Fluid venting structures of terrestrial mud volcanoes (Italy) and marine cold seeps (Black Sea) -Organo-geochemical and biological approaches

Abstract

Two different fluid venting structures, marine cold seeps of the Black Sea and terrestrial mud volcanoes of Italy, were part of this thesis. Both were formed by the expulsion of water, mud and gases, which consist mainly of methane and higher hydrocarbons. The methane gas acts as substrate for various microorganisms, which perform, amongst other, the anaerobic oxidation of methane (AOM). The AOM is one key process of the methane consumption in the oceans worldwide. According to phylogenetic and metagenomic analyses, the AOM is mediated by consortia of anaerobic methane oxidizing archaea (ANME) and sulfate reducing bacteria (SRB). To get a deeper insight into the methanotrophic consortia, microbial mats of the Black Sea were used to assign one of the specific key enzymes of the methanogenesis to the AOM-performing microorganism. By using a specific antibody as marker, the metabolic activity of one part of the syntrophic partners could be identified. The key enzyme methyl coenzyme M reductase (MCR) of the (reverse) methanogenic pathway was detected on cellular and sub-cellular level in the ANME cells. The study confirms the assumption of the reversed methanogenic pathway for the anaerobic oxidation of methane. Furthermore, trace element concentrations were measured in the microbial mats and the methane derived carbonates of the Black Sea to test if nickel could be a geochemical indicator for the anaerobic oxidation of methane or for methanogenesis. Nickel is part of the MCR cofactor F430 and the iron sulfide greigite (Fe3S4), which can be found in the microbial mats of the Black Sea. The results have shown that Ni together with stable carbon isotopic ratios could act as a geochemical tracer for methanogenesis or the anaerobic oxidation of methane in both recent and fossil environments. The second part of my thesis deals with the terrestrial mud volcanoes in Italy. Fluids expelled from this type of fluid venting structures were analyzed organo-geochemically (lipid biomarkers) and geochemically (composition of water and gas). In general, mud volcanoes also release a three phase mixture of gas (mainly methane and higher hydrocarbons), water and sediment particles from a source that is often associated with an active petroleum reservoir. Organo-chemical analyzes of the expelled fluids revealed that the biological signals are superimposed by higher hydrocarbons originating from associated active petroleum reservoirs and the underlying organic-rich geological formations. However, signals of various eukaryal, bacterial and archaeal organisms in the mud volcanoes were also found. In addition to signals from higher plants, most likely originated from the surrounding flora and soils, specific bacterial dialkyl glycerol diethers (DAGE) were found which are putatively sourced by sulfate-reducing bacteria (SRB). The presence of archaea is evidenced by archaeol and hydroxyarchaeol. The latter is indicative for the anaerobic oxidation of methane. The results have shown that the complexity of microbial communities in the different mud volcanoes is very high and that some of the microorganisms were involved in the anaerobic turnover of methane. Furthermore, analyzes of sediments from the underlying geological formations have shown that most of the biomarker signals found in the fluids derived from the sediments. Therefore, mud volcanoes could act as a window in the deep bio- and geosphere as well as into in situ microbial processes.