A microcosm in the seafloor

On July 20th, 2008, scientists from the Center of Marine Environmental Sciences (MARUM) at Bremen University and their Japanese colleagues published an article on microbial life deep beneath the seafloor.

The researchers show that – expressed in terms of carbon mass – this so-called deep biosphere contains about 90 billion tons of microbial organisms. That corresponds to about one tenth of the amount of carbon stored globally in tropical rainforests. Applying novel methods, the German-Japanese team concluded that about 87 percent of the deep biosphere consists of Archaea. This is in stark contrast to former reports, which suggested that Bacteria dominate the subseafloor ecosystem.

A team led by Prof. Kai-Uwe Hinrichs investigated sediment samples from several hundred meters beneath the seafloor. The sediment cores were retrieved in the Atlantic and the Pacific Oceans as well as in the Black Sea, most of them well below the ocean floor during expeditions of the Integrated Ocean Drilling Program (IODP). The scientists pursued two main objectives: “We wanted to find out which microorganisms can be found in the seafloor, and how many of them are living down there”, states biogeochemist Kai-Uwe Hinrichs.

For quite a long time, scientists believed that the extreme conditions such as high pressure, lack of oxygen, low supply of nutrients and energy would make deep sub-seafloor environments inhabitable for any life form. But now they know better: Sea-going expeditions have proven the existence of the deep biosphere. “In general, life at and below the seafloor is dominated by minute monocellular organisms. According to our analyses, Bacteria dominate the upper ten centimeters of the seafloor. Below this level, Archaea appear to take over the major fraction of the biomass pool”, says MARUM researcher Julius Lipp, who has just completed his PhD on this subject.

According to Lipp, Archaea make up at least 87 percent of organisms that colonize the deep biosphere. “These subsurface Archaea can be viewed as starvelings. Compared to Bacteria, Archaea appear to be better adapted to the extreme, chronic deficiency of energy that characterizes this habitat – a consequence of the only food being stable, fossil remnants of plants that were pre-digested by generations of other microorganisms””, says Lipp.

Next to Bacteria, Archaea represent one of three domains in the systematics of life. Both groups can be identified by fat-like molecules, so-called lipids that make up their cell membranes. To date, estimations of the deep biosphere biomass range from about 60 to 300 billion tons of carbon. “Our measurements determined by entirely independent means are with 90 billion tons of carbon right in this bracket”, says Prof. Hinrichs, head of the Organic Geochemistry Group at MARUM and the Department of Geosciences, Bremen University. The authors of the Nature paper assume that about 200 million cubic kilometers of mud below the ocean floor are inhabited by microorganisms – a volume roughly corresponding to a 600 kilometer-long cube.

Because all current techniques aimed at detecting biomass in the deep biosphere arrive at different conclusions regarding its quantity and composition, Prof. Hinrichs has initiated an international “ring experiment”. Currently, his colleagues in German, European, US-American, and Japanese laboratories are investigating standardized sediment samples from the seafloor with different methods. Moreover, they want to find out whether identical methods applied in different labs lead to dissimilar results. The aim is to gain a more reliable picture of life in the deep biosphere. In September, the researchers involved in the experiment will present and discuss their findings at MARUM. “All participants hope that this experiment will shed a bit more light on the dark deep biosphere” Hinrichs states.

Further information/interviews/photos:

Yasmin Khalil
MARUM Public Relations
Fon: +49 – 421 – 218-65541
Email: ykhalil@marum.de
Prof. Kai-Uwe Hinrichs
MARUM Organic Geochemistry
Fon: +49 – 421 – 218-65700
Email: khinrichs@uni-bremen.de