Gaseous hydrocarbon cycling and lipid biogeochemistry in cold and hot seep sediments

Abstract

The release of hydrocarbon gases at the seafloor is a rich energy source for chemosynthetic ecosystems, but can also impact the climate when entering the atmosphere. The overall objective of this thesis was to explore the formation and degradation pathways of hydrocarbon gases in marine sediments and to characterize the involved microorganism in these processes. Two contrasting seep systems were explored for this purpose: the cold, methane-laden US Atlantic Margin (USAM) sediments and the hot, methane and higher hydrocarbon impregnated hydrothermal sediments of the Guaymas Basin. The unusual isotopic relationships for C2-C5 hydrocarbon at the Guaymas Basin were inconsistent with thermogenic cracking of larger molecules as well as other known abiotic reactions as their major source. A novel formation pathway was proposed and thereby confirmed by isotope tracer experiments under hydrous pyrolysis conditions and by thermodynamic computations. This pathway involves abiotic reduction of volatile fatty acids, whose carboxyl groups are isotopically equilibrated with ambient dissolved inorganic carbon in the hot subsurface of the basin. Additionally, microbial regulation of methane and higher hydrocarbons in these two seep settings was characterized by means of intact polar lipid (IPL) biomarkers in parellel with geochemistry and gene-based analyses. At the USAM, the diversity and abundance of archaeal IPLs were closely linked to the in situ microbial community composition and was well-constrained by methane flux and organic carbon content. In the hydrothermal sediments of Guaymas Basin, distribution of microbial lipids was indicative of the existence of mesophilic and thermophilic methane and potential higher hydrocarbon degraders, and was mainly controlled by in situ temperature.