Living on gas – microbial degradation of gaseous alkanes, from mud to genes

The gaseous, short-chain alkanes (SCA) ethane, propane and butanes are constituents of natural gas and are also found in petroleum and petroleum-related gas. These hydrocarbons are released into the biosphere mainly through natural seepage of gas and crude oil in both terrestrial and marine environments, and as a result of anthropogenic activities. Other relevant sources are biogenic production (plants, oceans), and burning of biomass. Recent global estimates indicate that emissions of ethane and propane from all known sources are as high as ca. 10 Tg per year each. The SCA entering the atmosphere contribute to photochemical pollution and production of ozone. The release of SCA to the atmosphere is controlled by the microbial degradation of the gaseous alkanes, which occurs under both aerobic and anaerobic conditions. Aerobic biodegradation of SCA occurs in oxic water columns and in oxygenated surface sediment layers. Recent studies of the massive oil and gas spill from the Deepwater Horizon event showed ethane and propane-degrading microorganisms as the main hydrocarbon degraders, with a massive impact on the oxygen consumption in the water column associated with hydrocarbon plumes. The anaerobic biodegradation of SCA is an emerging research field. The first evidence of SCA degradation in situ was provided by geochemical studies of anoxic deep-sea sediments, including the enrichment in 13C of propane and butane in interstitial waters atop gas hydrates or at mud volcanoes, the uncoupling of sulfate reduction and anaerobic oxidation of methane rates in hydrothermal sediments or at cold seeps, the apparent consumption of SCA at mud volcanoes. Anaerobic microorganisms able to degrade SCA under anoxic, sulfate-reducing conditions were cultivated relatively recently from marine or terrestrial sediments associated with hydrocarbon seepage.

Further research is required to unravel to full potential of gaseous alkane-degrading microorganisms. What is the extent of SCA degradation in various environments affected by hydrocarbon seepage? What is the phylogenetic and functional diversity of both aerobic and anaerobic SCA-degrading microorganisms? To what extent do the gaseous alkane-degrades influence the elemental cycles (e.g. carbon, sulfur) in hydrocarbon-impacted environments? In addition, future (meta)genomic and (meta)proteomic analyses could shed light on the metabolic potential and could help in defining the ecological niches of SCA-degrading microoganisms. With the present research topic we aim to bring together several lines of research into the microbial degradation of gaseous alkanes: (1) in situ qualitative and/or quantitative evidence for microbial SCA degradation, (2) diversity of SCA degraders in oxic and anoxic, terrestrial or marine environments, including diversity of functional genes involved in the alkane metabolism, (3) physiology and phylogeny studies of novel pure or enriched cultures growing with gaseous alkanes, (4) functional genomic/metagenomic analyses of SCA degraders, and (5) further advances in the biochemistry of SCA degradation.