Atmospheric methane is contributing substantially to the present day global warming and likely has also been instrumental in past greenhouse periods. However, tools for constraining past methane concentrations in the atmosphere and the intensity of methane cycling are lacking. In this project we want to investigate and develop biomarker proxies for aerobic methane oxidizers (AMO), microorganisms that oxidise methane into CO2.
In particular we will focus on bacteriohopanepolyols (BHPs) as potential biomarkers because of their high structural diversity and potential selectivity for bacterial AMO. We will improve the currently used analytical method, which first derivatizes BHPs, to instead analyze intact BHPs directly using HPLC-high resolution mass spectrometry (HRAMS). The improved technology will be applied on biomass of a number of cultivated AMO to constrain the structural diversity in BHPs. In case of unknown or poorly characterized BHPs we will utilize prep-HPLC in combination with NMR to rigorously identify these compounds.
These analyses will be combined with stable carbon isotope analyses as methane oxidizers generally utilize 13C-depleted methane to build their biomass. However, one microbe on which we will also focus is Methylomirabilis oxyfera. M. oxyfera produces oxygen from the dismutation of nitrite in order to oxidize methane under anaerobic conditions. This intra-aerobic methane oxidizer potentially influences both the carbon and nitrogen cycles in natural environments. Initial screening of its BHPs and their isotopic composition shows that M. oxyfera are not depleted in 13C due to the utilization of CO2 for lipid production rather than methane.
We will further explore the lipid diversity of M. oxyfera, as well as those of other aerobic methane oxidizers, and screen for alternative lipid proxies other than BHPs, in particular in the high molecular weight region outside of the range of GC-MS (>800 Da) but inside the range of HPLC-HRAMS (>1000 to 3000). Newly developed (BHP) biomarkers for AMO will subsequently be applied to paleo-environments where methanotrophy is thought to have been an important process (e.g. periods of wetland expansion, gas hydrate destabilization zones).