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EDI
(Bio)geochemistry of redox sensitive elements in oxic, anoxic, and euxinic environments

Redox sensitive elements (RSE) are generally found in higher concentrations (compared to their detrital levels) within the sediments deposited in oxygen depleted environments. Although modern ocean waters are well oxygenated, with oxygen deprived settings occurring in isolated basins and high-productivity zones, oxygen-depleted environments were dominating through early Earth’s history. The degree of RSE enrichments in ancient sediments reflects the sedimentary environment that was oxygen-poor. However, we are still lacking tools for more precise description of ancient environments implying need for more extensive interdisciplinary research on mechanism of RSE accumulation in anoxic sediments. In order to understand fully RSE behaviour in anoxic settings one needs to have comprehensive understanding of the overall “chemistry behind”, not only in anoxic environments. This session presents new contributions on RSE present-day investigation in the environment using different techniques (mass spectrometry, isotopic analysis, elemental speciation, solid phase characterization, radiochemical methods) in settings (lakes, rivers, ocean) characterized with different oxygen concentration and oxygen penetration depth in water column and/or sediment, respectively. We encourage contributions using interdisciplinary and multi-analytical approaches targeting description of the RSE sedimentary phases, RSE speciation in aqueous phase, RSE cycling at the oxia/anoxia boundary, i.e., in general research leading towards better understanding of the (bio)geochemistry of RSE.

Convener: Elvira Bura-Nakić | Co-convener: Igor ZivkovicECSECS
Presentations
| Mon, 23 May, 13:20–14:50 (CEST)
 
Room 2.15

Mon, 23 May, 13:20–14:50

Chairpersons: Igor Zivkovic, Elvira Bura-Nakić, Lucija Knežević

13:20–13:24
Introduction

13:24–13:31
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EGU22-12143
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ECS
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Highlight
Amonda El Houssainy et al.

In the current context of climate change, the coastal area is exposed to an increasing pressure from hydrodynamic agents such as tide, flood and storm (Parry et al., 2007) and to enormous anthropogenic activities due to urbanization and industrialization of the coastline, which weakens the coastal ecosystem. In France, the Manche department presents more than a half of the Normandy coastline (330 km) and a great diversity of its shores. It is a key player in the preservation of the coastal environment. Among its conservation areas, two estuaries in Saint-Vaast-La-Hougue interested us: the Saire Estuary and Cul-de-Loup Bay, which both subjects to the impact of human activities (agriculture, shellfish farming, tourism, modification of the coastline, etc.). In order to quantify the chemical and biological exchanges in the mudflat of these two sites, we performed dissolved oxygen profiles in the sediment using a benthic microprofiler system (Unisense®). Moreover, sediment cores were collected and sliced under inert atmosphere, in order to measure diagenetic tracers (NH4+, PO43-, Fe2+ and ΣHS-) and trace metals levels, and to identify bacterial communities. The results of sediment cores and oxygen microprofiles taken from each of the mudflat indicate a greater dynamic degradation of organic matter in the superficial sediments of Saire estuary and in deep sediment of Cul-de-Loup Bay. The benthic microprofiler results show that oxygen penetration depth is around 1 mm and 1.4 mm respectively in Saire estuary (n=3) and Cul-de-Loup bay (n=5). This difference is marked by (i) an intense reduction of Fe (oxy)hydroxides at 4 cm of sediment depth in the Saire estuary, (ii) the appearance of ΣHS- from ~ 12 cm of sediment depth against 5 cm of sediment depth in the Cul-de-Loup Bay and (iii) a slight Fe(oxy)hydroxide zone at 3 cm of sediment depth. Metagenomics analysis confirm major differences between the two study sites.

How to cite: El Houssainy, A., Poirier, I., Bertrand, M., Verdier, L., and Cesbron, F.: Vertical distribution and aerobic degradation at the sediment-water interface in two urbans estuaries in Normandy, France, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12143, https://doi.org/10.5194/egusphere-egu22-12143, 2022.

13:31–13:38
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EGU22-2654
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ECS
Igor Zivkovic et al.

Rhenium (Re) is known as one of the rarest elements at Earth’s surface. Re enrichment in sediments deposited beneath anoxic and sulfidic water columns can be few orders of magnitude greater in comparison to the average crustal concentration of Re. The exact mechanism of Re transformation and transport from dissolved phase in oxic environments to anoxic sediments is still poorly understood. The hypothesis of Re enrichment involves the reduction of perrhenate to an insoluble Re(IV) product within the sediment-water interface or progressive thiolation of perrhenate anion that leads to the formation of particle-reactive thioperrhenates. Like molybdenum (Mo) and uranium (U), the analysis of vanadium (V) and Re enrichment covariations within anoxic sediments may also be potentially used as an important paleoredox tool. To broaden our understanding of Re geochemistry in hypoxic and euxinic marine lakes, we have performed sampling of seawater and sediments in two marine lakes in the Eastern Adriatic Sea (Small Lake on the Island of Mljet and Dragon Eye Lake near the town of Rogoznica). Samples were collected in April 2020 and November 2020 at the Small Lake, while those in the Dragon Eye were collected in July 2020. Seawater profiles were collected from the surface to near-bottom layer. Sediments were sampled using core-sampler and cut in 2-cm layers under nitrogen atmosphere. Porewater was separated from the sediment by centrifugation and filtered under nitrogen atmosphere. Re in sediments was determined using ID-ICP-MS following acid digestion, matrix substitution, and preconcentration on Dowex resin. Re in seawater and porewater was determined using ID-ICP-MS after preconcentration on Dowex resin. Multi-elemental analyses in waters and sediments were also performed to obtain insights into Re behavior in these compartments. The first results for the Small Lake (April sampling) showed that Re concentration in seawater is rather uniform (about 8 ng/L). Further on, Re concentrations in sediments were increasing with depth (from 3.5 to 9.7 ng/g), while the corresponding Re concentrations in porewater were decreasing (from 4.6 to 1.8 ng/L). Principal component analyses showed different behavior of Re in porewaters and sediments when compared to other redox sensitive elements. In sediments, Re was highly correlated with Mo and U, and usually without correlations with Mn or Fe. On the contrary, Re in porewaters was highly correlated to Mn and Fe, and negatively correlated with Mo and U. Regarding correlations between Re in V: in cores in which Re was negatively correlated with V in porewater, there were no significant correlations in sediments, and vice versa. Re in porewaters did not show correlations with sulfide. These first results indicate different geochemistry of Re in hypoxic and euxinic marine lakes when compared to Mo, U, and V. The appropriate data analysis will be evaluated following the analysis of other samples.

How to cite: Zivkovic, I., Knezevic, L., Omanovic, D., Jagodic Hudobivnik, M., Rovan, L., and Bura Nakic, E.: Rhenium geochemistry in hypoxic and euxinic marine lakes of the Eastern Adriatic Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2654, https://doi.org/10.5194/egusphere-egu22-2654, 2022.

13:38–13:45
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EGU22-680
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ECS
Lei Xia et al.

The reductive dissolution of iron (Fe) (oxy)hydroxides in sediments releases phosphorus (P) to the overlying water and may lead to eutrophication. Glauconite sands (GS) are rich in Fe and may be used as readily available P sorbents. This study was set up to test effects of dose and type of GS on the P immobilisation in sediments under hypoxic conditions. Three different GS were amended to a P-rich river sediment at doses of 0% (control), 5% and 10% (weight fractions) and incubated with overlying water in batch laboratory conditions. Glutamate was added to the solution after 15 days to deplete any residual dissolved oxygen from the sediment-water interface. In the first 15 days, the P concentration in the overlying water peaked to 1.5 mg P L-1 at day 9 in the control and decreased to 0.9 mg P L-1 at lowest Fe-dose and to 0.03 mg P L-1 at the highest Fe-dose, the effects of GS type and dose were explained by the Fe dose. After 15 days, the added glutamate induced a second, and larger peak of P in the overlying water in sediment, that peak was lower in amended sediments but no GS dose or type related effects were found. This suggests that freshly precipitated P species at the sediment-water interface can be remobilised. This study highlights the potential for using this natural mineral as a cheap and easily available sediment remediation material, but its longevity under rare extreme conditions needs to be further investigated.

How to cite: Xia, L., Verbeeck, M., Bruneel, Y., and Smolders, E.: Iron rich glauconite sand as an efficient phosphate immobilising agent in river sediments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-680, https://doi.org/10.5194/egusphere-egu22-680, 2022.

13:45–13:52
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EGU22-9462
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ECS
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Highlight
Alice Bosco Santos and Orit Sivan

Microbial metabolisms that attain close together different biogeochemical cycles, such as Fe, C, and N, introduce complexity to the traditional redox electron acceptors cascade in sediments, leading to spatial overlap between geochemical gradients. A good example of overlap when considering Fe geochemistry is the oftentimes peaks in dissolved Fe2+ observed below the sulfur-methane transitional zone (SMTZ) in different environments. While anaerobic methane oxidation mediated by Fe reduction (Fe-AOM) might explain the feature in deep lacustrine sediments, our preliminary data indicate that Fe-AOM is not significant in oligotrophic marine sediments. We described Fe speciation, nutrients, and microbiota composition in various sedimentary profiles from the Levantine Basin, Eastern Mediterranean Sea, Israel and observed coupled Fe and N cycling. In the ammonium-rich (2000 µmol L-1) deep methanic sediments, strongly positive correlated increases in dissolved Fe2+ and NO2- (and/or NO3-) via microbe-mediated ammonium oxidation coupled to Fe(III) reduction (Feammox) is proposed. In this environment, the deep availability of Fe2+ favors precipitation of authigenic Fe minerals below the SMTZ.

How to cite: Bosco Santos, A. and Sivan, O.: Who controls Fe cycling below the SMTZ of the Mediterranean Sea?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9462, https://doi.org/10.5194/egusphere-egu22-9462, 2022.

13:52–13:59
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EGU22-9099
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ECS
Lucija Knežević et al.

In the environment vanadium(V) is found in three oxidation states (III, IV and V) depending on physicho-chemical conditions (pH, Eh, ionic strength, its concentration, biology, organic matter content ect.) of certain environmental medium [1]. Knowledge of V species distribution is to this date very scarce due to its complex aquatic chemistry as well as modern analytical intrumentation constraints [2,3]. Especially, methods on accurate V(III) species determination in complex environmental matrices are poorly developed which makes estimation of V biogeochemical cycle uncomplete. It is known that V(III) is expected to be stable in strongly anoxic or euxinic conditions, such as sulphide-containing water samples or sediments. Furthermore, high affinity towards adsorption on various colloids found in natural aquatic systems as well as formation of strong complexes with various organic ligands is presumed. Possible formation of V(III) in certain aquatic environments can contribute to removal of V into sediments [2]. Therefore, determination of V(III) in natural environmental samples is highly needed for accurate estimation of V bioavailability, mobility and toxicitiy.

Method developed by Yatirajam et al. (1979) was used in order to establish V(III) stability in various model solutions [4]. Procedure is based on the formation of selective complexes of V(III) and picnolic acid in respect to V(IV) and V(V) species present in samples. Upon complexation, V(III) species are then extracted into chloroform. Extracts were evaporated to dryness and the remaining content was dissolved in 2% HNO3. Samples were then measured using HR ICP-MS analytical intrumentation. For measurements using spectrophotometry, samples were measured immidiately upon extraction. Studies so far show good selectivity, reproducibility and accuracy which offers a promising method for V(III) determination in natural samples. Stated findings are planned to be further applied on natural anoxic sediment samples of Mljet and Rogoznica lakes (Croatia).

Literature:

  • Huang, J.H.; Huang, F.; Evans, L.; Glasauer, S. Vanadium: Global (bio)geochemistry. Chem. Geol. 2015, 417, 68–89, doi:10.1016/j.chemgeo.2015.09.019.
  • Gustafsson, J.P. Vanadium geochemistry in the biogeosphere –speciation, solid-solution interactions, and ecotoxicity. Appl. Geochemistry 2019, 102, 1–25, doi:10.1016/j.apgeochem.2018.12.027.
  • Costa Pessoa, J. Thirty years through vanadium chemistry. J. Inorg. Biochem. 2015, 147, 4–24, doi:10.1016/j.jinorgbio.2015.03.004.
  • Yatirajam. V, Arya, S.P. EXTRACTION DETERMINATION AS VANADIUM ( II1 ) OF VANADIUM. Talanta 1978, 26, 60.

How to cite: Knežević, L., Mandić, J., Živković, I., Omanović, D., and Bura Nakić, E.: Preliminary studies on V(III) determination in the form of picnolate complex using HR ICP-MS , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9099, https://doi.org/10.5194/egusphere-egu22-9099, 2022.

13:59–14:07
Q&A Part 1

14:07–14:14
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EGU22-9601
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ECS
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Highlight
Nimet Alımlı and Mustafa Yücel

Iron is one of the most important redox-sensitive elements in marine systems. A better understanding of the marine iron cycle is urgently needed for many scientific questions, including the evolution of ancient co-factors under changing redox conditions, marine primary production, and global climate change. Given the scarcity of iron in oceans, the interplay between different iron pools in various redox settings is analytically challenging and poorly understood. In this study, we report on new downcore profiles of pore water iron species along with their size distributions across the oxic, suboxic, and sulfidic regions of the Black Sea and in the recently deoxygenated areas of the Sea of Marmara. The vertical distribution of dissolved iron (<0.45 µm) in sediment pore water showed strong subsurface iron peaks reaching maximum concentrations around 87 µM in the Sea of Marmara, resulting in high benthic iron fluxes and indicating high rates of bacterial iron mineral respiration under hypoxia. In the Black Sea, highly sulfidic sediment conditions appeared to limit dissolved iron mobility, with iron concentrations in pore water ranging from 0.3 to 0.05 µM.  We also performed additional experiments at selected sites to understand the nature of the colloidal fraction. Size fractions were obtained by sequential filtering and filtered samples were analyzed by the spectrometric ferrozine method. To achieve the low detection limits required for the water column samples, the spectrometer was used in conjunction with a 50cm liquid waveguide capillary cell, allowing rapid on-board detection of iron at nanomolar levels. The partitioning between soluble (<0.02 µm) and colloidal (0.02-0.2 µm) iron pools in the pore water showed that the dissolved iron was mainly dominated by the soluble fraction, while colloidal fraction behaved differently. The results suggest that the colloidal fraction may be more dependent on other biogeochemical characteristics of the environment in addition to redox conditions. We also applied to colloidal fraction a revised sequential acid leaching scheme originally developed for hydrothermal iron fractions. Preliminary results suggest that the characteristics of the colloidal iron pool in the pore waters of the Sea of Marmara and Black Sea sediments differ from the nanomineral-dominated vent iron, and that organic fractions may play a greater role in mobilizing iron colloids from sediments of deoxygenated basins.  

How to cite: Alımlı, N. and Yücel, M.: Seafloor iron mobilization across the deep-water redox gradients of the Black Sea and the Sea of Marmara, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9601, https://doi.org/10.5194/egusphere-egu22-9601, 2022.

14:14–14:21
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EGU22-183
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ECS
Daniel A. Petrash et al.

In the oligotrophic bottom waters of a post-mining lake (Lake Medard, Czechia), ferruginous conditions occur without quantitative sulfate depletion. The dissolved organic matter supply to the deep waters is small and, accordingly, sulfate reduction promoting precipitation of stable ferrous sulfides is limited. In line with these observations, an isotopically constrained estimate of the rates of planktonic sulfate reduction (SRR) suggests that despite a high genetic potential—as determined by genome analyses, SRR are limited by substrate competition exerted by nitrogen and iron respiring prokaryotes. The microbial succession across the nitrogenous and ferruginous zones of the bottom water column also indicates a sustained genetic potential for chemolithotrophic sulfur oxidation, probably accompanied by disproportionation of S intermediates[1].

The bottom waters displayed dissolved Fe concentrations (~0.1 to 33 µM) and δ56Fe values (-1.77 ± 0.03 ‰ to +0.12 ± 0.05 ‰) that increase across the redoxcline and towards the anoxic sediment-water interface (SWI). These parameters pinpoint diffusive transport and partial oxidation of dissolved ferrous iron (Fe(II)) sourced from the lakebed, depletion of the residual Fe(II) in heavy isotopes at the redoxcline and enrichment near the SWI linked to monosulfide precipitation. In the carbonate-buffered lake sediments, however, sulfur re-oxidation appears to prevent substantial stabilization of iron monosulfides as pyrite, but it enables the interstitial precipitation of small proportions of equant microcrystalline gypsum. This gypsum isotopically fingerprints sulfur oxidation proceeding at near equilibrium with the ambient anoxic waters, whilst authigenic pyrite-sulfur displays a 38 to 27 ‰ isotopic offset from ambient sulfate, suggestive of incomplete sulfate reduction and indicative of the openness of the system[1].

Overall, our results demonstrate that under transitional redox states producing the meromictic stability described here, the simple biogeochemical zonation models based on energetic considerations of pure phases at standard conditions may not accurately describe the overlapping zonation of dissimilatory iron and sulfur reduction. Vigorous sulfur and iron co-recycling in the water column can be fuelled by ferric and manganic particulate matter and notably by the redeposited siderite stocks of the upper anoxic sediments. In the absence of ferruginous coastal zones today, the current water column redox stratification in the post-mining Lake Medard has scientific value for (i) testing emerging hypotheses on how a few interlinked biogeochemical cycles operated in low productivity nearshore paleoenvironments during transitional states between ferruginous and euxinic conditions; and (ii) to acquire insight on potential avenues for early diagenetic overprinting of redox proxy signals in ferruginous-type sediments.

[1] Petrash, D. A., Steenbergen, I. M., Valero, A., Meador, T. B., Pačes, T., and Thomazo, C.: Aqueous system-level processes and prokaryote assemblages in the ferruginous and sulfate-rich bottom waters of a post-mining lake, Biogeosciences Discuss. [preprint], https://doi.org/10.5194/bg-2021-253, in review, 2021.

How to cite: Petrash, D. A., Steenbergen, I. M., Valero, A., Meador, T. B., Lalonde, S. V., and Thomazo, C.: Disentangling the overlapping zonation of dissimilatory iron and sulfate reduction in a carbonate-buffered sulfate-rich and ferruginous lake water column, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-183, https://doi.org/10.5194/egusphere-egu22-183, 2022.

14:21–14:28
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EGU22-10643
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ECS
David Yousavich et al.

Sulfate reduction, a crucial metabolic pathway for organic matter remineralization in marine sediments, produces hydrogen sulfide that can be subsequently utilized by chemoautotrophic organisms. When the water column above marine sediments becomes anoxic, microbial metabolisms at the sediment-water interface shift to take advantage of the electron donors and acceptors available in the new redox conditions. These processes were examined In November 2019 during the AT42-19 expedition aboard RV Atlantis. Samples were collected using ROV Jason at different depths along a transect traversing the Santa Barbara Basin between 440 and 600 m depth. Deeper parts of the basin experience transient deoxygenation that is sometimes associated with a nitrate-depleted zone. Under these conditions, large benthic microbial mats of sulfur-oxidizing bacteria form in the basin. To analyze the effect of these mats on the basin geochemistry, sulfur and nitrogen (SO42-, H2S, NO3-, NO2-, NH4+) consumption and production were examined using sediment push cores and benthic flux chambers. Other redox sensitive compounds (e.g. Fe and PO43-) were also measured using these methods. Areal sulfate reduction rates measured in push cores using the 35S-Sulfate radiotracer method were highest in the deepest, anoxic part of the basin (~4 mmol m-2 d-1) where microbial mats were most prevalent and the sediment-water interface was anoxic and low in nitrate (7.3 µM). Sulfate reduction was noticeably lower at shallow stations (~2 mmol m-2 d-1) with oxygenated water, signs of bioturbation, and without mats. Sulfate reduction below the sediment-water interface (0-1 cm sediment depth) was also an order of magnitude higher at deep stations (~120 nmol cm-3 d-1) compared to shallow stations (~18 nmol cm-3 d-1). Despite high sulfate reduction activity in areas covered by mats, sulfide concentrations were near-zero in the uppermost 2 cm of sediment. Nitrate flux into the sediment and ammonium flux out of the sediment was highest where mats were present (-2.93 mmol m-2 d-1 and 11.19 mmol m-2 d-1 respectively). Additionally, the anoxic depocenter of the basin contains a flux of ferrous iron (4.10 mmol m-2 d-1) and phosphate (3.18 mmol m-2 d-1) out of the sediment into the water column. Our results provide a direct comparison of redox cycling at the sediment-water interface under vastly different redox conditions within the same oceanic basin. These results also provide strong evidence that chemoautotrophic sulfur-oxidizing bacteria in sediments of the anoxic Santa Barbara Basin perform dissimilatory nitrate reduction to ammonium and are responsible for rapid sulfur cycling near the sediment-water interface with a concurrent flux of ammonium, iron, and phosphate into the water column.

How to cite: Yousavich, D., Robinson, D., Krause, S. J. E., Tarn, J., Liu, N., Janssen, F., Wenzhoefer, F., Valentine, D. L., and Treude, T.: Dissimilatory nitrate reduction to ammonium by benthic microbial mats fuels rapid sulfur oxidation and sediment ferrous iron release in the anoxic Santa Barbara Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10643, https://doi.org/10.5194/egusphere-egu22-10643, 2022.

14:28–14:35
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EGU22-13482
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ECS
Zohar Tik et al.

Anaerobic respiration is being observed in many aquatic environments as an outcome of oxygen depletion. Chemical profiles in porewater of organic-rich sediments indicate that various microbes use several electron acceptors in the anaerobic dissimilatory respiration. These electron acceptors are used in the order of decreasing chemical potential, beginning with nitrate, then manganese and iron oxides, then sulfate and finally carbon dioxide .1 During respiration, organisms consume and produce metabolites and thus are changing their environment. 
Some bacteria are capable of using various compounds as final electron acceptors (EA). One of those bacteria is Desulfuromonas carbonisis sp. nov., a Gram-negative, obligatory anaerobic, rod-shaped bacterium. This bacterium is closely related to bacteria from the Geobacter genus, which is well known as a major iron reducer through dissimilatory anaerobic respiration. Species from this genus are found in natural anaerobic systems and are capable of reducing Fe(III)-oxides, S0, and Mn(IV)-oxides. Here we investigated the change in Desulforomonas metabolites as a result of available EA. 
Bacterial cultures were extracted, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) data were analyzed using the global natural products social molecular networking (GNPS) online platform.2 Our results indicate that unique metabolites are produced by the bacteria depending on the presence of different EA in the culture, while some of the metabolites were shared by two groups or more. Indole-3-carboxaldehyde (I3C) was found almost exclusively in the iron-oxide containing cultures. This compound is known as part of tryptophan metabolism and is known to affect chemical communication of bacteria. To the best of our knowledge, I3C was not identified in the Desulfuromonas genus until now. 
We were able to detect this compound not only in pure cultures but also in cultures containing the bacterium, natural anoxic lake sediment and iron oxides. That establishes the potential of I3C to be involved in natural processes specific to dissimilatory iron reduction. We will continue to investigate these processes and the connection between I3C signaling and iron. 

1. Froelich, P. N. et al. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim. Acta 43, 1075–1090 (1979). 
2. Wang, M. et al. Sharing and community curation of mass spectrometry data with Global Natural Products Social Molecular Networking. Nature Biotechnology vol. 34 828–837 (2016). 

How to cite: Tik, Z., Vigderovich, H., Sivan, O., and Meijler, M. M.: Identification of Desulfuromonas carbonis sp. nov. Metabolites that are Secreted in Response to Different Electron Acceptors, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13482, https://doi.org/10.5194/egusphere-egu22-13482, 2022.

14:35–14:42
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EGU22-6209
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ECS
Andrew Grigg et al.

Jarosite is a ferric iron sulfate mineral [(KFe3(SO4)2(OH)6] that is commonly formed in acidic environments that are rich in iron and sulfate, such as acid-sulfate soils or acid mine drainage. The stability of jarosite is important because the mineral contains embodied acidity and may scavenge trace elements by sorption and co-precipitation. Although stable under high Eh and low pH conditions, previous studies have shown that jarosite is prone to transformation by hydrolysis at circumneutral pH, or may undergo Fe(II)-catalysed transformation where ferrous ions are present [1-3]. Jarosite may be exposed to Fe(II) at circumneutral pH in reducing environments, such as in flooded acid-sulfate soils [2]. Jarosite is a member of the alunite supergroup and forms a solid solution series with alunite by substitution of Al for Fe. However, the effect of Al substitution on the stability of jarosite in the presence of Fe(II) has not previously been investigated. Here, we performed batch experiments using samples of a synthetic jarosite without aluminium substitution, and synthetic jarosite containing 7.3% Al-for-Fe substitution. Mineral samples were reacted with 0.5 mM and 5 mM Fe(II) at pH 7.1 (50 mM MOPS buffer) for up to 24 hours. Rietveld analysis of X-ray diffraction patterns was used to quantify mineral transformations and to determine the crystallinity of, and Al substitution in, product phases. Complete transformation of jarosite to mixtures of ferrihydrite, goethite and lepidocrocite occurred within several hours for all jarosite samples and Fe(II) treatments. The 10-fold increase in Fe(II) concentration resulted in a 50% increase in jarosite transformation rate, and pure jarosite transformed 110% to 280% faster than Al-substituted jarosite. The transformation products of Al-substituted jarosite contained a smaller proportion of lepidocrocite than the products of pure jarosite transformation, and the unit cell size of the lepidocrocite that initially formed from Al-substituted jarosite indicates that Al was substituted into the structure. These results demonstrate that structural Al can stabilise jarosite against transformation, which has implications for understanding the longevity of jarosite, and its importance to trace element cycling, in reducing environments.

1. Welch, S. A., et al. (2008) Chem. Geol. 254: pp. 73-86
2. Karimian, N., et al. (2017) Environ. Sci. Technol. 51: pp. 4259-4268
3. Whitworth, A. J., et al. (2020) Chem. Geol. 554 

How to cite: Grigg, A., Notini, L., Kaegi, R., ThomasArrigo, L., and Kretzschmar, R.: Stability of Al-substituted jarosite in the presence of Fe(II), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6209, https://doi.org/10.5194/egusphere-egu22-6209, 2022.

14:42–14:50
Q&A Part 2