BG1.6

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.

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.

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.

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 (SO₄^2-, 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 PO₄^3-) 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.

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.