Microbial community characterization and carbon turnover in methane-rich environments - case studies in the Gulf of Mexico and the Black Sea

Abstract

This thesis investigated patterns in the distribution of intact polar membrane lipids (IPLs) in the marine environment. IPL analysis is a relatively new tool in microbial ecology to study (i) live microbial biomass and (ii) the dominating microbial players. This technique was applied for the first time to study the oxic and anoxic water column of the Black Sea and observed a stratification of IPLs according to geochemical zonation. Export of IPLs to the sediment was found to be selective and the distribution of IPLs in the upper 2 cm of the sediments reflects de novo production of IPLs by indigenous microbes, putatively identified as sulfate-reducing bacteria and benthic archaea. The distribution of archaeal IPLs in the anoxic water body did not indicate the abundant presence of methanotrophic archaea, which were presumed in earlier studies due to high estimated methane oxidation rates. The presence of betaine lipids and glycosidic sphingolipids in the anoxic water column could be linked to unknown anaerobic bacteria and is a novel finding as these lipids are primarily known to be produced by eukaryotes.Investigations of microbial communities associated to the Chapopote asphalt seep in the southern Gulf of Mexico revealed the presence of a diverse array of IPLs from both Bacteria and Archaea. IPL concentrations in the sediments were correlated with the abundant presence of oil and methane, indicating that the petroleum hydrocarbons are a major stimulant for microbial activity. Bacterial IPL concentrations decreased with decreasing sulfate concentrations over depth, whereas archaeal IPLs increased simultaneously and comprised up to 80% of total IPLs at ca. 15 cm sediment depth. Bacterial lipids mainly included phospholipids with the polar head groups phosphatidylethanolamine (PE), phosphatidyl-(N)-methylethanolamine (PME), and phosphatidylglycerol (PG). The assignment of these IPLs to sulfate-reducing bacteria (SRB) was confirmed by the presence of SRB-characteristic fatty acids. Polar head group-specific isotope analysis of the SRB core lipids revealed that the majority of the SRB population is autotrophic and involved in the anaerobic oxidation of methane. However, a large amount of SRB are heterotrophic hydrocarbon-degrading bacteria. The oil-degrading bacteria mainly contained PME as head group. Archaeal IPLs indicated the presence of ANME-1 archaea comprised of diglycosidic glyceroldibiphytanylglyceroltetraethers (2Gly-GDGT) accompanied by ANME-2 archaea suggested by phosphate-based hydroxyarchaeols. Polar head group-specific stable carbon isotope analysis of the archaeal IPLs confirmed the association of those lipids to methanotrophic archaea and could also show that phosphate-based archaeols and GDGTs with mixed glycosidic and phosphate-based head groups were mainly derived from methanogenic archaea. In subsurface sediments of the oil-influenced Chapopote asphalt seep abundant archaeal IPLs were detected close to a sulfate-methane transition zone. Here, bacterial lipids were only a minor part of the total IPLs and were dominated by diether lipids with PE headgroups. Phosphate-based hydroxyarchaeols and diglycosidic GDGTs could be assigned to both methanogenic and methanotrophic sources. Methane is thus a major intermediate in microbial metabolism at the Chapopote asphalt volcano. Investigations of biological and physical weathering of the deposited asphalts showedthat the asphalts are an important substrate for the microbial community. The removal of n-alkanes, branched alkanes, isoprenoids and low molecular weight polyaromatic hydrocarbon compounds could be primarily assigned to biodegradation. Biomarkers such as steranes and hopanes were most recalcitrant and were still observed in highly weathered brittle asphalts. Comparison of fresh and weathered asphalts allowed to estimate total petroleum hydrocarbon losses into the environment. Assessment of the potential of total hydrocarbon emission from the Chapopote asphalt seep amounts to up to 1,540 Ã ± 770 tons. However, there is indication that a large fraction of these hydrocarbons are already efficiently recycled by the indigenous microorganisms associated to the asphalts.