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- Organische Geochemie
- Forschung
- Biogeochemical processes
Hinrichs Lab - Biogeochemical processes
Introduction
In his number-one-bestseller The Swarm, Frank Schätzing depicted a fictitious “yrr”, which consist of single-cell organisms that operate in groups in the oceans. With a collective intelligence and inheritable memories that is passed on by manipulating parts of their DNA, the yrr rule the ocean biosphere and make attempts to eliminate the human race, which is devastating the Earth's oceans. Although bearing some physical resemblance to the yrr, the real marine microorganisms are far from such an aggressive character. Quietly and tenaciously, they regulate the cycling of matter in the oceans, which further controls the functioning of the Earth as a habitable planet for life. Once discovered and studied, they surprise even the most experienced scientists by their diverse metabolism and the immense influence of the biogeochemical processes in which they are involved.
The fact that the key microbial players in natural environments usually escape isolation attempts (Eilers et al., 2000) makes biogeochemistry a fundamentally different subject from biochemistry. While biochemistry utilizes model organisms to investigate subcellular processes, biogeochemistry aims at clarifying the basic relationships between matter and organisms: Do microorganisms participate in the turnover of a particular compound? If they do, who are they? What is the precursor of this compound, and what is its fate? What is the timescale of the involved processes, and what is their global relevance?
Studies in the Hinrichs Lab focus on biogeochemical processes associated with the carbon cycle.
The fact that the key microbial players in natural environments usually escape isolation attempts (Eilers et al., 2000) makes biogeochemistry a fundamentally different subject from biochemistry. While biochemistry utilizes model organisms to investigate subcellular processes, biogeochemistry aims at clarifying the basic relationships between matter and organisms: Do microorganisms participate in the turnover of a particular compound? If they do, who are they? What is the precursor of this compound, and what is its fate? What is the timescale of the involved processes, and what is their global relevance?
Studies in the Hinrichs Lab focus on biogeochemical processes associated with the carbon cycle.
Our research approach
In order to address these questions, we combine the analysis of pristine environmental samples with the investigation of processes in incubation experiments in our laboratory. In order to track processes, we study gases and watersoluble metabolites. However, not only metabolites but also membrane lipids provide information on biogeochemical processes. The latter allow us to identify the involved groups of microorganisms and metabolic capacities. Moreover, their stable isotopic composition encodes information on the used substrates and metabolic pathways.
For both the investigation of pristine environmental samples and incubation experiments, isotopic investigations are an extremely useful tool. They allow us to clarifying the relationship between precursors and products, to link processes to microorganisms, and to measure the rate of biogeochemical processes.
For both the investigation of pristine environmental samples and incubation experiments, isotopic investigations are an extremely useful tool. They allow us to clarifying the relationship between precursors and products, to link processes to microorganisms, and to measure the rate of biogeochemical processes.
Scientists in the Hinrichs Lab have combined investigation of environmental samples and laboratory incubation to examine the biogeochemical processes associated with low-molecular-weight organic compounds, currently including hydrocarbon gases, methylated sulfides and volatile fatty acids.
Example 1: Biogeochemistry of methane and higher hydrocarbon gases
In marine sediments, methane and other volatile hydrocarbons (C1 to C6) are formed during microbial and/or thermal degradation of organic matter (OM). In particular for methane it has long been known that the different sources and pahtways result in different stable isotopic compositions (e.g. Whiticar et al., 1986; Whiticar, 1990). The accumulation of volatile hydrocarbons in the form of dissolved, free and hydrate-bound gas in pore spaces has been extensively investigated in biogeochemical studies as well as in the exploration of gas and oil fields (see Reeburgh, 2007, for a review).
In the Hinrichs Lab, we routinely monitor the concentration and stable isotopic composition of methane and higher hydrocarbons (if present) in studies of pristine environmental samples and in incubations experiments.
During Leg 201 of the Ocean Drilling Program (ODP), the routine shipboard gas analysis resulted in a big surprise. When the sediment samples were treated with strong base, substantial amounts of ethane and propane were detected although the sampled sites were not in contact with fossil hydrocarbon reservoirs. Based on the evidence of carbon isotopic modeling and thermodynamic calculation, novel ethano- and propanogenic pathways involving acetate, bicarbonate and molecular hydrogen were proposed (Hinrichs et al., 2006).
This finding upsets the general belief in exploratory petroleum geochemistry that hydrocarbons larger than methane derive only from thermal degradation of fossil organic material.
During Leg 201 of the Ocean Drilling Program (ODP), the routine shipboard gas analysis resulted in a big surprise. When the sediment samples were treated with strong base, substantial amounts of ethane and propane were detected although the sampled sites were not in contact with fossil hydrocarbon reservoirs. Based on the evidence of carbon isotopic modeling and thermodynamic calculation, novel ethano- and propanogenic pathways involving acetate, bicarbonate and molecular hydrogen were proposed (Hinrichs et al., 2006).
This finding upsets the general belief in exploratory petroleum geochemistry that hydrocarbons larger than methane derive only from thermal degradation of fossil organic material.
In a following BMBF-funded project “METRO“, we further pursued the issue of sorbed hydrocarbon gas but turned our attention to methane. An extensive survey on a global set of marine sediment samples revealed the presence of substantial amounts of sorbed methane, which is largely of microbial origin based on carbon isotopic evidence (Ertefai et al., 2010). Results from high-pressure experiments suggested that variations in mineral composition are not controlling variations in quantities of sorbed methane.
Following the proposition of biological ethano- and propanogenesis, laboratory experiments have been designed and are being carried out to test these hypotheses.
The biological production of higher hydrocarbon gases on a technological scale would be highly interesting since these gases are more energy rich than methane and used as hydrocarbon fuels in the form of liquefied petroleum gas. We aim to study the biological formation of ethane and propane and the involved microorganisms in laboratory based experiments. Within the project BioFlüssigGas we have established an experimental set-up that allows us to simulate the high pressures and temperatures of subseafloor environments in a pressure-temperature block and to test different to investigate potential ethanogenic and propanogenic processes under different P-T conditions.
Following the proposition of biological ethano- and propanogenesis, laboratory experiments have been designed and are being carried out to test these hypotheses.
The biological production of higher hydrocarbon gases on a technological scale would be highly interesting since these gases are more energy rich than methane and used as hydrocarbon fuels in the form of liquefied petroleum gas. We aim to study the biological formation of ethane and propane and the involved microorganisms in laboratory based experiments. Within the project BioFlüssigGas we have established an experimental set-up that allows us to simulate the high pressures and temperatures of subseafloor environments in a pressure-temperature block and to test different to investigate potential ethanogenic and propanogenic processes under different P-T conditions.
Example 2: Biogeochemistry of volatile fatty acids
Volatile fatty acids (VFAs) are key compounds in anaerobic metabolism and in the cycling of carbon in marine sediments. Among the VFAs, acetate is particulare interesting. The water-soluble C2-compound is produced by fermentation of organic matter as well as by reduction of CO2 via the acetyl-CoA pathway (acetogenesis) and serves as an important substrate for a variety of microorganisms including sulfate reducing bacteria and methanogens. Stable isotopes provide a natural way to directly follow and trace details of carbon cycling, but a suitable technique to analyze the carbon isotopic composition of watersoluble metabolites has long been missing. We have overcome this problem with the development of a new analytical method to analyze the stable carbon isotope of volatile fatty acids (Heuer et al., 2006).
Following method development, we conducted incubation experiments to confirm our hypothesis that different processes in acetate production and conumption result in distinct isotopic compositions of the acetate pool (Heuer et al., 2010) and we employed the new technique to study the biogeochemistry of volatile fatty acids in the deep biosphere (Heuer et al., 2009; Lever et al., 2010).
Our experiments demonstrate that the stable carbon isotopic composition of acetate is a useful indicator for the relative importance of the different biogeochemical processes. In particular, we find (a) δ13C-values of acetate to closely resemble δ13C-values of total organic carbon (TOC) where fermentation is the dominant process, (b) a distinct 13C enrichment of acetate where a significant fraction of the acetate pool is consumed by acetoclastic methanogenesis, and (c) a distinct 13C depletion of acetate where high levels of H2 stimulate acetogenesis.
Outlook
The major goals of our next phase of biogeochemical studies include: (1) to improve the sensitivity of our methods and to develop new methods for analyzing environmental samples, particularly those from subseafloor sediment, (2) to explore the applicability of our existing analytical infrastructure for other trace, low-molecular-weight organic compounds, and (3) to continue investigating the mechanisms leading to production and consumption of these metabolites by laboratory experiments. The overall goal is to have a better understanding of the role of these organic compounds in the deep biosphere as well as in other natural systems.