A carbonate corrosion experiment at a marine methane seep: The role of aerobic methanotrophic bacteria.

Methane seeps are typified by the formation of authigenic carbonates, many of which exhibit corrosion surfaces and secondary porosity believed to be caused by microbial carbonate dissolution. Aerobic methane oxidation and sulfur oxidation are two processes capable of inducing carbonate corrosion at methane seeps. Although the potential of aerobic methanotrophy to dissolve carbonate was confirmed in laboratory experiments, this process has not been studied in the environment to date. Here, we report on a carbonate corrosion experiment carried out in the REGAB Pockmark, Gabon-Congo-Angola passive margin, in which marble cubes were deployed for 2.5 years at two sites (CAB-B and CAB-C) with apparent active methane seepage and one site (CAB-D) without methane seepage. Marble cubes exposed to active seepage (experiment CAB-C) were found to be affected by a new type of microbioerosion. Based on 16S rRNA gene analysis, the biofilms adhering to the bioeroded marble mostly consisted of aerobic methanotrophic bacteria, predominantly belonging to the uncultured Hyd24-01 clade. The presence of abundant 13 C-depleted lipid biomarkers including fatty acids (n-C16:1ω8c , n-C18:1ω8c , n-C16:1ω5t ), various 4-mono- and 4,4-dimethyl sterols, and diplopterol agrees with the dominance of aerobic methanotrophs in the CAB-C biofilms. Among the lipids of aerobic methanotrophs, the uncommon 4α-methylcholest-8(14)-en-3ß,25-diol is interpreted to be a specific biomarker for the Hyd24-01 clade. The combination of textural, genetic, and organic geochemical evidence suggests that aerobic methanotrophs are the main drivers of carbonate dissolution observed in the CAB-C experiment at the REGAB pockmark.

Variation of salinity and nitrogen concentration affects the pentacyclic triterpenoid inventory of the haloalkaliphilic aerobic methanotrophic bacterium Methylotuvimicrobium alcaliphilum.

The occurrence and activity of aerobic methanotrophs are influenced by environmental conditions, including pH, temperature, salinity, methane and oxygen concentrations, and nutrient availability. Aerobic methanotrophs synthesize a variety of lipids important for cell functions. However, culture-based experiments studying the influence of environmental parameters on lipid production by aerobic methanotrophs are scarce. Such information is crucial to interpret lipid patterns of methanotrophic bacteria in the environment. In this study, the alkaliphilic strain Methylotuvimicrobium alcaliphilum was cultivated under different salinities and different nitrate concentrations to assess the effect of changing conditions on the inventory of pentacyclic triterpenoids. The results indicate that hopanoid abundance is enhanced at lower salinity and higher nitrate concentration. The production of most pentacyclic triterpenoids was favored at low salinity, especially for aminotriol. Interestingly, 3-methyl-aminotetrol and tetrahymanol were favored at higher salinity. Bacteriohopanepolyols (BHPs), particularly aminotriol and 3-methyl-aminotriol, increased considerably at higher nitrate concentrations. Four novel N-containing BHPs-aminodiol, 3-methyl-aminodiol, and isomers of aminotriol and 3-methyl-aminotriol-were identified. This study highlights the significance of environmental factors for bacterial lipid production and documents the need for cultivation studies under variable conditions to utilize the full potential of the biomarker concept.