The oil that leaked out of the well following the explosion on April 20th 2010 at the Deepwater Horizon drilling platform in the Gulf of Mexico brought back memories of earlier oil spills. The publicity tends to concentrate on oiled beaches, seabirds struggling in black goo and a series of gloomy predictions about the long-term damage to commerce, tourism and wildlife.
The stories soon disappear from the headlines, but the effects of oil spills can be long-lasting, as can the effects of detergents used to ‘clean up’ the oil.
The Deepwater Horizon well was finally capped 85 days after the explosion and over that time about 780 000 cubic metres oil had escaped into the sea.
The Gulf of Mexico oil spill is different from most previous spills. In the past, most major spills have been at the surface. The Deepwater Horizon rig was drilling in seawater about 1500 metres deep, and the oil that gushed from the broken well on the sea bed was released at great depth.
A lot reached the surface, but some has probably travelled in deep water currents and it is hard to predict the effects that this might have, and how long the oil will take to decay. One of the agents of decay is microbial action, where bacteria and other microbes are able to utilise components of oil as a food source.
Crude oils vary in composition but all contain a mixture of hydrocarbons, molecules containing carbon and hydrogen only. Some hydrocarbons have ring shaped molecules, and include benzene, naphthalenes, xylenes and toluenes.
Hydrocarbon molecules containing two or more rings, are known as polycyclic aromatic hydrocarbons (PAHs), and they are highly toxic to marine organisms. Hydrocarbons made of straight chain molecules are also abundant. The longer the hydrocarbon chain the greater the viscosity - the thickness and resistance to flow).
The components of crude oil are separated by refining, and oil tankers may be carrying a refined fraction rather than crude oil.
Many marine microbes feed on oil and refined oil products, gaining energy for growth and reproduction by breaking down the hydrocarbons, using the process of respiration. These microbes are abundant where oil seeps naturally through fissures in the oceanic floor into seawater, as happens in the Gulf of Mexico.
After an accidental oil spill into open water, the numbers of oil eating microbes increase. Oxygen concentration is high in sea water close to the surface, so microbes break down oil at relatively rapid rates using aerobic respiration.
Oil eating bacteria include Alcanivorax borkumensis, which breaks down straight-chain and branched-chain hydrocarbons 1. As the oil is fragmented into droplets by wave action, Alcanivorax cluster on the surface of droplets forming a biofilm, and secrete biosurfactant which helps with ingesting the oil.
The effects of oil dispersants
Although, in theory, dispersants used to break up oil slicks into droplets could facilitate microbial degradation, there are concerns about their toxicity.
By July 30th 2010 about 7 million litres of the dispersant Corexit had been used to treat oil spilled from Deepwater Horizon, and claims were made that treated oil had disappeared.
However the discovery in August 2010 of an undersea plume of oil droplets 35 km in length at 110m depth in the Gulf suggested the oil had not disappeared.
Smaller undersea plumes had also been found. The oil droplets in the plumes probably derive from the action of dispersant.
Whether microbial breakdown of oil is diminished or enhanced by Corexit is not known, but species such as Alcanivorax which make their own surfactants may be affected adversely.
The Deepwater Horizon well is close to low energy tidal areas, estuaries, and salt marshes and, by May 2010, oil had coated and penetrated marshes on the East bank of the Mississippi in Louisiana.
Oxygen levels are highest near the surface of sediments and around burrows. Study of samples of sediment collected around worm and mollusc burrows in an intertidal mudflat (Lowes Cove, Maine), yielded a previously unknown bacterium, Lutibacterium anuloederans, LC8, which can break down PAHs aerobically.
Cycloclasticus species also break down PAHs in sediments, as well as in the open sea. However, oil pollution buried in deeper sediments can linger for many years.
Both wave action at sea and dispersants create oil droplets, which when washed ashore, seep into the sediment with the water. The accumulated droplets clog the sediment particles, blocking the though flow of oxygen rich seawater.
So shallow sediment layers become anaerobic, depleted of oxygen, and only microbes that can break down oil anaerobically, without oxygen, can survive. Anaerobic respiration is much slower than aerobic, so oil can persist in sediments.
Examination of the effects of previous oil spills can indicate the possible long term effects of the Deepwater Horizon oil spill in coastal marshes and estuary ecosystems.
Many fish and birds feed on animals living in sediments, especially molluscs, shrimps and worms. Their vulnerability to oil spills was shown by the effects of the explosion in June 1979 of the IXTOC 1 oil well in the Gulf of Mexico.
The IXTOC 1 well head was located at 50 metres depth and 50 miles offshore. Over 10 months, about 525 000 cubic metres of crude oil flowed into the sea. Oil degrading microbes increased rapidly but were limited by lack of phosphates and nitrates in the seawater.
Currents and wind action spread the oil over a wide area, and after two and a half months, a floating mousse of oil had drifted 750 -1000 km distance from the oil well. Studies on how the oil spill impacted on marine life report immediate serious effects.
Viscous oil coated the Gulf marshes, and seagrasses died. Animals were oiled and shrimp and other fisheries were devastated. Yet one year later, the seagrass beds were recovering, and oil on the surface was weathered and contained no volatile or water soluble chemicals.
Shrimp fisheries also recovered quickly. 30 years on, IXTOC 1 oil lingers in the mangrove swamps in the Bay of Campeche. Fishermen living have not found any mangrove oysters on the Isla Arena since IXTOC 1 exploded. Both the adult oysters and their free swimming larvae were probably extirpated by the oil. IXTOC 1 oil trapped in sediments was shown to have toxic effects on recently settled marine worms, shrimps, barnacles and bivalve molluscs.
The long term effects of the 700 cubic metres of fuel oil spilled from the barge Florida in 1969 also provides insights into what may happen to residues of Deepwater Horizon oil. The Florida ran aground in Buzzard’s Bay, Massachusetts and the leaked oil flooded salt marsh at Wild harbour killing grasses, seaweeds, fish, crustaceans, marine worms and molluscs.
More than 40 years on, an estimated 100 kilogrammes of part-degraded petroleum hydrocarbons persist at 8 – 20 centimetres depth. This residue affects ribbed mussels, Geukensia demissa and fiddler crabs Uca pugnax, both pivotal components of the food web.
Mussels use a siphon to take in seawater which filters through their gills, where food particles, plankton and particles of oil residues are trapped for feeding. 'Waste' water flows out of a second siphon. The protein rich mussels are eaten by birds, crustaceans and other predators. Long-term effects of the oil on the mussels at Wild Harbour included depressed growth rates, and decreased filtration rate.
Fiddler crabs also show the effects of the lingering oil as they burrow and turn over sediment foraging for food. Moulds of their burrows were made by pouring plaster of Paris into 19 burrows at Wild harbour, and 12 at Sippewissett marsh which had not been oiled.
The latter burrows were on average 14.8cm deep, whereas those at Wild harbour were 6.8 centimetres deep, close to the depth at which oil is in the sediment.
Shallow burrows reduce aeration of the sediments, so slowing down microbial degradation of the oil.
The risks of removing oiled sediments
Nevertheless, removal of oiled sediments soon after an oil spill is not always wise, as shown by the experience at the Ile Grande salt marsh in Brittany, coated in oil in 1978 after the Amoco Cadiz ran aground on rocks.
Vigorous removal of oiled silt from the marsh increased erosion and delayed regeneration of marsh plants, samphire, sea purslane and sea blite. Yet where the sediments were not removed, oil persisted and sea rush plants growing in the oiled marsh were yellow and weakened.. So it is impossible to find an ideal solution in the short term.
While much can be learned from previous experience of oil spills, it is difficult to predict how long it will take for the Deepwater Horizon oil in sediments to break down. It’s possible that the warm water temperatures in the Gulf may speed up the rate of decomposition of oil.
We’ve learned from previous oil spills that buried oil persists for a long time in deeper sediment layers where there is very little oxygen. So far no biotechnical solution is available to overcome this. However, bacteria in the open sea and at shallow depth in sediments break down hydrocarbons aerobically and so play an important part in clearing oil from salt marsh and estuaries.
The aftermath of IXTOC1 suggests that recovery is possible and can be quite rapid. However research has shown that oil buried in sediments can continue to affect animals and saltmarsh grasses for a long time.
Large oil spills can have long-term effects for a species if a generation of larvae and adults are lost, as may have happened with mangrove oysters9 after IXTOC 1 In the meantime, ecologists are investigating and recording the impact of the Deepwater Horizon oil on coastal marshes and reefs. We can only wait and see what happens.
References and further reading
Schneiker S, Martins de Santos V A P, Bartels D, Bekel Th, Brecht M, Buhrmester J, Chernikova T N, Denaro R, Ferrer M, Gertler C, Goesmann A, Golyshina O V, Kaminski F, Khachane A, Lang S, Linke B, Mchardy A, Meyer F, Nechitaylo, Puhler, A, Regenhardt D, Rupp O, Sabirova J S, Selbitschka W, Yakimov MM, Timmis K N, Vorholer F-J, Weidner S, Kaiser O, P N Golyshin (2006) Genome sequence of the ubiquitous hydrocarbon degrading marine bacterium Alcanivorax borkumensis sheds light on marine oil degradation. Nature Biotechnology 24 997 – 1004
Camilli R, Reddy C M, Yoerger D R, Van Mooy B A S, Jakuba M V, Kinsey J C, McIntyre C P, Sylva S P and Maloney J V (2010) Tracking hydrocarbon plume transport and biodegradation at deepwater Horizon Science online August 19th 2010 Accessed August 20th 2010.
Kintisch E (2010) Toxicity aside dispersants could undermine natural oil eaters. Science Insider Science May 26th
Chung W K and King G M (2001) Isolation, characterization andPAH degradation potential of aerobic bacteria frommarine macrofaunal sediments and description of Lutibacterium anuloederans and Cycloclasticus spirillensus. Applied and environmental microbiology 5585 – 5592
Kasai Y, Kishira H and S Harayama (2002) Bacteria belonging to the genus Cycloclasticus play a primary role in the degradation of primary hydrocarbons released in a marine environment. Applied environmental microbiology 68 (11) 5625 -5633
Patton J S, Rigler M W, Boem P D and Fiest D L (1981) Ixtoc 1 oil spill: flaking of surface mousse in the gulf of Mexico Nature 290 235 – 238
Baca B J, Schmidt T M and Tunnell J W (1979) Ixtoc oil in seagrass beds surrounding Isla de media. Unpublished paper delivered at International symposium IXTOC-1
Schrope (2010) The lost legacy of the last great oil spill Nature 466 304 – 305
Kalke R D, Duke T A and Flint R W (1982) Weathered IXTOC 1 Oil effects on estuarine benthos. Estuarine and shelf science 15 75 - 84
Culbertson J B, Valiela I, Peacock E E, Reddy M C, Carter A and VanderKruik R (2007) Long-term biological effects of petroleum residues on fiddler crabs in salt marshes Marine Pollution Bulletin 54 955 -962
Culbertson J B, Valiela I, Olsen, Y S and Reddy C M (2008) Effect of field exposure to 38 year old residual petroleum hydrocarbons on growth, condition index, and filtration rate of the ribbed mussel Geukensia demissa. Environmental Pollution 154 312 – 319
Siep K L (1984)The Amoco Cadiz oil spill – At a glance. Marine pollution bulletin 15 218 -220
Grundlach E R, Biehm P D, Marchand M, Atlas R M, Ward D M and Wolfe D A (1983) The fate of Amoco Cadiz oil Science 221 122 - 130