Exploring the upper temperature limit and biosignatures of archaea and bacteria involved in anaerobic hydrocarbon degradation

The Guaymas Basin in the Gulf of California is a deep-sea hydrothermal vent area. There, the combination of high pressure and heat transforms complex organic compounds into natural gas and petroleum. These thermocatalytic processes resemble processes taking place in petroleum reservoirs that are deeply buried in the subsurface. Because of the easier accessibility on the seafloor, such processes can be examined much easier in the Guaymas Basin. In sediment close to the seafloor surface, the produced hydrocarbons become available to anaerobic microorganisms as energy and carbon sources. The aim of my dissertation was to culture novel hydrocarbon-degrading microorganisms from Guaymas Basin sediment which are active at high temperatures. I also examined the membrane lipids of alkane-degrading cultures to determine the influence of different factors on membrane lipid composition. In chapter 2 I examined the anaerobic degradation of mid-chain linear alkanes. I was able to enrich two archaea of the novel genus Candidatus Alkanophaga, which degrade alkanes between pentane (C5 alkane) and tetradecane (C14) at 70 °C. These archaea activate the alkanes with highly transcribed alkyl-coenzyme M reductases. They form syntrophic relationships with the previously unknown sulfate-reducing bacterium Ca. Thermodesulfobacterium syntrophicum. In chapter 3 I established anaerobic cultures, in which the unsubstituted small aromatic hydrocarbons benzene and naphthalene were degraded at 50 °C and 70 °C. In these cultures, different microorganisms became enriched with different substrate and temperature combinations. Of particular interest are two new species of the sulfate-reducing bacterial order Desulfatiglandales, which were highly abundant and dispose of almost complete metabolic pathways for the degradation of the aromatic hydrocarbons. In chapter 4 I studied the core membrane lipids of twelve anaerobic cultures consisting of alkane-oxidizing archaea and sulfate-reducing bacteria. The cultures grew between 20-70 °C. The lipid composition depended on the incubation temperature, the growth substrate, and the prevalent archaeon and bacterium. Several lipids with peculiar modifications were identified, and especially at 70 °C the lipid composition was considerably different compared to the compositions at other incubation temperatures. The enriched microorganisms might be involved in hydrocarbon degradation in heated petroleum reservoirs, and contribute to reservoir souring through sulfide formation. Especially the growth temperature of 70 °C approaches the upper limit of the temperature range in which hydrocarbon degradation in reservoirs was reported.