Alum­ni se­mi­nar

Ma­xi­mi­li­an Vah­len­kamp

A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the orbital pacing of climate

International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for the Eocene astrochronology. A 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity is supplemented with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM-ca. 56 Ma) into the middle Eocene (ca. 40 Ma) and revealing the orbital phasing at the PETM.

Ste­fan Be­cker

Abundance and dynamics of laminarin - a key molecule in the marine carbon cycle

Marine algae sequester as much CO₂ into carbohydrate as terrestrial plants, yet the polysaccharide composition of the surface ocean remains unknown due to analytic challenges. Here we used a biocatalytic assay to quantify algal laminarin in which bacterial enzymes specifically cleave this polysaccharide into analyzable fragments. We measured laminarin in the Arctic, North-, Central- and South-Atlantic, coastal Pacific and in two North Sea time series and found that it accounts on average for 37±17% of the carbon in algae derived, particulate organic matter. The correlation between laminarin and chlorophyll suggests that 18±9 gigatons of laminarin are produced annually. Moreover, laminarin accounted for up to 50% of the carbon in sinking, diatom containing particles and thus contributed significantly to carbon export from surface waters. Spatially and temporally variable laminarin concentrations of up to 80% of algal particulate organic carbon in the sunlit ocean were driven by light availability. We were able to measure the first laminarin diel cycle of microalgae in nature, which suggests that laminarin is accumulated by photosynthetic activity over the day and consumed in low light conditions over night. The overall abundance of laminarin and its variations indicate that the molecule plays a key role for carbon export and energy flow to higher trophic species and point to an important role in marine ecosystems.

Rafael Laso-Pérez

Unravelling the molecular basis of the anaerobic degradation of hydrocarbons in archaea

Crude oil and natural gas are formed due to the degradation of the organic matter in deep subsurface layers. From there, they can migrate towards the sediment surface, where they represent an energy source for microbial communities. Anaerobic methanotrophic archaea (ANME) are the main responsible of anaerobic oxidation of methane in anoxic environments. For methane oxidation, they use a reversal of the methanogenesis, whose key enzyme is the methyl-coenzyme M reductase (MCR). However, little is known about the role of other archaea in the anaerobic oxidation of other hydrocarbons. Apart from ANME, only three archaea are described as anaerobic hydrocarbon degraders, although the three use mechanisms of bacterial origin. The aim of my PhD was to shed light on the role of archaea in anaerobic hydrocarbon degradation. I would present the results of my first Chapter during the talk, where I described a new pathway to degrade butane and propane in two archaeal strains of the newly described clade Candidatus Syntrophoarchaeum, which we cultivated in consortia with syntrophic partner bacteria. Using meta-omics approaches, we discovered that Ca. Syntrophoarchaeum use before uncharacterized divergent MCRs to activate butane and propane. The produced alkyl-CoM units are then fully oxidized using a combination of different pathways and the reducing equivalents are transferred to the partner bacteria for sulfate reduction. The discovery of this novel MCRs is the foundation of a novel pathway in archaea to degrade hydrocarbons that is present in different archaeal clades and might play a role in carbon cycling in deep reservoirs.