Phosphorus cycling in anoxic sediments

Worldwide oxygen minimum zones (OMZs) as well as coastal oxygen-deficient regions have been shown to be expanding during recent decades. When such oxygen minima impinge on the sea floor, the retention capacity of sediments for phosphate (TPO4), ferrous iron (Fe2+), as well as ammonium (NH4+) is strongly reduced, resulting in high sea-bed release rates of these key nutrients into the bottom water. Despite the significance of the benthos exerting a major positive feedback on surface-water primary productivity and in turn maintenance of oxygen (O2) deficiency, the nutrient release in OMZ and coastal O2-deficient regions has hardly been quantified. The aim of this study was to investigate the benthic nutrient turnover in two different highly O2-deficient systems: i. the intense OMZ off Peru and ii. the landlocked Gotland Basin, Baltic Sea, which suffers from anthropogenically induced eutrophication. The focus was on the phosphorus (P) cycle but associated cycles of iron (Fe) and nitrogen (N) were also included. Off the coast of Peru, benthic fluxes of TPO4 and Fe2+ were quantified in situ using benthic landers and were calculated from pore-water profiles across a latitudinal depth transect at 11°S. This transect extended from 80 m to 1000 m water depth and covered anoxic to oxic bottom-water conditions. The working area was divided into three different zones: the shelf that is subjected to periodically fluctuating bottom-water O2 conditions, the core of the OMZ where anoxia can be assumed to be permanent, and the depth range below 500 m where O2 levels increased again. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and in the core of the OMZ. In contrast, Fe2+ fluxes were high on the shallow shelf (maximum 316 mmol m-2 yr-1) but moderately low (15.4 mmol m-2 yr-1) in water depths between 250 m and 600 m due to the continuous reduction of Fe oxides and Fe hydroxides (henceforth referred to as Fe oxyhydroxides). Below 600 m, where O2 concentrations increased, Fe2+ fluxes became negligible due to the precipitation of Fe2+ in the oxic sediment surface. Ratios between organic carbon degradation and TPO4 flux indicated an excess release of P over carbon (C) when compared to Redfield stoichiometry. This was most likely caused by preferential P release during organic matter degradation, dissolution of fish debris, and/or P release from sulfide-oxidizing microbial mat communities. Fe oxyhydroxides were relevant as a P source only on the shallow shelf. The benthic fluxes are among the highest reported from similar O2-deficient continental margin systems, and highlight the efficiency of OMZ sediments returning TPO4 and Fe2+ to the bottom water. The shelf region is particularly important in this regard since O2 fluctuations likely trigger a complex biogeochemical reaction network of P, Fe and sulfur turnover resulting in transient, high TPO4 and Fe2+ release under anoxia. Sources for P release were further constrained by combining P speciation data, based on sequential extraction of sediment samples, with a mass balance and benthic modeling. P speciation revealed that authigenic calcium phosphate (Ca-P; including carbonate fluorapatite, biogenic apatite from fish remains, and calcium carbonate-bound P), was the major fraction along the transect. It accounted for 35 to 47% of the depth-averaged total extracted P on the shelf and upper slope, but for > 70% below 300 m water depth. Further extraction of fish-P showed that below 259 m water depth this fraction dominated the authigenic Ca-P pool by 60 to 69%. Organic P was present in considerable amounts (18 to 37%) only at the shelf and the upper slope, whereas detrital P and P bound to Fe oxyhydroxides was generally of minor importance at all sites. Organic matter in surface sediments was highly depleted in P relative to Redfield stoichiometry with C:P ratios of up to 516. The benthic model found preferential P mineralization in the water column or, alternatively, preferential P release during organic matter degradation in the sediment surface as possible pathways explaining such high C:P ratios. Nevertheless, both model and mass balance calculations revealed that irrespective of which pathway prevails, organic P was only of minor importance for the benthic P budget of Peruvian OMZ sediments. According to the solid phase speciation, authigenic Ca-P, with a high contribution of fish debris, is a likely candidate for the missing source of P required to close the P budget. These sediments were identified as weak sinks for P, as more than 80% of the imported P was recycled back into the water column. In the Gotland Basin, TPO4 and DIN fluxes were quantified in situ across an oxic to anoxic depth-transect using benthic landers. A CTD-water sampling rosette was deployed to record the nutrient and O2 distribution in the water column and thereby investigate the benthic-pelagic coupling because of its significance for the euthrophication state of the Baltic Proper. The study area was divided into three different zones: the oxic zone at 60 m to 80 m and 120 m, and the deep anoxic and sulfidic basin at > 120 m. The hypoxic transition zone was characterized by fluctuating O2 levels as well as the occurrence of extended mats of sulfur bacteria. Beside the deep anoxic basin, the hypoxic transition zone was revealed as a major release site for TPO4 and NH4+ with rates of up to 0.2 mmol m-2 d-1 and 1 mmol m-2 d-1, respectively. There are clear indications that the bacterial mats converted NO3-/NO2- into NH4+ during dissimilatory nitrate reduction to ammonium (DNRA), thereby retaining reactive N in the ecosystem. The transient release and uptake of TPO4 during oscillating anoxic and oxic conditions by these bacteria, however, can only be speculated as the entire TPO4 release from the sediment could be potentially covered by preferential P release during organic matter degradation. Extrapolation of benthic fluxes to the Baltic Proper resulted in internal TPO4 and DIN loads of 109 kt yr-1 and 295 kt yr-1, respectively, which is significantly higher than external P and DIN loads. This up-scaling of fluxes revealed the importance of the hypoxic transition zone for the internal nutrient loading, which only covered 51% of the total considered area, but released as much as 70% of the total TPO4 load. Likewise, 75% of the internal NH4+ load (200 kt yr-1) was released from this particular environment; however, this NH4+ did not reach the surface mixed layer. This resulted in the supply of water with a low N:P ratio to the euphotic zone. In summertime, such low N:P ratios favor the development of N2-fixing cyanobacterial blooms which, by different feedback processes, counteract the recovery of the Baltic Proper from eutrophication

Spatial and temporal trends of iron and iron isotope cycling in the Peruvian oxygen minimum zone

Iron (Fe) is a key element in the global ocean’s biogeochemical framework because of its essential role in numerous biological processes. A poorly studied link in the oceanic Fe cycle is the reductive release of Fe from sediments in oxygen depleted ocean regions - the oxygen minimum zones (OMZs). Changing rates of Fe release from OMZ sediments may have the potential to modulate ocean fertility which has far-reaching implications considering the high amplitude oxygen fluctuations throughout earth history as well as the ongoing ocean deoxygenation projected for the near future. In order to explore spatial and temporal trends of Fe cycling in OMZs, we present here Fe isotope and speciation data for surface sediments from a transect across the Peruvian upwelling area, one of the most pronounced OMZs of the modern ocean. Because of continuous dissimilatory Fe reduction and diffusive loss across the benthic boundary, sediments within the OMZ are strongly depleted in reactive Fe components, and the little reactive Fe left behind has a heavy isotope composition. In contrast, surface sediments below the OMZ are enriched in reactive Fe, with the majority being present as Fe oxides with comparably light isotope composition. This lateral pattern of Fe depletion and enrichment indicates that Fe released from sediments within the OMZ is reoxidized and precipitated at the oxycline. First-order calculations suggest that the amount of Fe mobilized within the OMZ and that accumulated at the boundaries are largely balanced. Therefore, benthic Fe fluxes in OMZs should be carefully evaluated prior to incorporation into global models, as much of the initially released Fe may be reprecipitated prior to vertical or offshore transport. First XRF core scanning results for partly laminated piston cores from the OMZ boundaries reveal downcore oscillations in the content of reactive Fe and redox-sensitive trace metals that are attributed to past changes in OMZ extension. Ongoing work on these cores will focus on their dating and the downcore investigation of Fe and trace metal records in order to better understand past Fe cycling within the Peruvian OMZ and potential interactions with climate variability

Early diagenesis of trace metals (V, Mo, U) in sediments of the Peruvian upwelling area: response to oxygen dynamics in the water column

The upwelling area in the eastern equatorial Pacific off Peru is one of the most pronounced oxygen minimum zones (OMZs) of the modern ocean. Modeling scenarios predict an expansion of the OMZs in the course of global change in the coming decades. As a consequence, the Peruvian continental margin represents a key locality for studies on biogeochemical dynamics in the future ocean. We present pore water and sediment data for redox-sensitive metals (Fe, Mn, V, Mo, and U) that have been collected along a transect across the Peruvian margin at 11°S. The results are used to evaluate the behavior of trace metals in a wide range of biogeochemical and hydrodynamic settings. In the core of the OMZ, where permanently anoxic conditions prevail, redox sensitive metals exhibit diagenetic behaviors largely consistent with previous studies. Vanadium and Mo are released from Fe oxihydroxides and subsequently recycled through diffusion across the benthic boundary or trapped through formation of authigenic V phases and sequestration of Mo by authigenic pyrite. Some U is delivered through diffusion across the benthic boundary, reduction and precipitation of UO2 and incorporation into phosphorites. The utmost part of the buried U, however, is delivered in particulate form, most likely as bioauthigenic U which cannot be recycled in the suboxic waters overlying the anoxic sediments. In contrast to sediments in the core of the OMZ, sediments on the shelf experience frequent oxygenation episodes related to the passage of internal waves and the regular recurrence of El Niño events. These oxygenation episodes lead to the re-oxidation and remobilization of authigenic U and V. In contrast to that, the authigenic accumulation of Mo is favored by the occasional occurrence of slightly oxidizing conditions. This is most likely due to enhanced formation of sulfur intermediates necessary for pyrite formation and the increased stability of pyrite, the major Mo sink, under oxidizing conditions, compared to authigenic V and U phases. Redox oscillations in the Peruvian OMZ thus lead to a discrimination of U against Mo, a mechanism that should be considered in the interpretation of U/Mo systematics in paleo redox studies. Overall our results provide valuable constraints on how trace metal inventories of marginal sediments may respond to expanding shelf anoxia and to short term perturbations of sediment redox conditions

Early diagenesis of redox-sensitive trace metals in the Peru upwelling area – Response to ENSO-related oxygen fluctuations in the water column

Pore water and solid phase data for redox-sensitive metals (Mn, Fe, V, Mo and U) were collected on a transect across the Peru upwelling area (11°S) at water depths between 78 and 2025 m and bottom water oxygen concentrations ranging from ~0 to 93 µM. By comparing authigenic mass accumulation rates and diffusive benthic fluxes, we evaluate the respective mechanisms of trace metal accumulation, retention and remobilization across the oxygen minimum zone (OMZ) and with respect to oxygen fluctuations in the water column related to the El Nino Southern Oscillation (ENSO). Sediments within the permanent OMZ are characterized by diffusive uptake and authigenic fixation of U, V and Mo as well as diffusive loss of Mn and Fe across the benthic boundary. Some of the dissolved Mn and Fe in the water column re-precipitate at the oxycline and shuttle particle-reactive trace metals to the sediment surface at the lower and upper boundary of the OMZ. At the lower boundary, pore waters are not sufficiently sulfidic as to enable an efficient authigenic V and Mo fixation. As a consequence, sediments below the OMZ are preferentially enriched in U which is delivered via both in situ pre-cipitation and lateral supply of U-rich phosphorites from further upslope. Trace metal cycling on the Peruvian shelf is strongly affected by ENSO-related oxygen fluctuations in bottom water. During periods of shelf oxygenation, surface sediments receive particulate V and Mo with metal (oxyhydr)oxides that derive from both terrigenous sources and precipitation at the retreating oxycline. After the recurrence of anoxic conditions, metal (oxyhydr)oxides are reductively dissolved and the hereby liberated V and Mo are authigenically removed. This alternation between supply of particle-reactive trace metals during oxic periods and fixation during anoxic periods leads to a preferential accumulation of V and Mo compared to U on the Peruvian shelf. The decoupling of V, Mo and U accumulation is further accentuated by the varying susceptibility to re-oxidation of the different authigenic metal phases. While authigenic U and V are readily re-oxidized and recycled during periods of shelf oxygenation, the sequestration of Mo by authigenic pyrite is favored by the transient occurrence of oxidizing conditions.Our findings reveal that redox-sensitive trace metals respond in specific manner to short-term oxygen fluctuations in the water column. The relative enrichment patterns identified might be useful for the reconstruction of past OMZ extension and large-scale redox oscillations in the geological record

Benthic iron and phosphorus fluxes across the Peruvian oxigen minimum zone

Benthic fluxes of dissolved ferrous iron (Fe2+) and phosphate (TPO4) were quantified by in situ benthic chamber incubations and pore-water profiles along a depth transect (11°S, 80–1000 m) across the Peruvian oxygen minimum zone (OMZ). Bottom-water O2 levels were < 2 µmol L-1 down to 500-m water depth, and increased to ~40 µmol L-1 at 1000 m. Fe2+ fluxes were highest on the shallow shelf (maximum 316 mmol m-2 yr-1), moderate (15.4 mmol m-2 yr-1) between 250 m and 600 m, and negligible at deeper stations. In the persistent OMZ core, continuous reduction of Fe oxyhydroxides results in depletion of sedimentary Fe :Al ratios. TPO4 fluxes were high (maximum 292 mmol m-2 yr-1) throughout the shelf and the OMZ core in association with high organic carbon degradation rates. Ratios between organic carbon degradation and TPO4 flux indicate excess release of P over C when compared to Redfield stoichiometry. Most likely, this is caused by preferential P release from organic matter, dissolution of fish debris, and/or P release from microbial mat communities, while Fe oxyhydroxides can only be inferred as a major P source on the shallow shelf. The benthic fluxes presented here are among the highest reported from similar, oxygen-depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean. The shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom-water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of Fe2+ and TPO4

Benthic phosphorus cycling in the Peruvian oxygen minimum zone

Oxygen minimum zones (OMZs) that impinge on continental margins favor the release of phosphorus (P) from the sediments to the water column, enhancing primary productivity and the maintenance or expansion of low-oxygen waters. A comprehensive field program in the Peruvian OMZ was undertaken to identify the sources of benthic P at six stations, including the analysis of particles from the water column, surface sediments, and pore fluids, as well as in situ benthic flux measurements. A major fraction of solid-phase P was bound as particulate inorganic P (PIP) both in the water column and in sediments. Sedimentary PIP increased with depth in the sediment at the expense of particulate organic P (POP). The ratio of particulate organic carbon (POC) to POP exceeded the Redfield ratio both in the water column (202 ± 29) and in surface sediments (303 ± 77). However, the POC to total particulate P (TPP = POP + PIP) ratio was close to Redfield in the water column (103 ± 9) and in sediment samples (102 ± 15). This suggests that the relative burial efficiencies of POC and TPP are similar under low-oxygen conditions and that the sediments underlying the anoxic waters on the Peru margin are not depleted in P compared to Redfield. Benthic fluxes of dissolved P were extremely high (up to 1.04 ± 0.31 mmol m−2 d−1), however, showing that a lack of oxygen promotes the intensified release of dissolved P from sediments, whilst preserving the POC / TPP burial ratio. Benthic dissolved P fluxes were always higher than the TPP rain rate to the seabed, which is proposed to be caused by transient P release by bacterial mats that had stored P during previous periods when bottom waters were less reducing. At one station located at the lower rim of the OMZ, dissolved P was taken up by the sediments, indicating ongoing phosphorite formation. This is further supported by decreasing porewater phosphate concentrations with sediment depth, whereas solid-phase P concentrations were comparatively high

Ruthenium piano-stool complexes bearing imidazole-based PN ligands

A variety of piano-stool complexes of cyclopentadienyl ruthenium(II) with imidazole-based PN ligands have been synthesized starting from the precursor complexes CpRu(C10H8)]PF6, CpRu(NCMe)(3)]PF6 and CpRu(PPh3)(2)Cl]. PN ligands used are imidazol-2-yl, -4-yl and -5-yl phosphines. Depending on the ligand and precursor different types of coordination modes were observed; in the case of polyimidazolyl PN ligands these were kappa P-1-monodentate, kappa P-2,N-, kappa N-2,N- and kappa N-3,N,N-chelating and mu-kappa P:kappa N-2,N-brigding. The solid-state structures of CpRu(1a)(2)Cl]center dot H2O (5 center dot H2O) and {CpRu(mu-kappa(2)-N,N-kappa('1)-P-2b)}(2)](C6H5PO3H)(2)(C6H5PO3H2)( 2), a hydrolysis product of the as well determined {CpRu(2b)} (2)](PF6)(2)center dot 2CH(3)CN (7b center dot 2CH(3)CN) were determined (1a = imidazol-2-yldiphenyl phosphine, 2b = bis(1-methylimidazol-2-yl) phenyl phosphine, 3a = tris(imidazol-2-yl) phosphine). Furthermore, the complexes CpRu(L)(2)]PF6 (L = imidazol-2-yl or imidazol-4-yl phosphine) have been screened for their catalytic activity in the hydration of 1-octyne. (C) 2011 Elsevier B. V. All rights reserved

Designing organometallic compounds for catalysis and therapy

Bioorganometallic chemistry is a rapidly developing area of research. In recent years organometallic compounds have provided a rich platform for the design of effective catalysts, e.g. for olefin metathesis and transfer hydrogenation. Electronic and steric effects are used to control both the thermodynamics and kinetics of ligand substitution and redox reactions of metal ions, especially Ru II. Can similar features be incorporated into the design of targeted organometallic drugs? Such complexes offer potential for novel mechanisms of drug action through incorporation of outer-sphere recognition of targets and controlled activation features based on ligand substitution as well as metal- and ligand-based redox processes. We focus here on η 6-arene, η 5-cyclopentadienyl sandwich and half-sandwich complexes of Fe II, Ru II, Os II and Ir III with promising activity towards cancer, malaria, and other conditions. © 2012 The Royal Society of Chemistry

Conjugation of a Ru(II) Arene Complex to Neomycin or to Guanidinoneomycin Leads to Compounds with Differential Cytotoxicities and Accumulation between Cancer and Normal Cells

A straightforward methodology for the synthesis of conjugates between a cytotoxic organometallic ruthenium(II) complex and amino- and guanidinoglycosides, as potential RNA-targeted anticancer compounds, is described. Under microwave irradiation, the imidazole ligand incorporated on the aminoglycoside moiety (neamine or neomycin) was found to replace one triphenylphosphine ligand from the ruthenium precursor [(η6-p-cym)RuCl(PPh3)2]+, allowing the assembly of the target conjugates. The guanidinylated analogue was easily prepared from the neomycin-ruthenium conjugate by reaction with N,N′-di-Boc-N″-triflylguanidine, a powerful guanidinylating reagent that was compatible with the integrity of the metal complex. All conjugates were purified by semipreparative high-performance liquid chromatography (HPLC) and characterized by electrospray ionization (ESI) and matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) and NMR spectroscopy. The cytotoxicity of the compounds was tested in MCF-7 (breast) and DU-145 (prostate) human cancer cells, as well as in the normal HEK293 (Human Embryonic Kidney) cell line, revealing a dependence on the nature of the glycoside moiety and the type of cell (cancer or healthy). Indeed, the neomycin-ruthenium conjugate (2) displayed moderate antiproliferative activity in both cancer cell lines (IC50 ≈ 80 μM), whereas the neamine conjugate (4) was inactive (IC50 ≈ 200 μM). However, the guanidinylated analogue of the neomycin-ruthenium conjugate (3) required much lower concentrations than the parent conjugate for equal effect (IC50 = 7.17 μM in DU-145 and IC50 = 11.33 μM in MCF-7). Although the same ranking in antiproliferative activity was found in the nontumorigenic cell line (3 2 > 4), IC50 values indicate that aminoglycoside-containing conjugates are about 2-fold more cytotoxic in normal cells (e.g., IC50 = 49.4 μM for 2) than in cancer cells, whereas an opposite tendency was found with the guanidinylated conjugate, since its cytotoxicity in the normal cell line (IC50 = 12.75 μM for 3) was similar or even lower than that found in MCF-7 and DU-145 cancer cell lines, respectively. Cell uptake studies performed by ICP-MS with conjugates 2 and 3 revealed that guanidinylation of the neomycin moiety had a positive effect on accumulation (about 3-fold higher in DU-145 and 4-fold higher in HEK293), which correlates well with the higher antiproliferative activity of 3. Interestingly, despite the slightly higher accumulation in the normal cell than in the cancer cell line (about 1.4-fold), guanidinoneomycin-ruthenium conjugate (3) was more cytotoxic to cancer cells (about 1.8-fold), whereas the opposite tendency applied for neomycin-ruthenium conjugate (2). Such differences in cytotoxic activity and cellular accumulation between cancer and normal cells open the way to the creation of more selective, less toxic anticancer metallodrugs by conjugating cytotoxic metal-based complexes such as ruthenium(II) arene derivatives to guanidinoglycosides