Speaker:

Dr. Gunter Wegener, Max Planck Institute for Marine Microbiology, Bremen, Germany MARUM, Center for Marine Environmental Sciences, Univ. Bremen, Germany

Abstract:

Microorganisms love hydrocarbons and they make use of these compounds under oxic and anoxic conditions. Aerobic hydrocarbon degraders and their mechanisms to activate and oxidize their substrates are well-known for long, yet in marine sediments most hydrocarbons are already oxidized in anoxic layers, coupled to sulfate reduction. Until recently, the anaerobic oxidation of saturated higher hydrocarbons (≥C3) was solely described for bacteria that are capable of substrate activation via alkyl-succinate formation, its complete oxidation, and couple it with sulfate reduction. Instead, methane, the most abundant hydrocarbon in the seafloor, is consumed by anaerobic methane-oxidizing archaea (ANME). They use the same enzymes as their methanogenic relatives but in a reverse order with methyl-coenzyme M reductase (MCR) as central element. To yield energy from these reactions, the reducing equivalents need to be efficiently removed. However ANME themselves cannot dispose the electrons themselves, but they team up with sulfate-reducing bacteria forming iconic microbial consortia. For long the necessary interaction mechanisms were not understood, which was due to their extremely slow in vitro growth. I will talk about microbial enrichment cultures that my team and I obtained from the gas-rich hydrothermal vent sediments of the Guaymas Basin. We yielded sediment-depleted microbial enrichment cultures that oxidize methane, butane and various other hydrocarbons. In the thermophilic AOM culture we found that ANME-1 forms consortia with a novel type of partner bacterium, the deep-branching Ca. Desulfofervidusauxilii. Both organisms are connected by myriads of nanowire-like structures that apparently allow direct electron transfer between the ANME and their partner bacteria. Ca. Desulfofervidusalso appeared in the other enrichments. For instance using butane as substrate consortia with the before undescribed Ca. Syntrophoarchaeum(GoMArc87 clade) were formed. In a so far unprecedented case these archaea use a novel, highly modified variant of methyl-CoMreductase for the activation of the C4-compound butane to butyryl-CoM. This compound is fully oxidized by Ca. S. butanivorans, yet not all steps in this oxidation are understood. Similar to ANME, Syntrophoarchaeumcannot perform sulfate reduction, hence it transfers electrons to its partner bacterium. We now find that the non-methane activating type of methyl-CoMreductases has many different variants – that allow the activation of various compounds. The discovered new groups of archaea have some things in common: They grow slowly, form consortia and require partner bacteria, they interact by forming pili-like connections and, to our experience, demand high temperatures to compete over bacteria that use the same substrates. I will introduce ways to retrieve those cultures, and their analysis using microscopy, -omics approaches, metabolite analysis and will end with further perspectives in the field of anaerobic hydrocarbon oxidation.