Unchaining the biomarker potential of aerobic methanotrophic bacteria

Abstract

The aim of this project was to increase the knowledge on the lipid biomarker inventory of aerobic methanotrophic bacteria, regarding their lipid biomarker patterns and their stable carbon isotopic fingerprint, facilitating the recognition and interpretation of this important group of bacteria in marine and other highly alkaline and saline environments. To fulfil this objective, the biomarker distribution and compound-specific carbon isotopic composition of the strains Methylotuvimicrobium alcaliphilum and Methylotuvimicrobium kenyense were studied. Both strains can tolerate high alkalinity and salinity conditions similar to those typifying marine methane seeps. Their unique cyclic terpenoid inventory comprises 4-methyl steroids, 3-methyl- and desmethyl bacteriohopanepolyols (aminotetrol and aminotriol), and tetrahymanol, all of which are 13C-depleted. The average carbon isotope fractionation between methane and the respective lipid (Δδ13Cterpenoid-methane) is found to be –25‰ for M. kenyense and –16‰ for M. alcaliphilum. These data shed new light on the previously reported compound and carbon stable isotope patterns of cyclic terpenoids from methane-seep environments, particularly, 13C-depleted tetrahymanol and its degradation product gammacerane, which are reinterpreted as biomarkers of aerobic methanotrophic bacteria based on their occurrence in methane-seep deposits in association with other biomarkers of aerobic methanotrophs.

In culture experiments at varying conditions, the hopanoid abundance of M. alcaliphilum was increased at lower salinity and higher nitrate concentration. The production of pentacyclic triterpenoids, but especially aminotriol, was favored at low salinity. Interestingly, 3-methyl-aminotetrol and tetrahymanol were favored at higher salinity. Bacteriohopanepolyols (BHPs) increased considerably at higher nitrate concentrations, particularly aminotriol and 3-methyl-aminotriol. The changes on the hopanoid production of M. alcaliphilum under varying conditions highlights the significance of environmental factors for bacterial lipid production.

Aerobic methanotrophs have been considered to cause carbonate dissolution in natural environments. To understand the role of aerobic methanotrophic bacteria on the biocorrosion of carbonates, a carbonate corrosion experiment was performed at a modern marine methane seep (REGAB Pockmark, Congo area). Carbonate cubes exposed to active seepage showed visible traces of microbioerosion, visualized by epoxy casts. The corrosion traces are accompanied by unique and strongly 13C-depleted fatty acids, 4-methyl sterols and diplopterol on the corroded surface of the carbonates, most likely produced by aerobic methanotrophs. 16S rRNA analyses confirm the predominance of methanotrophs among bacteria and support aerobic methanotrophic bacteria as potential cause of biocorrosion. The outcome of the obtained data demonstrate that carbonate corrosion is potentially caused by aerobic methanotrophic bacteria, supporting the hypothesis that aerobic methanotrophy can be a trigger for carbonate biocorrosion at marine methane seeps.