Kleindienst, a molecular ecologist from the Centre for Applied Geoscience at the University of Tübingen, has now shown that chemical dispersants do not stimulate oil biodegradation. In cooperation with an international team of researchers, Kleindienst simulated the Deepwater Horizon oil well blowout in the Gulf of Mexico and obtained unexpected results on the degradation of harmful substances following oil spills.
Crude oil can also enter marine environments through natural oil and gas wells. Associated with these wells is a natural microflora consisting of hydrocarbon-degrading bacteria, so-called oil-degrading microorganisms. Dr. Sara Kleindienst started studying organisms that live under such extreme conditions during her doctorate at the Max Planck Institute for Marine Microbiology in Bremen. Kleindienst's research focus was mainly on the microbial species found in deep-sea oil and gas wells, including those found in the area of the oil drilling platform Deepwater Horizon. In April 2010, the explosion of Deepwater Horizon led to one of the biggest environmental disasters ever. More than 750 million litres of oil were spilled into the sea. Immediate measures to contain the spread of oil involved the application of seven million litres of chemical dispersants on the surface of the water and in deeper water regions in an attempt to disperse hydrocarbons and stimulate the biodegradation of the oil. “I was very interested in the effects the disaster had on oil-degrading microorganisms. After my doctorate, I was offered the opportunity to study this at the University of Georgia in the USA, which I gladly accepted,” says Kleindienst.
Together with Prof. Samantha Joye and her team at the University of Georgia, Kleindienst studied the consequences of the Deepwater Horizon disaster on the microbial ecosystem by analysing water samples collected in the Gulf of Mexico at the time of the accident. “As luck would have it, Prof. Joye was travelling with a research vessel and collecting water samples when the blowout occurred. We were extremely lucky, if I may put it that way, as Prof. Joye and her team got the chance to collect numerous seawater samples for subsequent molecular and geochemical analyses,” says Kleindienst.
“We studied the microbial composition of the water column before, during and after the disaster and found that certain groups of microorganisms, for example, Colwellia species, had outgrown other species. This clearly showed that the diversity of the microbial community had declined,” says Kleindienst. The researchers analysed the DNA sequences in detail using a method known as oligotyping. “We discovered something completely new,” says Kleindienst. “What we found was that the microorganisms that grew when an oil spill had occurred differed from those found in natural oil and gas wells.”
Virtually all oil spills, caused for example by tanker accidents, leaking pipelines or incidents on oil drilling platforms, are usually cleaned with chemicals, especially in the USA. Oil dispersants are solvents that break up oil into small droplets, just like detergents do. “The oil does not disappear, but is very finely dispersed in the water column,” says Kleindienst. “Dispersants emulsify surface oil slicks and prevent them from being transported across long distances or, worse, washed up on shorelines. Dispersants increase the surface area of the oil, so that oil-degrading microorganisms find it easier to access and degrade the oil. This stimulates the biodegradation of the oil.” Kleindienst adds: “The effect of such dispersants on microorganisms has not yet been studied in detail. Numerous tests have been carried out with animals and microorganisms, albeit microorganisms found in Gulf of Mexico ecosystems. This is why we decided to focus especially on these microorganisms."
Dispersants were also applied in the Gulf of Mexico following the Deepwater Horizon disaster around six years ago. The chemicals were applied onto the seawater surface by plane, and for the first time ever also at greater depths. “At that time, a deepwater plume, enriched with dispersant components and hydrocarbons formed early on over an area of many kilometres. The concentration of oil and dispersant components was particularly high in this deepwater plume,” says Kleindienst. “We therefore asked the question as to what might have happened with the microorganisms at these depths during the Deepwater Horizon oil spill.
In order to find out whether dispersants actually altered the microbial community composition in deepwater samples, the researchers analysed a total of 130 litres of seawater collected at a depth of 1,200 metres in the Gulf of Mexico. The water contained natural oil-degrading microorganisms. In the laboratory, the researchers mixed the water with either oil, dispersants, or both, as well mixtures containing nutrients in order to recreate the environmental conditions that prevailed in the depths of the Gulf when the Deepwater Horizon disaster happened. The microcosms were then studied according to various criteria. For example, the researchers studied how the oil and the chemical dispersants were degraded, whether the microbial community composition altered, whether the selection of one bacterial population over another would lead to differences in hydrocarbon-degradation rates and alter the oil-degradation efficiency. Radioactively labelled hydrocarbons were used to quantify and monitor the degradation rate of selected oil component classes.
The researchers found that – unlike previously believed – dispersants do not stimulate the biodegradation of hydrocarbons*. It turned out that the natural oil degraders of the genus Marinobacter were the dominant microbial responders in seawater samples to which oil, but no dispersant, was added. However, in the presence of dispersant, the relative abundance of Marinobacter decreased and bacteria of the genus Colwellia became more abundant. The same was observed when dispersant, but no oil, was added to the sample. “We therefore assume that Colwellia bacteria use the complex dispersant mixture as growth substrate, i.e. they degrade it. We still have to investigate this in greater detail,” says Kleindienst. Even more important for the researchers was their observation that the depressant suppressed oil-degrading bacteria. “In many cases, we also found a higher microbial activity in microcosms when oil, but no dispersant, was present. We have therefore been able to show that dispersants do not stimulate microbial hydrocarbon-degradation rates in deeper water areas, and that the oil is degraded just as effectively without dispersants.
The researchers will now carry out further tests to find out why the genus Colwellia grew far better than the genus Marinobacter. Over the next few years, Kleindienst will also study whether the growth of natural oil degraders is stimulated or inhibited, and whether the chemicals have a toxic effect on the organisms. Her American colleagues will focus on the situation in surface seawater where conditions differ from those of deepwater areas. They assume that light and higher temperatures favour the growth of different microorganisms. The researchers have already conducted a preliminary experiment. “My American colleagues observed that the surface water effects were the same as in deeper water regions. But we need to carry out a detailed experiment before we can say more,” says Kleindienst. In any case, the researchers hope that their research results will be taken into account when the next oil spill occurs. “We hope that we will have more data available before the next disaster occurs,” concludes Kleindienst.
* Original publication: S. Kleindienst , M. Seidel , K. Ziervogel , S. Grim , K. Loftis , S. Harrison , S. Malkin , M. J. Perkins, J. Field , M.L. Sogin , T. Dittmar , U. Passow , P.M. Medeiros , S.B. Joye. Chemical dispersants can suppress the activity of natural oil-degrading microorganisms. Proceedings of the National Academy of Sciences, vol. 112 no. 48, pp. 14900–14905 (doi: 10.1073/pnas.1507380112)