Thiel, Volker
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Thiel, Volker
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Thiel, Volker
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Item type:Publication, Microbial activity in marine sediment constrained via lipid-based stable isotope probing(2018-01-30); ; ; Microbial-driven organic matter mineralization in marine sediment plays a vital role in the global carbon cycle. However, microbial activities and metabolisms in this environment are poorly constrained. This thesis aimed to quantitatively assess microbial activity by using a lipid-based stable isotope probing (SIP) method. Compared with 13C- or 15N-labelled organic substrates, dual labelling with D2O and 13C-inorganic carbon (IC) substrates serves to estimate in situ microbial activity with minimal distortion of in situ nutrient conditions. This approach provides an estimate of total microbial activity and can distinguish microbial metabolisms based on the ratio of incorporation of 13C-IC vs D2O (R13C/D) into microbial lipids (cf. Wegener et al., 2012). However, the current working hypothesis is based solely on the cultivation of the Deltaproteobacterium Desulfosarcina variabilis and thus poorly constrained. Therefore, additional empirical evidence is required for broad scale application of this proxy, e.g., to archaea. To extend the application of this dual SIP method, the methanogen archaeon Methanosarcina barkeri was incubated with D2O and 13C-IC under different conditions (H2/CO2, acetate and methanol; Chapter II). Incorporation of IC and water-derived H were dependent on the carbon substrates used for growth. The relatively low water H contribution ( 40%) to lipids yielded an increased R13C/D ratio that exceeded the assumed upper limit of 1 (cf. Wegener et al., 2012). This finding invokes more investigation of water H contribution to microbial lipids in order to quantitatively estimate microbial activity in environmental settings. By using the dual SIP method, this thesis further investigated the microbial activities and metabolisms in surface costal sediments under in situ sulfate-reducing conditions including an intertidal sediment from Janssand (Wadden Sea, Germany; Chapter III) and estuarine sediment from the Rhone River prodelta (Gulf of Lions, France; Chapter IV). Average turnover times of bacteria and archaea populations were 7.4 and 110 years, respectively. Compared with archaea, IC was significantly incorporated into bacterial fatty acids in surface sediments, with the assimilation rates ranging from 0.15 to 1.3 A g gdw-1 yr-1. Furthermore, R13C/D of bacterial fatty acids ranged from 0.07 to 0.23, indicating that heterotrophic bacteria were dominant and measured inorganic carbon fixation rates likely resulted from their anapleurotic reactions. To further explore the potential for IC assimilation among Archaea, a surface estuarine sediment (upper 10 cm) from Hangzhou Bay (East China Sea, China; Chapter V) was incubated anaerobically with different organic substrates (i.e., casein, oleic acids, cellulose, phenol and lignin) and 13C-IC. Only the addition of lignin stimulated the growth of Bathyarchaeota (formerly referred as Miscellaneous Crenarchaeotal Group, MCG) and resulted in 13C-labelling of their putative lipids. Together, these results suggest that Bathyarchaeota can use lignin as an energy source and IC as a carbon source. The demonstration of Bathyarchaeota metabolism of lignin sheds new light into microbial carbon cycling and important energy substrates for microbial populations in marine sediments. This thesis quantitatively assessed microbial activity and compared the difference between bacterial and archaeal activity in surface marine sediment via lipid-based SIP method. The findings suggest that dark inorganic carbon fixation may be an important consequence of heterotrophic microbial activity in marine sediment and should be considered as important role for constraining sedimentary C fluxes in the biogeochemical carbon cycle.doctoral thesis541 602 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Stable carbon isotopic studies of microbial lipids from distinct geochemical marine environments(2020-01-27); ; ; Sedimentary microbial lipids are widely used as sensitive indicators of environmental conditions in paleoenvironmental and biogeochemical studies. Nonetheless, systematic isotopic investigations of microbial lipids to constrain their sources, modes of production, and turnover remain fragmentary. In order to decipher the information encoded in sedimentary archaeal and bacterial lipids in different marine depositional environments, I conducted carbon isotopic analysis of marine environmental samples (Chapter III and IV) and laboratory-based stable isotope probing (SIP) experiments (Chapter V). Chapter III provides fundamental constraints on archaeal activity, sources of archaeal lipids, and preservation signatures in diverse marine sediments. This is, to my knowledge, the first systematic comparison of stable carbon isotopic compositions of archaeal core lipids (CLs) and intact polar lipids (IPLs). Results from lipid analysis are interpreted in the context of geochemical data in contrasting depositional regimes, including a transect from the Rhone River delta into the western Mediterranean Sea, the anoxic Black Sea, Marmara Sea, and sapropel layers in the Eastern Mediterranean Sea. Mass balance calculation along the transect of the Western Mediterranean Sea proved that terrestrial input of archaea into marine sediments can be substantial (up to 42%) and suggests caution when reconstructing such inputs based on existing molecular proxies, such as BIT (Branched isoprenoid tetraether index). Similar I 13C values of core and intact polar crenarchaeol strongly suggest that the alkyl moieties are not synthesized de novo, thus indicating that intact polar crenarchaeol is either a fossil relic from planktonic archaea or a product of lipid recycling by benthic archaea. By contrast, an average offset in I 13C values of 2.6a degree between core and intact polar caldarchaeol indicates active in-situ activity of benthic archaea. To further constrain the sources and turnover of sedimentary archaeal lipids, we compared the stable carbon isotopic compositions of archaeal IPLs and CLs through the oxic, suboxic and anoxic water column and in high resolution within an 8-m deep sediment core in the Black Sea (Chapter IV). The comparison between the water column and surface samples suggests that archaea residing in the lower suboxic zone are the main source of lipids found in surface sediments, including a large fraction of IPLs. The isotopic offset between core and intact caldarchaeol indicates sedimentary in-situ production by benthic archaea. Based on a two endmember mixing model, an average of 34% sedimentary intact polar caldarchaeol is likely produced by benthic archaea. Two independent isotope mass balance calculations suggested that on average 35% and 18% of CL-caldarchaeol are derived from IPL degradation in sediment. In addition to the analysis of environmental samples, laboratory-based SIP provides us another distinct perspective to understand the microbial-mediated processes in marine sediments, for example, anaerobic oxidation of methane (AOM). Their enrichment cultures have been increasingly purified but still contain additional community members, even after years of repeated dilution and inoculation. In order to investigate the potential heterotrophic activity of the microorganisms coexisting with AOM consortia, I performed an L-leucine-3-13C (13C-leu) labeling study on a mesophilic AOM enrichment culture (Chapter V). Our results showed that most 13C-Leu incorporation was observed in bacterial fatty acids, especially iso and anteiso-branched C15:0 and C17:0, but very limited in archaeal ether lipids. Interestingly, these incorporation patterns were independent of the addition of methane to the enrichments, suggesting the ancillary heterotopic bacteria (e.g. Anaerolineae and Spirochaetes) are the main producers of these 13C-labeled lipids rather than AOM consortia. These ancillary heterotopic bacteria probably thrive on the amino acids derived from AOM necromass and likely explain the common absence of AOM lipid signals in sedimentary records. This thesis highlights the presence of various archaeal lipid inputs from terrestrial, planktonic and sedimentary sources into marine sediments. New lipids are continuously produced and degraded by microorganisms in the active subseafloor biosphere. The incubation experiment suggests that heterotrophic bacteria may play a pivotal role in necromass turnover in marine sediments.doctoral thesis578 305