Ocean beds are the habitat of choice for a multitude of living microbial-organisms. Their occurrence in deeply buried sediments of the depths of the ocean remained undetected for a long time, despite estimations showing that the biomass in the "deep biosphere" makes up between a third and a tenth of the total biomass of our planet. Microbial diversity in the depths of the biosphere is huge and numerous groups of metabolic systems have already been identified. In the overall picture of things they are extremely important. Reactions, such as the formation of methane–a major climate warming gas–but also the oxidation of methane, play an important role in conditions on the surface of the Earth. "Until recently not much was known about these organisms," says Patrick Meister.

Collecting data in subsea floor sediments

The PhD student from the Geological Institute at ETH Zurich (1) took part in the international deep sea "Ocean Drilling Program“ (ODP) (2), which is co-funded by the Swiss National Science Foundation (3), on board the research vessel "JOIDES Resolution“. The goal of the drilling expedition, entitled "Leg 201“, was to explore the deep biosphere in the eastern Pacific ocean and in the continental margin of Peru. The research vessel is equipped with a 63-metre high bore drill, which makes it possible to take sediment samples from areas as far down as 1000 metres below the subseafloor. The structure and composition of the drilling cores can be analysed on the spot in fully equipped laboratories on the ship. Meister brought conserved samples back to Zurich with him, in order to carry out further analyses.

The research vessel JOIDES is equipped with a 63-metre high drilling rig. large

Great diversity

The results of the expedition, which took place in spring 2002, have now been published in "Science“ (4). Above all, they show how micro-organisms are spread throughout the sedimentary layers of the ocean bed. The activity of the organisms can be ascertained by a study of the chemical processes

Investigator of dolomite formation in the deep biosphere: ETH geologist and PhD student Patrick Meister. large

and the formation of gradients in the sediment. Just as the oceanographic conditions on the Peru margin are different from those in the accompanying shelves or on the open sea, so too are the communities of microbes and their metabolic reactions in these surroundings. Surprisingly more single cell organisms were found 100 metres below the subseaflor than in sediment nearer the surface. This shows that the occurrence of cells does not decrease exponentially as hitherto supposed but that, under certain circumstances, the number increases as, for example, when the deep water is provided with sulphate rich water.

From microbes to dolomites

In addition to the recently published data Meister's particular interest is directed at the geomicrobiological processes that take place in the deep biosphere. He works on the team of Professor Judith McKenzie ,which examines microbiotic processes that lead the formation of carbonate rocks. In earlier experiments in the laboratory the team had already verified that dolomite, a calcium-magnesium carbonate, can ensue with the help of microbes. "But isotopic examinations of the carbonic samples from ODP Leg 201 now demonstrate more clearly how these processes unfold in nature," explains Meister. The "isotopic fingerprints" of strontium, carbon and oxygen in the subseafloor sediment samples show that dolomite forms in relative low depths, just below the surface. Now, and in addition, the new findings indicates that dolomite layers also evolve in sulfate-methane transition zones within the ocean floor sediment. This hypothesis is supported by the especially high number of single cell organisms determined during Leg 201 in this neighbouring region. It is probable that these bacteria are involved in a decisive manner in the formation of dolomites. Meister sees the next step in identifying the single cell organisms that favour the formation of dolomite–two other members of the geomicrobiological team have already taken up this challenge.

Researchers on board JOIDES taking drilling samples from 1000 metres below the sea bed. large