Sometimes during an oil spill, anoxic zones will even appear directly in the water, since the aerobic oil-degraders draw the essential oxygen from the seawater. In order to break down one drop of 0.2 millimeters of oil completely, aerobes require the oxygen from up to 80 liters of water. In the case of large amounts of oil in the environment, the aerobes themselves will deplete their “life elixir.” Once the oxygen has been completely consumed, the anaerobes become active. They will go through the oil constituents unhurriedly.
If the oil comes in masses, this may cause an additional problem. Because of the depletion of oxygen and the anaerobic degradation, hydrogen sulfide is formed, which is toxic even in low concentrations. “The more you study the different aspects of oil degradation, the more you realize that we need to handle the valuable resource that is oil with much greater care and responsibility,” Widdel says. “An oil spill should concern us all; after all, we all use it.”
The degradation of thick crude oil masses in anoxic or near-anoxic conditions can take years or even decades, and even then the results will probably be incomplete. That is why, long after a tanker accident, a black, smeary oil residue can still be found deep down in a sandy beach. Nevertheless, the anaerobes are in no hurry. For millions of years now, they have been feeding on the oil that is available in natural reservoirs in the ocean. Indeed, anaerobic oil-degrading bacteria can be found anywhere in ocean sediment containing hydrocarbons, whether in the silt of a Frisian marina or in the Gulf of California, in Western Mexico.
Here, in a sea basin around 2,000 meters deep, hot water rises through cracks in the Earth’s crust. The remains of large amounts of dead algae that have made their way down from the surface simmer in the hot water. This is a place where oil forms unusually fast. Normally, crude oil is produced far below the sea, under hundreds of meters of heavy sediment layers, where high temperatures and pressures prevail. In such a geological pressure cooker, sugars or fatty acids, which are rich in oxygen, are boiled down to hydrocarbons from the dead biomass.