Carsten Schubert

Last Name

Schubert

First Name

Carsten

ORCID

0000-0003-1668-5967

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01709 - Lehre Umweltsystemwissenschaften

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Publications 1 - 10 of 31

Radiocarbon Stocktaking of Swiss Lakes: Relevance to Carbon Budgets and Dynamics
Mittelbach, Benedict V.A.; Blattmann, Thomas; Haghipour, Negar; et al. (2022)
Inland waters play a crucial role in the global carbon cycle, with organic carbon (OC) burial in lake sediments constituting a removal of carbon from rapidly cycling Earth surface pools. However, the nature of sequestered OC has different climatic implications. The burial of recently synthesized terrestrial and aquatic biospheric OC represents a drawdown of atmospheric carbon. In contrast, the reburial of petrogenic OC exerts no net effect on atmospheric CO2 levels, while oxidation of petrogenic carbon releases CO2. Therefore, it is crucial to distinguish the origin of OC when quantifying lake sediment carbon budgets. Radiocarbon (14C) is a powerful tool to distinguish between recent, pre-aged, and fossil carbon sources. Moreover, the 20th-Century radiocarbon “bomb spike” offers the possibility of constraining carbon dynamics and deconvolute inputs on decadal time scales through down-core investigation of lacustrine records including quantitatively disentangling different sources of OC, which exhibit contrasting susceptibilities to remineralization. Switzerland hosts various lake catchments within different biogeographical ecoregions, experiencing contrasting degrees of climate change. We use these natural gradients to assess controls on the abundance, sources, and dynamics of OC accumulating within Swiss lake sediments over the last century. We combine 14C and stable δ13C isotope signatures of bulk OC from sediment cores from a range of lake systems to constrain the nature and dynamics of OC accumulation. These data form the foundation for more in-depth investigations using 14C measurements on source-specific biomarkers to constrain the temporal dynamics and transport pathways of biospheric carbon and to refine carbon stocktaking assessments.
Hydrogen isotopes from sedimentary lipid biomarkers record changes in algal community assemblages
Ladd, Nemiah; Nelson, Daniel B.; Matthews, Blake; et al. (2022)
Goldschmidt 2022 Abstracts
A new lipid-based proxy for the reconstruction of past phytoplankton ecological dynamics
Klatt, Antonia; De Jonge, Cindy; Nelson, Daniel B.; et al. (2024)
EGUsphere
Methanogenesis by CO₂ reduction dominates lake sediments with different organic matter compositions
Su, Guangyi; Tolu, Julie; Glombitza, Clemens; et al. (2025)
Biogeosciences
Microbial methane production is a key reaction involved in the terminal step of anaerobic degradation of organic matter. The energy substrates of methane-producing microorganisms are largely generated during the breakdown of larger organic molecules by fermentative microorganisms, wherein the products of fermentation may vary with the chemical compositions of these larger molecules. Due to differences in energy substrates among methane-producing microorganisms, it is thus possible that organic matter compositional variations select for different communities of methane producers Here, we investigate the sources and compositions of OC in sediments of Lake Geneva and how both are potentially linked to methane production. Differences in dominant long-chain fatty acid abundances and carbon isotopic compositions suggest the predominance of diagenetically altered phytoplankton-derived OC at a profundal site (PS) and temporally highly variable sources of both aquatic and terrestrial OC in a deltaic location. Despite these differences, radiotracer-based methanogenesis rate measurements and stable isotopic signatures of methane indicate significant methane production that is dominated by CO₂ reduction (>95% of total methanogenesis) in both locations. Matching this interpretation, members of well-known CO₂-reducing Methanoregula sp. dominate both sites. Similarly, no clear effect of OC source on methane production rates was evident. Our data demonstrate that OC of diverse sources and diagenetic states supports microbial methane production, but the data do not indicate a clear impact of the OC source on the dominant methanogenic pathway or the community structure of methanogenic microorganisms in lacustrine sediments.
Hydrodynamic regimes modulate nitrogen fixation and the mode of diazotrophy in Lake Tanganyika
Ehrenfels, Benedikt; Baumann, Kathrin B.L.; Niederdorfer, Robert; et al. (2023)
Nature Communications
Long-term preservation of biomolecules in lake sediments: potential importance of physical shielding by recalcitrant cell walls
Han, Xingguo; Tolu, Julie; Deng, Longhui; et al. (2022)
PNAS Nexus
Even though lake sediments are globally important organic carbon (OC) sinks, the controls on long-term OC storage in these sediments are unclear. Using a multiproxy approach, we investigate changes in diatom, green algae, and vascular plant biomolecules in sedimentary records from the past centuries across five temperate lakes with different trophic histories. Despite past increases in the input and burial of OC in sediments of eutrophic lakes, biomolecule quantities in sediments of all lakes are primarily controlled by postburial microbial degradation over the time scales studied. We, moreover, observe major differences in biomolecule degradation patterns across diatoms, green algae, and vascular plants. Degradation rates of labile diatom DNA exceed those of chemically more resistant diatom lipids, suggesting that chemical reactivity mainly controls diatom biomolecule degradation rates in the lakes studied. By contrast, degradation rates of green algal and vascular plant DNA are significantly lower than those of diatom DNA, and in a similar range as corresponding, much less reactive lipid biomarkers and structural macromolecules, including lignin. We propose that physical shielding by degradation-resistant cell wall components, such as algaenan in green algae and lignin in vascular plants, contributes to the long-term preservation of labile biomolecules in both groups and significantly influences the long-term burial of OC in lake sediments.
Mineralization pathways of organic matter deposited in a river-lake transition of the Rhone River Delta, Lake Geneva
Randlett, Marie-Ève; Sollberger, Sébastien; Del Sontro, Tonya; et al. (2015)
Environmental Science: Processes & Impacts
During the éLEMO endeavour (a research project in which the Russian MIR submersibles were used for studying Lake Geneva) four sediment cores were retrieved on a transect from the delta of the Rhone River towards the profundal part of the lake. The degradation pathways of organic material (OM) were investigated considering different electron acceptors. Essentially, OM at the delta sites had a higher fraction of terrestrial material than the lake sites indicated by higher C/N ratios, and higher long-chain n-alkane and alcohol concentrations. The concentrations of chlorins were higher at the distant sites indicating more easily degradable OM in the sediments. However, the chlorin index that was used to determine the degradation state of the OM material indicated that pigment derived OM of deltaic sediments was less degraded than that of the profundal sediments. The fluxes of reduced species from the sediments decreased from the delta to the profundal for CH4 (from 2.3 to 0.5 mmol m−2 d−1) and NH4+ (from 0.31 to 0.13 mmol m−2 d−1). Fluxes of Fe(II) and Mn(II), however, increased although they were generally very low (between 9 × 10−5 and 7.6 × 10−3 mmol m−2 d−1). Oxygen concentration profiles in the pore waters revealed lower fluxes close to the river inflow with 4.3 and 4.1 mmol m−2 d−1 compared to two times higher fluxes at the profundal sites (8.8 and 8.2 mmol m−2 d−1). The rates for totally mineralized OM (Rtotal) at the shallower sites (4.7 mmol C m−2 d−1) were only half of those of the deeper sites (9.7 mmol C m−2 d−1). Accordingly, not only the rates but also the mineralization pathways differed between the shallow and profundal sites. Whereas only 0–6% of the OM was mineralized aerobically at the shallow sites (since almost all O2 was used to oxidize the large flux of CH4 from below) the situation was reversed at the deeper sites and the fraction of aerobically degraded OM was 72–78%. We found a better efficiency in CH4 production per carbon equivalent deposited at the deeper sites as a result of the higher degradability of the mainly autochthonous OM in spite of the lower deposition rate and the higher degradation state of the OM compared to the delta sites.
Persistent activity of aerobic methane-oxidizing bacteria in anoxic lake waters due to metabolic versatility
Schorn, Sina; Graf, Jon S.; Littmann, Sten; et al. (2024)
Nature Communications
Lacustrine methane emissions are strongly mitigated by aerobic methane-oxidizing bacteria (MOB) that are typically most active at the oxic-anoxic interface. Although oxygen is required by the MOB for the first step of methane oxidation, their occurrence in anoxic lake waters has raised the possibility that they are capable of oxidizing methane further anaerobically. Here, we investigate the activity and growth of MOB in Lake Zug, a permanently stratified freshwater lake. The rates of anaerobic methane oxidation in the anoxic hypolimnion reached up to 0.2 µM d−1. Single-cell nanoSIMS measurements, together with metagenomic and metatranscriptomic analyses, linked the measured rates to MOB of the order Methylococcales. Interestingly, their methane assimilation activity was similar under hypoxic and anoxic conditions. Our data suggest that these MOB use fermentation-based methanotrophy as well as denitrification under anoxic conditions, thus offering an explanation for their widespread presence in anoxic habitats such as stratified water columns. Thus, the methane sink capacity of anoxic basins may have been underestimated by not accounting for the anaerobic MOB activity.
Microbial Nitrogen Transformation Potential in Sediments of Two Contrasting Lakes Is Spatially Structured but Seasonally Stable
Saribekyan-Baumann, Kathrin; Thoma, Raoul; Callbeck, Cameron M.; et al. (2022)
mSphere
The nitrogen (N) cycle is of global importance, as N is an essential element and a limiting nutrient in terrestrial and aquatic ecosystems. Excessive anthropogenic N fertilizer usage threatens sensitive downstream aquatic ecosystems. Although freshwater lake sediments remove N through various microbially mediated processes, few studies have investigated the microbial communities involved. In an integrated biogeochemical and microbiological study on a eutrophic and oligotrophic lake, we estimated N removal rates from pore water concentration gradients in sediments. Simultaneously, the abundance of different microbial N transformation genes was investigated using metagenomics on a seasonal and spatial scale. We observed that contrasting nutrient concentrations in sediments were associated with distinct microbial community compositions and significant differences in abundances of various N transformation genes. For both characteristics, we observed a more pronounced spatial than seasonal variability within each lake. The eutrophic Lake Baldegg showed a higher denitrification potential with higher nosZ gene (N2O reductase) abundances and higher nirS:nirK (nitrite reductase) ratios, indicating a greater capacity for complete denitrification. Correspondingly, this lake had a higher N removal efficiency. The oligotrophic Lake Sarnen, in contrast, had a higher potential for nitrification. Specifically, it harbored a high abundance of Nitrospira, including some with the potential for comammox. Our results demonstrate that knowledge of the genomic N transformation potential is important for interpreting N process rates and understanding how the lacustrine sedimentary N cycle responds to variations in trophic conditions.
Algal lipid distributions and hydrogen isotope ratios reflect phytoplankton community dynamics
Klatt, Antonia; De Jonge, Cindy; Nelson, Daniel B.; et al. (2025)
Geochimica et Cosmochimica Acta
Reconstructions of past changes in algal community composition provide important context for future alterations in biogeochemical cycling. However, many existing phytoplankton proxies are indicative of individual algal groups and are not fully representative of the whole community. Here, we evaluated hydrogen isotope ratios of algal lipids (δ2HLipid) as a potential proxy for phytoplankton community composition. We sampled the water column of Rotsee, a small eutrophic lake in Switzerland, every second week from January 2019 to February 2020 and analyzed distributions and the relative offsets between δ2HLipid values (δ2HLipid1/Lipid2) from short-chain fatty acids, phytosterols and phytol. Comparing these data with phytoplankton cell counts, we found that δ2HC16:0 Acid/Sterol and δ2HSterol/Phytol values reflect shifts within the eukaryotic algal community. To assess whether the selected phytoplankton groups were the main sources of the selected lipids, we further modeled algal δ2HLipid1/Lipid2 values based on δ2HC16:0 Acid, δ2HSterol and δ2HPhytol values from batch cultures of individual algal groups and their biovolume in Rotsee and evaluated the role of heterotrophy on δ2HLipid1/Lipid2 values using a model incorporating δ2HC16:0 Acid and δ2HSterol values from microzooplankton. Annually-integrated and amount-weighted δ2HLipid1/Lipid2 values measured in Rotsee were within 2 to 20 ‰ of the mean of modeled algal δ2HLipid1/Lipid2 values, demonstrating a strong link with the phytoplankton community composition, while δ2HLipid1/Lipid2 values including microzooplankton lipids had a larger offset. Additionally, cyanobacterial biovolume was positively correlated with the ratio of phytol and phytosterols (phytol:sterol ratio) as well as the ratio of unsaturated C18 and C16:0 fatty acids (C18:C16 ratio). Our results support the application of sedimentary δ2HLipid1/Lipid2 values in eutrophic lakes as a proxy for past phytoplankton community assemblages. Moreover, the calculation of sedimentary phytol:sterol and C18:C16 ratios provides an additional proxy for reconstructing cyanobacterial blooms.

Publications 1 - 10 of 31

Carsten Schubert

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