Organic-geochemical studies of microbial lipids and carbon flow in oxygen-deficient marine environments

Abstract

In order to get a better understanding of microbes role in marine element cycles, organic-geochemical studies targeting microbial lipids and metabolic products in oxygen-deficient oceanic water column and sediments were carried out. Intact polar membrane lipids (IPLs) as biomarker for living biomass have been established as a tool in microbial ecology and already successfully used in a variety of surface ecosystems. In the Eastern Tropical North Pacific Ocean (ETNP), the oxygen minimum zone (OMZ) presents between 100~800 m depths characterized by dissolved oxygen concentration of less than 20 µM. IPLs were predominant by eukaryotic and bacterial IPLs. Intact polar isoprenoid glycerol dialkyl glycerol tetraethers (IP GDGTs), the biomarker for living Archaea, were detected after purification of the total lipids extract (TLE) using preparative HPLC. Glycolipids which are mainly derived from photosynthetic membranes were dominant in the euphotic zone. With increasing depth, phospholipids and betaine lipids (BL) became dominant components in the OMZ and deep oxycline layers. In the surface layers where light and oxygen could penetrate, photosynthetic organisms, such as photosynthetic algae and cyanobacteria, produced abundant glycolipids. Glycolipids were decreased quickly with increasing depth probably due to remineralization. In the oxygen minimum zone, eukaryotic and bacterial organisms which could survive under oxygen limitation condition accumulated and produced abundant phospholipids and BL. Ratios between phosphorous-containing lipids and their corresponding non-phosphorous-containing substitute lipids, e.g., SQDG/PG and BL/PC (SQDG: sulfoquinovosyldiacylglycerol, PG; phosphatidylglycerol, PC: phosphatidylcholine) were high at depths where phosphate was abundant suggesting that not only phosphate limitation but also the microbial community inhabiting in the oceanic water impact enrichment of substitute lipids. Archaeal IP GDGTs peaked in the upper layers of the OMZ, which exhibited different from peaks of most glycerol ether core lipids (glycerol ether lipids without head groups representing fossil signal) in the deeper depths of the OMZ indicating that IP GDGTs represented an in-situ contribution from the planktonic archaeal community whereas core lipids were exported downward and accumulated in the mid OMZ with a longer residence time. After exported to the sediment, IPLs derived from the upper water column would either rapidly degrade or bury as fossil components. Degradation kinetics of IPLs could influence the interpretation of abundant observed archaeal IPLs in the deep biosphere. Based on a radiotracer experiment and a new comprehensive modeling work, half-life of model archaeal IPL increased with depth from 20 to 310 kyrs, which was relatively longer than the microbial community turnover times of 1.6 to 73 kyrs. It is suggested that a substantial fraction of the archaeal IPLs in marine sediments were fossil components of past microbial populations. Based on the observed IPL concentration and their degradation kinetics, the in-situ synthesis rates of archaeal IPL fell into a range of 1000 pg ml-1 yr-1 to 0.2 pg ml-1 yr-1 from surface to 1 km depth. Such a result is equivalent to the annual production of 7× 105 to 140 archaeal cells ml-1 sediment. Due to the high fossil proportion of archaeal IPLs of probably more than 80%, previously estimated subseafloor living biomass were probably too high. Therefore, the abundant archaeal IPL in subsurface sediments may not reflect a dominant archaeal community of deep biosphere. Ethane and propane as metabolic products of microorganisms are widely detected in the anoxic cold marine sediments. Through a test of several C-2 and C-3 compounds for their alkane-producing potential in anoxic Wadden Sea sediment, alkane production could be observed from ethylene, ethanethiol and propanethiol. Among these three substrates, ethylene had the maximum conversion efficiency for alkane production. Compared to the incubation with sterilized sediment, methanogens were involved in the alkane production. The initial H2 concentration required to stimulate ethanogenesis from ethylene was lower than 0.01% H2. After 80-days of incubation, an ethane-producing enrichment with ethylene as the substrate was used for molecular characterization. Methanocalculus and sequences belonging to the Methanomicrobiales were the dominant groups in the archaeal 16S rRNA gene library and the mcrA gene library, respectively. Methanocalculus is a candidate responsible for ethanogenesis from ethylene.