Archaic one-celled organisms may live up to 2,000 years. Low-nutrient concentrations, slow metabolism and unusual metabolic pathways are key to this longevity. A German-American team of geochemists and microbiologists studied a heretofore poorly understood group of subsurface archaea and gained insights into their physiology and the role they play in the deep biosphere. The study was part of the Ocean Drilling Program. The team led by geochemist Prof. Kai-Uwe Hinrichs from the DFG Research Center Ocean Margins (RCOM) at the University of Bremen published its results in the renowned Proceedings of the National Academy of Sciences, U.S.A.
Only recently, we have learned that sediments far below the seafloor harbor a unique ecosystem the so-called deep biosphere. This biosphere of Archaea and Bacteria may comprise one-tenth's of Earth's living biomass. "Archaea are single-celled organisms that have as much in common with Bacteria as Bacteria have with us. They comprise one of the three domains of life besides Bacteria and Eukarya. The latter encompass plants and animals," explains PhD student Julius Lipp, RCOM.
Together with PhD student Jennifer F. Biddle from Pennsylvania State University, he is joint first-author of the study. "Up to now, we knew archaea mostly form inhospitable places like hot vents on land and in the sea, hypersaline solutions, oil fields, or as in this case under extremely high-pressure and low-nutrient conditions deep in the seafloor," explains team leader Hinrichs. "The organisms down there seem to mediate familiar processes differently than well-studied organisms in shallower environments, e.g., they appear to metabolize methane completely differently."
A very important process indeed: deep within the seafloor, Archaea produce huge amounts of methane. Other Archaea feed on this methane and turn it into carbon dioxide. Since carbon dioxide is a 25 times less powerful greenhouse gas than methane, this dampens the overall impact on global climate. The scientists targeted deeply buried sediment layers in which Archaea anaerobically oxidize methane to carbon dioxide. "Until now, we knew these types of organisms only from methane-rich areas. But the targeted areas are very low in methane," says Hinrichs. "Comparisons of genetic material showed that we dealt with species that had up to now not been associated with this process. Also, the metabolic turnover of the entire ecosystem is so low that the cells may only divide every 100 to 2,000 years."
The scientists were fascinated by their findings: "Our studies of the Peruvian continental margin indicate that the bulk of the energy in the system is derived from the degradation of methane to carbon dioxide. But the carbon that the Archaea incorporate into their cell comes from fossil organic material, not from methane," Hinrichs says. "Quite different from what we've known so far."
A new combination of methods made it possible: a strategy designed together with his Penn State colleague Prof. House, enabled Hinrichs, House and co-workers to analyze the ratio of the carbon isotopes 12C and 13C in living archaeal cells. The results provided clues on the source of the carbon incorporated into cells fossil material rather than methane. In addition to the analysis of specific lipids in Bremen, the team by Prof. Andreas Teske from the University of North Carolina, Chapel Hill, provided a special genetic fingerprint. Together, these techniques showed how many and what types of microorganisms where not only present but alive. "Organic material is degraded so slowly in the deep biosphere that, if you were to analyze all genetic material present, you may end up with information of fossil organisms," Hinrichs says.
The deep biosphere is largely untapped: "There are heavenly bodies about which we know more. In addition to financial support from DFG (Deutsche Forschungsgemeinschaft) and other agencies, we received funding from NASA because our techniques and research questions are relevant for the search of life on other planets. That's how strange the deep biosphere is," Hinrichs points out.
Although the described processes are observed deep underneath the seafloor, they influence the global balance of climatically active elements. "Even though the processes are extremely slow, the area of reactive ocean margins is enormous. That way the Archaea metabolize immense amounts of methane to carbon dioxide and therefore have a great influence on our climate," Hinrichs stresses.