As a rule the remains of living organisms that have died do not last very long. Organisms consist largely of carbohydrates and proteins, which are readily destroyed by degradation processes. Nonetheless organic material is found in rocks that are millions of years old, for example in the form of coal or petroleum. The reason for this is that lipids with their carbon double bonds are converted relatively quickly into saturated compounds that do not have any double bonds and are more resistant to decomposition. Up to now there were only vague ideas as to precisely how and under what circumstances this transformation takes place. It was agreed that preservation takes place preferentially in what is called an anaerobic environment, i.e. one that is free from oxygen, and that micro-organisms are responsible for the conversion.

A team of researchers from ETH Zurich and the Louis Pasteur University in Strasbourg has now succeeded in gaining a better understanding of this mechanism. As the researchers report in Science Express (1) this week, a purely chemical process plays a decisive role in stabilising the organic compounds in the early phase of sediment deposition. Consequently it is necessary to revise the thesis that micro-organisms are the main agent responsible for converting the unsaturated carbon chains into stable forms.

An unusual body of water

For the purposes of their study, the researchers examined samples of water and sediments from Lake Cadagno in the Canton of Ticino. This mountain lake, which lies a short distance above the water supply dam, Lake Ritom, has several special features, as Stefano Bernasconi from the Geological Institute of ETH Zurich explains. The body of water, which is about 20 metres deep, has an extremely stable stratification: at the top the water is well oxygenated, whereas anaerobic conditions prevail at the bottom. The two bodies of water are separated from one another by a sharply-defined layer about one metre thick which is the habitat of highly specialised reddish-coloured bacteria.

The lower layer is anaerobic because underwater springs introduce sulphate-containing water into the lake. The sulphate is converted into hydrogen sulphide by bacteria in the sediment and in the lower layer of water. This creates the conditions that enable dead organic material to be better preserved. The reddish-coloured bacteria in the boundary layer utilise the hydrogen sulphide for a special form of photosynthesis, thus preventing it from escaping into the upper layer of water.

An ideal model system

Because of this special situation, Lake Cadagno is an ideal model system for geologically important habitats. Similar conditions would have been present in many marine basins during the geological history of the Earth. This is precisely where, over the course of time, the source rocks were formed in which the deposited organic material matured to form petroleum. Nowadays a comparable situation is found only in a few locations, for example in the Black Sea and in certain fjords in Norway.

The researchers have now discovered that in the presence of hydrogen sulphide, the conversion of certain organic compounds that are typical for bacterial and algae obviously commences shortly after the living organisms have died. Partially saturated carbon chains are already found in the uppermost layers of sediment that were deposited only a short time ago. Another conspicuous feature is that the opening up of the double bonds takes place at random positions along the carbon chains. “This suggests that the conversion is not caused by micro-organisms,” says Bernasconi, “because as a rule their metabolism causes them to attack at specific positions.”

A view of Lake Cadagno (foreground) and the water supply dam Lake Ritom at a slightly lower level. The Leventina valley is visible in the background. (Photo: Mauro Tonolla) large

Replication in the laboratory

The group was able to confirm its hypothesis by laboratory studies. The scientists allowed artificial solutions of the corresponding organic compounds to react for several weeks at 50 to 90°C. Analysis showed that the conversion occurring in the laboratory was exactly the same as the transformation that can be observed in the lake sediments. This chemical reaction takes place in two steps, as Bernasconi explains: first of all a single sulphydryl (-SH) group bonds to the carbon chain. In a second step this group is then reduced; the sulphur atom is removed so that only a hydrogen atom remains.

Bernasconi is convinced that the team has discovered an important mechanism in the global carbon cycle. He is convinced that “This helps us to gain a better understanding of how petroleum was formed.” The newly discovered process could also be significant in relation to what are known as “Black Smokers”. These are submarine springs venting hot water containing sulphur on the ocean floor. It is suspected that life on Earth might have originated in the region surrounding these springs. The chemical reactions taking place around these “Black Smokers” could be similar to those occurring in the depths of the inconspicuous Lake Cadagno.