The ocean floor is covered with sediments ranging from a few meters to more than 10 km thickness. Biological, geological, chemical and physical processes within marine sediments shape the composition of the natural environment. We study the cycles of chemical elements such as carbon, nitrogen and sulfur and their interaction with microorganisms.

Microorganisms in marine sediments derive energy by transferring electrons to a variety of external electron acceptors depending on their availability and energy yield and the availability of a suitable organic carbon substrate. Using a multitude of techniques we conduct direct measurements of microbially mediated transformation rates of specific compounds such as sulfate, iron and nitrogen. We measure the solid phase concentrations and isotopic signature of key elements and of the dissolved compounds in the interstitial water. This allows us to describe the partitioning of microbially mediated pathways of carbon mineralization and to model reaction rates throughout the sediment column. Our research takes us from the organic rich coastal sediments, over the continental shelf and into the nutrient-poor sediments of the deep oceans, and ranges from the very young to millions of years old deposits.

Carbon mineralization

The total microbial respiration in sediments can in principle be determined from either the consumption of electron acceptors or from the production of CO₂. Due to slow conversion of large pools it is generally not possible to determine concentration changes over time directly on deeply buried sediments. We develop new modeling approaches combining reverse modeling of porewater profiles, direct rate measurements based on radiotracers, and natural isotopic signature. In addition we have developed a D:L-amino acid racemization model by which it is possible to quantify living microbial biomass and necromass, turnover times of these pools as well as the carbon oxidation rate. We have a keen interest in microbial processes that linger on at extremely slow rates across immense time spans in deeply buried deposits.

Microbially mediated turnover of acetate in the sulfate reduction zone

Acetate is the main substrate for sulfate reducing bacteria in marine sediments. We developed a new method based on Two-Dimensional Ion Chromatography Mass Spectrometry (2D-IC-MS) for analyzing acetate at in situ levels. We wish to resolve a long standing uncertainty by the traditional acetate analysis that is suspected to flaw existing estimates of acetate turnover.

Cryptic sulfur cycling

The traditional view of marine geomicrobiology is that the sediment column is divided into compartments of different electron acceptor utilization. According to theory, a more energy efficient respiration excludes less efficient pathways by depleting the electron donors below the threshold for energy conservation for organisms with less efficient metabolism. This produces a characteristic sequence where oxygen is respired in the first few millimeter, then nitrate, manganese, iron, and sulfate. The organic matter below the penetration depth of sulfate is consumed by methane producing organisms. Nevertheless, the population of sulfate reducing organisms is about as high in the metanogenic zone as in the sulfate reducing zone. We investigate this phenomenon under the assumption that sulfate is continuously produced by oxidation of sulfide with ferric iron in the metanogenic zone, and that this sulfate is immediately consumed by an active population of sulfate reducing microorganisms. We study the turnover of sulfate based on radiotracer techniques and natural stabile isotopes as well as the natural populations from environmental RNA and DNA; and we study the metabolic capability of high affinity sulfate reducers.

KEY PUBLICATIONS FROM CfG

• Røy, H., Kallmeyer, J., Adhikari, R. R., Pockalny, R., Jørgensen, B. B. & D'Hondt / Aerobic Microbial Respiration in 86-Million-Year-Old Deep-Sea Red Clay. Science, vol. 336, s. 922-925, 2012.
• Karsten Alexander Lettmann, Natascha Riedinger, Ronny Ramlau, Nina Knab, Michael Ernst Böttcher, Arzhang Khalili, Jörg-Olaf Wolff, Bo Barker Jørgensen. / Estimation of biogeochemical rates from concentration profiles: A novel inverse method. Estuarine, Coastal and Shelf Science, Volume 100, 26–37, 2012.
• Jørgensen, Bo Barker ; Parkes, R. John. / Role of sulfate reduction and methane production by organic carbon degradation ineutrophic fjord sediments (Limfjorden, Denmark).. In: Limnology and Oceanography. 2010 ; Vol. 55, No. 3, p. 1338–1352
• Lever, Mark ; Heuer, Verena ; Morono, Yuki ; Masui, Noriaki ; Schmidt, Frauke ; Alperin, Marc ; Inagaki, Fumio ; Hinrichs, Kai-Uwe ; Teske, Andreas. / Acetogenesis in deep subseafloor sediments of the Juan de Fuca Ridge Flank: a synthesis of geochemical, thermodynamic, and gene-based evidence.. In: Geomicrobiology Journal. 2010 ; Vol. 27, No. 183-211