Klaus-Holger Knorr

Sulfate-Reducing Microorganisms in Wetlands – Fameless Actors in Carbon Cycling and Climate Change

Freshwater wetlands are a major source of the greenhouse gas methane but at the same time can function as carbon sink. Their response to global warming and environmental pollution is one of the largest unknowns in the upcoming decades to centuries. In this review, we highlight the role of sulfate-reducing microorganisms (SRM) in the intertwined element cycles of wetlands. Although regarded primarily as methanogenic environments, biogeochemical studies have revealed a previously hidden sulfur cycle in wetlands that can sustain rapid renewal of the small standing pools of sulfate. Thus, dissimilatory sulfate reduction, which frequently occurs at rates comparable to marine surface sediments, can contribute up to 36–50% to anaerobic carbon mineralization in these ecosystems. Since sulfate reduction is thermodynamically favored relative to fermentative processes and methanogenesis, it effectively decreases gross methane production thereby mitigating the flux of methane to the atmosphere. However, very little is known about wetland SRM. Molecular analyses using dsrAB [encoding subunit A and B of the dissimilatory (bi)sulfite reductase] as marker genes demonstrated that members of novel phylogenetic lineages, which are unrelated to recognized SRM, dominate dsrAB richness and, if tested, are also abundant among the dsrAB-containing wetland microbiota. These discoveries point toward the existence of so far unknown SRM that are an important part of the autochthonous wetland microbiota. In addition to these numerically dominant microorganisms, a recent stable isotope probing study of SRM in a German peatland indicated that rare biosphere members might be highly active in situ and have a considerable stake in wetland sulfate reduction. The hidden sulfur cycle in wetlands and the fact that wetland SRM are not well represented by described SRM species explains their so far neglected role as important actors in carbon cycling and climate change.

Isotopic evidence for condensed aromatics from non‐pyrogenic sources in soils – implications for current methods for quantifying soil black carbon

Black carbon (BC) is a complex continuum of partly charred organic matter predominantly consisting of condensed aromatic and graphitic moieties and it has high potential for long-term carbon sequestration in soils and sediments. There has been common agreement that BC is exclusively formed by incomplete combustion of organic matter, while non-pyrogenic sources are negligible. In this study, we investigated the stable carbon isotope signature of benzenepolycarboxylic acids (BPCAs) as molecular markers for BC to test if there is also a significant contribution of non-pyrogenic carbon to this fraction in soils. BPCAs were formed by hot nitric acid oxidation of different soils and analyzed by three different procedures: (i) elemental analysis - isotope ratio mass spectrometry (EA-IRMS) of bulk BPCAs and gas chromatography - combustion - isotope ratio mass spectrometry (GC-C-IRMS) of (ii) BPCA trimethylsilyl (TMS) derivatives, and (iii) BPCA methyl derivatives. Best accuracy and precision of isotope measurements were obtained by EA-IRMS of bulk BPCAs although this method has a risk of contamination by non-BC-derived compounds. The accuracy and precision of GC-C-IRMS measurements were superior for methyl derivatives (+/-0.1 per thousand and 0.5 per thousand, respectively) to those for TMS derivatives (+3.5 per thousand and 2.2 per thousand, respectively). Comparison of BPCA delta(13)C values of soil samples prior to and after laboratory and field incubations with both positive and negative (13)C labels at natural and artificial abundances revealed that up to 25% of the isolated BC fraction in soils had been produced in situ, without fire or charring. Commonly applied methods to quantify BC exclusively formed by pyrogenic processes may thus be biased by a significant non-pyrogenic fraction. Further research is encouraged to better define isolated BC fractions and/or understand mechanisms for non-pyrogenic BC production in soils.

N2O concentration and isotope signature along profiles provide deeper insight into the fate of N2O in soils

Nitrous oxide is an important greenhouse gas and its origin and fate are thus of broad interest. Most studies on emissions of nitrous oxide from soils focused on fluxes between soil and atmosphere and hence represent an integration of physical and biological processes at different depths of a soil profile. Analysis of N2O concentration and isotope signature along soil profiles was suggested to improve the localisation of sources and sinks in soils as well as underlying processes and could therefore extend our knowledge on processes affecting surface N2O fluxes. Such a mechanistic understanding would be desirable to improve N2O mitigation strategies and global N2O budgets. To investigate N2O dynamics within soil profiles of two contrasting (semi)natural ecosystem types (a temperate acidic fen and a Norway spruce forest), soil gas samplers were constructed to meet the different requirements of a water-saturated and an unsaturated soil, respectively. The samplers were installed in three replicates and allowed soil gas sampling from six different soil depths. We analysed soil air for N2O concentration and isotope composition and calculated N2O net turnover using a mass balance approach and considering diffusive fluxes. At the fen site, N2O was mainly produced in 30–50 cm soil depth. Diffusion to adjacent layers above and below indicated N2O consumption. Values of δ15N and δ18O of N2O in the fen soil were always linearly correlated and their qualitative changes within the profile corresponded with the calculated turnover processes, suggesting further reduction of N2O. In the spruce forest, highest N2O production occurred in the topsoil, but there was also notable production occurring in the subsoil at a depth of 70 cm. Changes in N2O isotope composition as to be expected from local production and consumption processes within the soil profile did hardly occur, though. This was presumably caused by high diffusive fluxes and comparatively low net turnover, as isotope signatures approached values measured for ambient N2O towards the topsoil. Our results demonstrate a highly variable influence of diffusive versus production/consumption processes on N2O concentration and isotope composition, depending on the type of ecosystem. This finding indicates the necessity of further N2O concentration and isotope profile investigations in different types of natural and anthropogenic ecosystems in order to generalise our mechanistic understanding of N2O exchange between soil and atmosphere. Revised version of a paper presented at the 30th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 8–10 October 2007, Bayreuth, Germany.

Consortia of low-abundance bacteria drive sulfate reduction-dependent degradation of fermentation products in peat soil microcosms.

Dissimilatory sulfate reduction in peatlands is sustained by a cryptic sulfur cycle and effectively competes with methanogenic degradation pathways. In a series of peat soil microcosms incubated over 50 days, we identified bacterial consortia that responded to small, periodic additions of individual fermentation products (formate, acetate, propionate, lactate or butyrate) in the presence or absence of sulfate. Under sulfate supplementation, net sulfate turnover (ST) steadily increased to 16–174 nmol cm–3 per day and almost completely blocked methanogenesis. 16S rRNA gene and cDNA amplicon sequencing identified microorganisms whose increases in ribosome numbers strongly correlated to ST. Natively abundant (⩾0.1% estimated genome abundance) species-level operational taxonomic units (OTUs) showed no significant response to sulfate. In contrast, low-abundance OTUs responded significantly to sulfate in incubations with propionate, lactate and butyrate. These OTUs included members of recognized sulfate-reducing taxa (Desulfosporosinus, Desulfopila, Desulfomonile, Desulfovibrio) and also members of taxa that are either yet unknown sulfate reducers or metabolic interaction partners thereof. Most responsive OTUs markedly increased their ribosome content but only weakly increased in abundance. Responsive Desulfosporosinus OTUs even maintained a constantly low population size throughout 50 days, which suggests a novel strategy of rare biosphere members to display activity. Interestingly, two OTUs of the non-sulfate-reducing genus Telmatospirillum (Alphaproteobacteria) showed strongly contrasting preferences towards sulfate in butyrate-amended microcosms, corroborating that closely related microorganisms are not necessarily ecologically coherent. We show that diverse consortia of low-abundance microorganisms can perform peat soil sulfate reduction, a process that exerts control on methane production in these climate-relevant ecosystems.

Electron Transfer Budgets and Kinetics of Abiotic Oxidation and Incorporation of Aqueous Sulfide by Dissolved Organic Matter

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227-0.325 h(-1). The main transformation products were elemental S (S0) and thiosulfate (S2O3(2-)), yielding electron accepting capacities of 2.82-1.75 μmol e- (mg C)(-1). Native iron contents in the HA could account for only 6-9% of this electron transfer. About 22-37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.

Investigating speciation and toxicity of heavy metals in anoxic marine sediments—a case study from a mariculture bay in Southern China

PurposeThe pollution of marine sediments by heavy metals is still a major concern, especially in zones affected by industry or mariculture. Toxicity of sediment heavy metal contents may be assessed using sequential extraction (SE) procedures, minding inherent constraints of such approaches. In this study, we investigated heavy metal speciation and toxicity in anoxic marine sediments in Zhelin Bay, a mariculture bay in Southern China, using an SE and acid volatile sulfur-simultaneously extracted metals (AVS-SEM) approach.Materials and methodsSpeciation of Cd, Cu, Ni, Pb, and Zn were studied by a modified SE of five fractions, adapted to separate organic and sulfidic metal fractions in anoxic sediments: F1 weak acid soluble (readily available), F2 reducible fraction, F3 organic matter-bound fraction, F4 sulfide-bound fraction, and F5 residually bound fraction. Toxicity predictions based on the sum of non-residual (NR) metal fractions from sequential extraction were compared to predictions based on AVS-SEM.Results and discussionResults showed that Cd, Ni, and Pb predominantly occurred in the weak acid soluble fraction (F1), residual fraction (F5), and sulfide-bound fraction (F4), respectively; Cu and Zn were mainly obtained in F4 and F5. Based on the distribution of indicator elements for metal fractions, the SEM from AVS extraction included different yields of non-residual and residual fractions besides the sulfidic fraction. Estimates for potential heavy metal toxicity based on NR metals of the SE procedure were thus based on a better-defined speciation compared to the simplistic approach of the AVS-SEM method.ConclusionsBased on the contents of NR metals and normalizing them by organic matter content, toxic effects are not expected for any of the sampling sites, irrespective of the presence or absence of mariculture. Using Pearson correlation analysis to identify predominant fractions influencing toxicity, we conclude that toxicity of heavy metals in anoxic sediments can be well predicted by their non-residual heavy metal contents.

Enhanced silicon availability leads to increased methane production, nutrient and toxicant mobility in peatlands

Peatlands perform important ecosystem functions, such as carbon storage and nutrient retention, which are affected, among other factors, by vegetation and peat decomposition. The availability of silicon (Si) in peatlands differs strongly, ranging from <1 to >25 mg L−1. Since decomposition of organic material was recently shown to be accelerated by Si, the aim of this study was to examine how Si influences decomposition of carbon and nutrient and toxicant mobilization in peatlands. We selected a fen site in Northern Bavaria with naturally bioavailable Si pore water concentrations of 5 mg/L and conducted a Si addition experiment. At a fourfold higher Si availability, dissolved organic carbon, carbon dioxide, and methane concentrations increased significantly. Furthermore, dissolved nitrogen, phosphorus, iron, manganese, cobalt, zinc, and arsenic concentrations were significantly higher under high Si availability. This enhanced mobilization may result from Si competing for binding sites but also from stronger reducing conditions, caused by accelerated respiration. The stronger reducing conditions also increased reduction of arsenate to arsenite and thus the mobility of this toxicant. Hence, higher Si availability is suggested to decrease carbon storage and increase nutrient and toxicant mobility in peatland ecosystems.

Potential effects of sediment processes on water quality of an artificial reservoir in the Asian monsoon region

Abstract Sediment processes in lakes may affect water chemistry through the internal loading of phosphorus, ammonia, and sulfides released under anoxic conditions. Lake Soyang is a deep warm monomictic reservoir with a dendritic shape, located in the Asian summer monsoon region, South Korea. During summer, the lake is stratified and receives a large nutrient input via storm runoff, which forms a turbid intermediate layer with high concentrations of suspended particles. The lake water, the main inflowing stream (the Soyang River), bottom sediment, and porewater of the lake sediments were studied over a 2-year period (2012–2013). After intensive monsoon rain events, particulate organic carbon (POC), total phosphorus (TP), and turbidity were high in the inflowing water (C: 1.21 mg L−1 in June 2013) and in the metalimnion (2.8 mg L−1, 17.6 μg L−1, and 58.5 NTU, respectively in July 2013). Higher concentrations of iron (Fe) and manganese (Mn) were also associated with the turbid intermediate layer (37 and 8 μg L−1, respectively, in July 2013). During the summer stratification period, oxygen started to deplete in the hypoliminion (down to 0.5 mg L−1 in September 2013), and sediment became anoxic, showing negative oxidation redox potential (ORP) in core samples. Diffusion of dissolved inorganic P and ammonia from sediment to the water column can be substantial, considering the concentration difference between the porewater and hypolimnetic water. Fe and Mn were abundant in the sediment porewater at the dam site, implying inorganic nutrients and minerals are well transported along the 60 km long lake axis by the density current of storm runoff. Sulfate and reduced sulfur were larger in the porewater of the top sediment than in the lower layer of the sediment core (below 10 cm). The results show that substantial amounts of inorganic nutrients and minerals are supplied to the lake by storm runoffs during monsoon and distributed through the lake by a density current, controlling the material cycle and flux at the sedimen tsurface.

Organic sulfur and organic matter redox processes contribute to electron flow in anoxic incubations of peat

Environmental context The extent to which organic matter decomposition generates carbon dioxide or methane in anaerobic ecosystems is determined by the presence or absence of particular electron acceptors. Evaluating carbon dioxide and methane production in anaerobic incubation of peat, we found that organic matter predominated as an electron acceptor over considered inorganic electron acceptors. We also observed changes in organic sulfur speciation suggesting a contribution of organic sulfur species to the electron-accepting capacity of organic matter. Abstract An often observed excess of CO2 production over CH4 production in freshwater ecosystems presumably results from a direct or indirect role of organic matter (OM) as electron acceptor, possibly supported by a cycling of oxidised and reduced sulfur species. To confirm the role of OM electron-accepting capacities (EACOM) in anaerobic microbial respiration and to elucidate internal sulfur cycling, peat soil virtually devoid of inorganic electron acceptors was incubated under anaerobic conditions. Thereby, production of CO2 and CH4 at a cumulative ratio of 3.2:1 was observed. From excess CO2 production and assuming a nominal oxidation state of carbon in OM of zero, we calculated a net consumption rate of EACOM of 2.36µmol electron (e–)cm–3day–1. Addition of sulfate (SO42–) increased CO2 and suppressed CH4 production. Moreover, subtracting the EAC provided though SO42–, net consumption rates of EACOM had increased to 3.88–4.85µmol e–cm–3day–1, presumably owing to a re-oxidation of sulfide by OM at sites otherwise not accessible for microbial reduction. As evaluated by sulfur K-edge X-ray absorption near-edge structure spectroscopy, bacterial sulfate reduction presumably involved not only a recycling of inorganic sulfur species, but also a sulfurisation of OM, yielding reduced organic sulfur, and changes in oxidised organic sulfur species. Organic matter thus contributes to anaerobic respiration: (i) directly by EAC of redox-active functional groups; (ii) directly by oxidised organic sulfur; and (iii) indirectly by re-oxidation of sulfide to maintain bacterial sulfate reduction.

Occurrence and fate of colloids and colloid-associated metals in a mining-impacted agricultural soil upon prolonged flooding

Colloids formed during soil flooding can potentially facilitate the mobilization of metal contaminants. Here, laboratory batch incubations with a contaminated soil were performed to monitor temporal changes in the porewater dynamics of metals, the morphology and composition of colloids, and the speciation of colloids-associated metals during 30 days of flooding. The concentrations of colloidal and dissolved metals increased initially and peaked at a certain time, but then decreased with the on-going sulfate reduction. The combined analysis of spectrometric, spectroscopic, and size-fractionation results revealed that the dynamics of Cu were dominated by microbe-associated colloids and were mediated largely by Cu(0) biomineralization and subsequent sulfidation, while the microbe-associated and freely dispersed colloids were equally relevant for governing the dynamics of Cd and Pb. Mobilization of Zn, on the other hand, was dominated by its dissolved form, probably due to the low thermodynamic stability of Zn-sulfide. Additionally, adsorption via organic functional groups was another mechanism for metal incorporation into colloids. We also provided direct spectroscopic evidence for the formation and persistence of dispersed heterocolloids consisting of CuxS and CdS during flooding. Our findings suggest that colloids-induced metal mobilization should be considered in assessing bioavailability and risks of metals in contaminated soils upon flooding.