Particulate Trace Metals Research Articles

Overview and Research Progresses in Chemical Speciation and In Vitro Bioaccessibility Analyses of Airborne Particulate Trace Metals

Overview and Research Progresses in Chemical Speciation and In Vitro Bioaccessibility Analyses of Airborne Particulate Trace Metals

Mechanisms and heterogeneity of in situ mineral processing by the marine nitrogen fixer Trichodesmium revealed by single-colony metaproteomics

The keystone marine nitrogen fixer Trichodesmium thrives in high-dust environments. While laboratory investigations have observed that Trichodesmium colonies can access the essential nutrient iron from dust particles, less clear are the biochemical strategies underlying particle–colony interactions in nature. Here we demonstrate that Trichodesmium colonies engage with mineral particles in the wild with distinct molecular responses. We encountered particle-laden Trichodesmium colonies at a sampling location in the Southern Caribbean Sea; microscopy and synchrotron-based imaging then demonstrated heterogeneous associations with iron oxide and iron-silicate minerals. Metaproteomic analysis of individual colonies by a new low-biomass approach revealed responses in biogeochemically relevant proteins including photosynthesis proteins and metalloproteins containing iron, nickel, copper, and zinc. The iron-storage protein ferritin was particularly enriched implying accumulation of mineral-derived iron, and multiple iron acquisition pathways including Fe(II), Fe(III), and Fe-siderophore transporters were engaged. While the particles provided key trace metals such as iron and nickel, there was also evidence that Trichodesmium was altering its strategy to confront increased superoxide production and metal exposure. Chemotaxis regulators also responded to mineral presence suggesting involvement in particle entrainment. These molecular responses are fundamental to Trichodesmium’s ecological success and global biogeochemical impact, and may contribute to the leaching of particulate trace metals with implications for global iron and carbon cycling.

Contamination level and ecological risk assessment of particulate trace metals in Southwestern Mediterranean Sea

In an effort to assess the coastal water quality in an Algerian Bay, suspended particulate matter (SPM) was analyzed for trace metals and various environmental parameters in Bou Ismail Bay. The trace metal concentrations analyzed by absorption atomic spectrometry followed the decreasing order Al > Fe > Zn > Mn > Ni > Cu > Pb > Co > Cr > Cd > Hg > As (p<0.001). To assess risk, a multiple risk assessment approach was used. The enrichment of particulate metals followed a descending order of Cd > Zn > Ni > Fe > Cu > Co > Hg > Pb > Cr > As. Cadmium and nickel, may enter the bay from anthropogenic non-point sources, had the highest potential ecological risk and biological effect. Nickel contributes 46% to the potential acute toxicity (ΣTUi) of the SPM. The combined metals Pb, Cd, Cu, Zn, Cr, Ni, Hg and As had 25%–30% probability of biotoxicity but did not indicate a likelihood of any significant toxic effects based on results from previous amphipod toxicity tests. Generally, the SPM was categorized as being slightly toxic (ΣTUi<4).

Household solid waste combustion with wood increases particulate trace metal and lung deposited surface area emissions

In households, municipal solid waste (MSW) is often burned along with wood to get rid of waste, to help in ignition or simply to reduce fuel costs. The aim of this study was to characterize the influence of household waste combustion, along with wood, on the physical and chemical properties of particulate emissions in a flue gas of a masonry heater.The MSW burning alongside wood increased average particulate matter (PM) mass (65%), lung deposited surface areas (LDSA, 15%), black carbon (BC, 65%) concentrations and the average particle size in the flue gas. The influence of MSW was smaller during ignition and burning phases, but especially during fuel additions, the mass, number, and LDSA concentrations increased significantly and their size distributions moved towards larger particles. For wood burning the trace metal emissions were relatively low, but significant increase (3.3–179 -fold increase over cycle) was seen when MSW was burned along the wood. High ratios were observed especially during fuel addition phases but, depending on compounds, also during ignition and burning end phases. The highest ratios were observed for chloride compounds (HCl, KCl, NaCl). The observed increase in light-absorbing particle, trace metal and BC concentrations in flue gas when adding wood with MSW are likely to have negative impacts on air quality, visibility, human health and climate. Furthermore, metals may also affect the condition and lifetime of the burning device due to corrosion.

HVAC filtration of particles and trace metals: Airborne measurements and the evaluation of quantitative filter forensics.

Filters installed in the heating, ventilation, and air-conditioning (HVAC) systems can serve as air-cleaning and sampling devices for indoor particles. The purpose of this article is to evaluate these dual roles. An occupied home with a central HVAC system equipped with a Minimum Efficiency Reporting Value (MERV, from ASHRAE Standard 52.2) 11 filter was monitored for six weeks. Weekly airborne gravimetric and real-time sampling was performed to measure the particle size distribution and the concentration of total suspended particles (TSP), PM10, PM2.5, PM1, and 12 trace metals. The weekly system runtimes were intentionally changed to provide a wide range of weekly filtration volumes. The quantitative filter forensics (QFF) concentrations of particulate matter (PM) and trace metals were calculated using the analysis of the dust collected on the HVAC filter, the filtration volume, and filter in-situ efficiency. The results indicated that filtration was not influential to remove PM and trace metals as the concentrations during the weeks with continuous HVAC operation were not consistently lower than those during the other weeks. This suggests the dominance of other particle and trace metal source and loss mechanisms weakens the influence of filtration in this home. The QFF evaluation results indicated that the concentration of TSP and over half of the tested trace metals (e.g., Pb, Cd, Ni, V, Sb, K, and Sr) could be estimated by QFF within a factor of two when compared to airborne sampling results. PM10, PM2.5, and PM1 concentrations were significantly underestimated by QFF potentially due to the limitations of size distribution analysis by a laser diffraction particle sizer (LDPS) for the detection of <1μm particles. Overall, while QFF was promising for TSP and some trace metals, improvement in size distribution analysis could extend the application of QFF for airborne sampling.

Potential Sources of Particulate Iron in Surface and Deep Waters of the Terra Nova Bay (Ross Sea, Antarctica)

The distribution of particulate Fe (pFe), suspended particulate matter (SPM), and other particulate trace metals were investigated in Terra Nova Bay as part of CDW Effects on glaciaL mElting and on Bulk of Fe in the Western Ross sea (CELEBeR) and Plankton biodiversity and functioning of the Ross Sea ecosystems in a changing Southern Ocean (P-ROSE) projects. Variable concentrations of SPM (0.09–97 mg L−1), pFe (0.51–8.70 nM) and other trace metals were found in the Antarctic Surface waters (AASW) layer, where the addition of meltwater contributed to the pool with both lithogenic and biogenic forms. The deeper layer of the water column was occupied by High Salinity Shelf Water (HSSW) and Terra Nova Bay Ice Shelf Water (TISW) encompassing glacial water as confirmed by the lightest δ18O measured values. The concentration of pFe in TISW (11.7 ± 9.2 nM) was higher than in HSSW samples (5.55 ± 4.43 nM), suggesting that the drainage of material released from glaciers surrounding the area is relevant in terms of pFe contribution. Particulate Fe/Al and Mn/Al ratios were substantially in excess compared with the mean crustal ratios. Microscopic analyses confirmed that more labile Fe oxyhydroxides and authigenic MnO2 phases were present together with biogenic sinking material. Future expected increasing melt rates of these glaciers enlarge Fe input, thus having a greater role in supplying iron and counteracting the reductions in sea ice cover around Terra Nova Bay.

Large global variations in measured airborne metal concentrations driven by anthropogenic sources

Globally consistent measurements of airborne metal concentrations in fine particulate matter (PM2.5) are important for understanding potential health impacts, prioritizing air pollution mitigation strategies, and enabling global chemical transport model development. PM2.5 filter samples (N ~ 800 from 19 locations) collected from a globally distributed surface particulate matter sampling network (SPARTAN) between January 2013 and April 2019 were analyzed for particulate mass and trace metals content. Metal concentrations exhibited pronounced spatial variation, primarily driven by anthropogenic activities. PM2.5 levels of lead, arsenic, chromium, and zinc were significantly enriched at some locations by factors of 100–3000 compared to crustal concentrations. Levels of metals in PM2.5 and PM10 exceeded health guidelines at multiple sites. For example, Dhaka and Kanpur sites exceeded the US National Ambient Air 3-month Quality Standard for lead (150 ng m−3). Kanpur, Hanoi, Beijing and Dhaka sites had annual mean arsenic concentrations that approached or exceeded the World Health Organization’s risk level for arsenic (6.6 ng m−3). The high concentrations of several potentially harmful metals in densely populated cites worldwide motivates expanded measurements and analyses.

Geochemical factors affecting the solubility of copper in seawater

Environmental contextMany trace metals, including copper, are only sparingly soluble in seawater and may exist in both dissolved and particulate forms (e.g. as precipitates). Aquatic organisms may experience different toxic effects from exposure to dissolved and particulate trace metals. This study investigates how concentration, reaction time and changes to precipitate composition/mineral formation affect copper solubility in seawater, thus influencing metal bioavailability and toxicity in the field and laboratory. AbstractA lack of knowledge on the solubility of metals such as copper affects the ability to predict the forms (dissolved and particulate) that organisms are exposed to in field and laboratory waters. Laboratory tests were conducted where copper (total concentrations of 0.5 to 20mg L−1) was added to natural and artificial seawater (pH 8.15, 22°C), equilibrated for 28 days and dissolved copper monitored at periodic intervals. At 0.5mg L−1, dissolved copper concentrations remained stable over 28 days and no precipitates were detected. However, at higher total copper concentrations, an initial rapid precipitation phase was followed by the establishment of a metastable equilibrium that persisted for periods of days to weeks, and whose solubility concentrations and duration were influenced by the total copper concentration and typically in the range 0.6 to 0.9mg L−1. After 5 to 15 days, a step change decrease in dissolved copper concentration followed by a slow decline was observed in the &amp;gt;2mg L−1 total copper treatments. The minimum solubility measured after 28 days was 0.053mg L−1. Elemental and X-ray diffraction analyses indicated that the copper precipitates comprised similar proportions of amorphous copper hydroxycarbonate and amorphous dicopper trihydroxide chloride after 1 day and transformed to predominantly mineralised dicopper trihydroxide chloride in the clinoatacamite polymorph form after 28 days. These observations have particular relevance for toxicity tests of less sensitive organisms and highlight the need to consider metal solubility, exposure to precipitates and changes in precipitate mineral phases.

Characterization of occupational smoke exposure among wildland firefighters in the midwestern United States

Wildland firefighters are repeatedly exposed to elevated levels of wildland fire smoke (WFS) while protecting lives and properties from wildland fires. Studies reporting personal exposure concentrations of air pollutants in WFS during fire suppression or prescribed burn activities have been geographically limited to the western and southeastern United States. The objective of this study is to characterize exposure concentrations of air pollutants in WFS emissions among wildland firefighters who conducted prescribed burns in the Midwest. Between 2016 and 2019, a total of 35 firefighters (31 males and 4 females, age of 35.63 ± 9.31 years) were recruited to participate in this study. Personal particulate matter 2.5 (PM2.5) and carbon monoxide (CO) exposure concentrations were measured during prescribed burns. The level of black carbon (BC) in WFS particulates was determined using the light transmission technique, while trace metal composition was analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The results showed geometric means for PM2.5, CO, and BC concentrations were 1.43 ± 0.13 mg/m3, 7.02 ± 0.69 ppm, and 58.79 ± 5.46 μg/m3, respectively. Although no occupational exposure limits (OELs) were exceeded by 8-h time-weighted average (TWA) exposure concentration observed in the firefighters, a total of 28 personal CO exposure concentrations were above the National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL) Ceiling (200 ppm) for CO. PM2.5 and CO concentrations were about 2–7 times higher in the Midwest than the other regions. Firefighters who performed holding had higher CO exposure concentrations compared to firefighters who performed lighting (p < 0.01), while lighters were exposed to higher level of BC in the smoke particulates (p < 0.01), possibly due to the domination of exposure by different combustion sources and stages. The levels of trace metals in WFS particulates were well below the corresponding OELs and no task-related difference was observed except for manganese. Our results suggest that wildland firefighters in the midwestern region have higher WFS exposures while working at prescribed burns compared to those western and southeastern United States.

Investigation on trace metal speciation and distribution in the Scheldt estuary

The biogeochemical behavior of Cd, Co, Cr, Cu, Ni and Pb along the historically polluted Scheldt estuary (Belgium - The Netherlands) was investigated in this study. As never studied before in this area, labile trace metals were measured using the passive sampling technique of Diffusive Gradients in Thin-films (DGT), while total dissolved and particulate trace metal concentrations were assessed using classic active sampling techniques. This dual approach allowed us to highlight the variations of trace metal speciation and distribution in the estuarine surface waters, considering environmental and physicochemical gradients along the transect. The large data set obtained was then compared with literature data of historical measurements along the Scheldt (from 1980 until now), but also from other estuaries. As emphasized by our results, trace metal mobility and partitioning along the Scheldt estuary was mainly driven by biogeochemical reactions which were strongly influenced by gradients of specific estuarine physico-chemical parameters, such as salinity, turbidity, temperature and so on. Hence, all species of trace metals displayed a non-conservative behavior. More precisely, dissolved labile fractions of trace metals showed higher levels in the middle estuary, where many solubilization and remobilization processes occurred due to turbulent mixing mechanisms and an increasing salinity. Our study confirmed the decreasing trend historically observed for particulate metals along the Scheldt, as well as the rising concentrations recorded for dissolved trace metals which might also lead to an increase of their labile fraction measured by the DGT. Finally, these preliminary results suggested that a more regular monitoring of labile metal along the Scheldt estuary is essential to have an in-depth understanding of trace metal speciation and to review bioavailability of trace metals within estuarine ecosystems.

Particulate Trace Metals in Arctic Snow, Sea Ice, and Underlying Surface Waters during the 2015 US Western Arctic GEOTRACES Cruise GN01

Sea ice, an intrinsic component of the Arctic system, is undergoing rapid change. Sea ice dynamics play a role in the distributions of trace metals in Arctic waters through the incorporation, trans...

Particulate trace metal fluxes in the center of an oceanic desert: Northeast Atlantic subtropical gyre

For this study particle flux material collected over 13 years period starting in February 2002 until summer 2015 with the long-term mooring Kiel 276 (33°N, 22°W) in the subtropical Northeast Atlantic was analyzed. Our focus was set on the elemental composition and trace element fluxes which we analyzed with the help of ICP-OES and ICP-MS. The particle flux has a mean of 60 mg m −2 d −1 (range: 1–365 mg m −2 d −1 ) with peak fluxes during February and April shortly after the winter bloom. Especially in 2004 high fluxes occurred through the year when the Azores Front was located south of the mooring. In this year enhanced nutrient supply and high dust concentration in the atmosphere were measured. The analyzed trace elements of Cd, Co, Cr, Cu, Ni, V, Zn, Zr, and Mn have mean fluxes of 43.4 μg Cd m −2 d −1 , 205.7 μg Co m −2 d −1 , 1.4 μg Cr m −2 d −1 , 3.2 μg Cu m −2 d −1 , 1.6 μg Ni m −2 d −1 , 1.0 μg V m −2 d −1 , 5.1 μg Zn m −2 d −1 , 1.3 μg Zr m −2 d −1 , and 7.8 μg Mn m −2 d −1 . Biogenic trace elements (Cd, Zn, Ni) show maximum fluxes in winter and spring while no seasonality was present for refractory (Zr) and redox-sensitive elements (V, Mn, Co). Fe, Ti, Co, Mn, V, Mn, and Zr are only little enriched towards the upper continental crust (EF < 2) as these elements originate from a lithogenic source and are little prone to dissolution or scavenging while Cd, Cu, Cr, Ni, and Zn are more enriched with EF of 3.0, 9.6, 2.4, 6.2, and 5.0, respectively. • 13 years long particle flux time series from the subtropical NE Atlantic • Atmospheric pathways and Azores Front modulate particle and trace elemental fluxes. • Deep ocean particulate trace elemental fluxes depend on the biogenic matter cycle.

Health risk-oriented source apportionment of PM2.5-associated trace metals

In health-oriented air pollution control, it is vital to rank the contributions of different emission sources to the health risks posed by hazardous components in airborne fine particulate matters (PM2.5), such as trace metals. Towards this end, we investigated the PM2.5-associated metals in two densely populated regions of China, the Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions, across land-use gradients. Using the positive matrix factorization (PMF) model, we performed an integrated source apportionment to quantify the contributions of the major source categories underlying metal-induced health risks with information on the bioaccessibility (using simulated lung fluid) and speciation (using synchrotron-based techniques) of metals. The results showed that the particulate trace metal profiles reflected the land-use gradient within each region, with the highest concentrations of anthropogenically enriched metals at the industrial sites in the study regions. The resulting carcinogenic risk that these elements posed was higher in the YRD than in the PRD. Chromium was the dominant contributor to the total excessive cancer risks posed by metals while manganese accounted for a large proportion of non-carcinogenic risks. An elevated contribution from industrial emissions was found in the YRD, while traffic emissions and non-traffic combustion (the burning of coal/waste/biomass) were the common dominant sources of cancer and non-cancer risks posed by metals in both regions. Moreover, the risk-oriented source apportionment of metals did not mirror the mass concentration-based one, suggesting the insufficiency of the latter to inform emission mitigation in favor of public health. While providing region-specific insights into the quantitative contribution of major source categories to the health risks of PM2.5-associated trace metals, our study highlighted the need to consider the health protection goal-based source apportionment and emission mitigation in supplement to the current mass concentration-based framework.

Particulate trace metal dynamics in response to increased CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and iron availability in a coastal mesocosm experiment

Abstract. Rising concentrations of atmospheric carbon dioxide are causing ocean acidification and will influence marine processes and trace metal biogeochemistry. In June 2012, in the Raunefjord (Bergen, Norway), we performed a mesocosm experiment, comprised of a fully factorial design of ambient and elevated pCO2 and/or an addition of the siderophore desferrioxamine B (DFB). In addition, the macronutrient concentrations were manipulated to enhance a bloom of the coccolithophore Emiliania huxleyi. We report the changes in particulate trace metal concentrations during this experiment. Our results show that particulate Ti and Fe were dominated by lithogenic material, while particulate Cu, Co, Mn, Zn, Mo and Cd had a strong biogenic component. Furthermore, significant correlations were found between particulate concentrations of Cu, Co, Zn, Cd, Mn, Mo and P in seawater and phytoplankton biomass (µgC L−1), supporting a significant influence of the bloom in the distribution of these particulate elements. The concentrations of these biogenic metals in the E. huxleyi bloom were ranked as follows: Zn &lt; Cu ≈ Mn &lt; Mo &lt; Co &lt; Cd. Changes in CO2 affected total particulate concentrations and biogenic metal ratios (Me : P) for some metals, while the addition of DFB only significantly affected the concentrations of some particulate metals (mol L−1). Variations in CO2 had the most clear and significant effect on particulate Fe concentrations, decreasing its concentration under high CO2. Indeed, high CO2 and/or DFB promoted the dissolution of particulate Fe, and the presence of this siderophore helped in maintaining high dissolved Fe. This shift between particulate and dissolved Fe concentrations in the presence of DFB, promoted a massive bloom of E. huxleyi in the treatments with ambient CO2. Furthermore, high CO2 decreased the Me : P ratios of Co, Zn and Mn while increasing the Cu : P ratios. These findings support theoretical predictions that the molar ratios of metal to phosphorous (Me : P ratios) of metals whose seawater dissolved speciation is dominated by free ions (e.g., Co, Zn and Mn) will likely decrease or stay constant under ocean acidification. In contrast, high CO2 is predicted to shift the speciation of dissolved metals associated with carbonates such as Cu, increasing their bioavailability and resulting in higher Me : P ratios.

Trace Metals in Fine and Respirable Ambient Air Particulates on Trinidad’s West Coast

The paper analyzed the concentrations of trace metals in fine and respirable particulates (fine-PM1 and PM2.5; respirable-PM10) to determine baseline concentrations in the ambient air and the factors impacting its distribution such as land use and time of year when levels may be concerning to public health. Measurements of particulates along with meteorological parameters were made at four sites over the heavily populated west coast of Trinidad (10&#17632'N, 61&#17618'W) during March ’15-May ’16, representing rural, urban, mixed background and industrial land uses. The study found mean levels of trace metals to be highest at the industrial and urban stations. Public health exceedances (referenced to the Canadian AAQ public health standards (Ontario-MoE, 2012)) were measured for beryllium, cadmium, chromium, iron, manganese and nickel (in PM10). Iron, manganese and nickel, most associated with particulates at the industrial station, were in frequent exceedance. Beryllium—concentrated in coarse PM (PM2.5-10) with only a single measured exceedance at the mixed background station likely poses minimal threat to the health of the nearby population. Cadmium—concentrated in fine PM which peaked once only at the rural station was likely due to an irregular event within a narrow timeframe during the time of sampling. Iron and manganese were frequently above the Canadian public health threshold, but predominated in the coarse PM fraction, suggesting localised sources. Nickel, concentrated in the fine PM fraction, was frequently in exceedance particularly at the industrial station. Cadmium and nickel are genotoxic and should be regulated in order to reduce the burden of toxic carcinogens to which the population can be exposed.

Trace metal geochemistry of remote rivers in the Canadian Arctic Archipelago

Trace metals are essential micronutrients for phytoplankton, as well as useful tracers of biogeochemical processes in aquatic systems. The largest Arctic rivers are known to play an important role in the cycling of trace metals in the Arctic Ocean; however, these systems account for little more than half of Arctic freshwater discharge, leaving a major gap in our understanding of the trace metal geochemistry from smaller river systems. This is especially problematic for rivers draining the Arctic coastal margins, as these continuous permafrost systems are anticipated to be particularly sensitive to climate change. This study presents the first observations of dissolved and particulate trace metal concentrations and lead isotopes in 14 rivers in the Canadian Arctic Archipelago (CAA). Dissolved concentrations of aluminum, iron, manganese, nickel, copper, zinc, cadmium, and lead display clear spatial variability in their distributions. Small rivers located in the central islands of the CAA have remarkably low concentrations of these trace metals, while the rivers draining the southern continental landmass have higher values. This geographic variability in dissolved metal concentrations reflects both differences in bedrock geology and the South-North environmental gradients in the Archipelago. We extrapolate these findings to estimate the dissolved trace metal concentrations for other rivers draining the CAA based on their geographic location. Elemental ratios measured in the suspended particulate matter in several rivers indicates a significant enrichment of nickel, copper, zinc, and lead over the crustal composition. In particular, lead isotope composition shows an extremely large range, varying from the highly radiogenic Tree River (206Pb/207Pb: 1.5121) in the south, to the relatively unradiogenic Glacier River (206Pb/207Pb: 1.0153) in the central CAA. Lack of correlation between the Pb isotopic composition and Pb enrichment, suggests Pb is derived from within the watershed. This work presents the first observations of dissolved and particulate trace metal concentrations in small coastal draining rivers from the CAA, an important first step in developing a baseline against which to evaluate future change in the high Arctic.

Trace metal speciation in North Sea coastal waters.

Most trace metals exhibit a dual role in marine waters, acting as nutrients at low concentration and being toxic at high concentration. But besides concentration range, speciation is also an important factor. They both show both seasonal and spatial variations. A thorough comparison between total dissolved and particulate concentrations estimated from manual sampling and an assessment of the bioavailability using Diffusive Gradients in Thin Films (DGT) has been performed in this work for Cd, Co, Cu, Ni and Pb, at several sampling points of the Belgian Coastal Zone (BCZ). Additional information to trace back the origin and identify the anthropogenic fingerprint of Suspended Particulate Matter (SPM) was measured using stable carbon isotope measurements in particulate organic matter. Our results show that: (i) particulate and total dissolved metal concentrations are higher at two stations, one in the harbor of Oostende and one offshore; (ii) dissolved and particulate trace metal concentrations do not correlate with the dissolved labile fractions; and (iii) SPM in the harbor zone is likely from allochthonous sources, while in the offshore station marine origin has been evidenced. Our results indicate that, even though contamination is higher in the harbor zones, the trace metal toxicity, which is linked to the metal bioavailability, is most likely not higher than in the open sea. However, with increasing acidification of the ocean, a shift from particulate to dissolved phase might lead to increasing adverse effects on the coastal environment.

Big Data Help Paint a New Picture of Trace Element Cycling

A new framework for understanding the suite of processes acting on marine particulate trace metals exemplifies how alternative analyses can maximize the information that large data sets provide.

A nutrient limitation mosaic in the eastern tropical Indian Ocean

The Indian Ocean accounts for about one fifth of global ocean net primary production but remains undersampled relative to other major ocean basins. The eastern tropical Indian Ocean is characterized by extremely low concentrations of both macronutrients and the micronutrient iron. We measured concentrations of dissolved and particulate trace metals (Fe, Mn, Zn, Pb) in the upper ocean along the GO-SHIP IO9N transect (28˚S to 17˚N, mostly along the 95˚E meridian) during a cruise in April 2016. Cellular quotas (metal/C) of Fe, Mn, Co, Ni, Cu, and Zn were measured in small eukaryotic flagellates (2–7 μm). Deckboard bottle incubation experiments were conducted at one station in each of three putative biogeochemical regions: southern Indian Ocean gyre (SIO, 28-10˚S); equatorial Indian Ocean (EqIO, 10˚S - 5˚N); Bay of Bengal (BoB, 5-17˚N). Nitrate and phosphate were below detection limits in surface waters across the transect. Dissolved and particulate Fe were <0.2 nM south of 10˚N and lowest in the EqIO. Cellular Fe/C quotas were approximately 6 μmoL/mol and did not vary along the transect, nor did cellular Mn/C or Co/C quotas. Cellular Ni/C and Zn/C quotas were significantly higher at the southern terminus. Nutrient addition experiments indicated that N was the primary limiting nutrient for autotrophs using chlorophyll a as a proxy, but biomass measurements of specific phytoplankton groups pointed to a more complex nutrient limitation mosaic. Prochlorococcus was limited by N in the EqIO but by multiple nutrients (N, P, and/or Fe) in the BoB. Synechococcus was limited by N in the EqIO and BoB, while small (<20 μm) eukaryotic phytoplankton were limited by N in the EqIO and by multiple nutrients in the BoB. Stoichiometric comparisons of cells and underlying source waters indicate a gradient of N and Fe stress along the transect. These data demonstrate that autotroph communities are poised near multiple nutrient limitation horizons in extremely oligotrophic waters far from micronutrient sources.

Iron isotope transformations in the meromictic Lake Cadagno

Abstract The iron isotope composition of sedimentary deposits is a key tool for tracking changes in the biogeochemical and redox conditions of modern and geologically ancient aquatic systems. The use of iron isotopes to reconstruct oxic, anoxic and redox conditions is based on iron isotope fractionation associated with the iron(II)–iron(III) redox reaction and the formation of various iron oxy, hydroxy and sulfur species. However, the degree of iron isotope fractionation varies within the sedimentary record, and the processes leading to isotope fractionation within aquatic systems also vary. Here we investigate iron isotope fractionation within the water column of Lake Cadagno, Switzerland. Lake Cadagno is a 21 m deep alpine meromictic lake that is permanently stratified. It has two chemically distinct water layers: an oxic mixolimnion separated by a narrow chemocline from an anoxic monimolimnion. The chemocline is located across a narrow band between 10 and 13.5 m and is defined by steep chemical gradients in dissolved oxygen, redox potential, nutrients (nitrite + nitrate, ammonia), dissolved and particulate trace metals (iron and manganese) and sulfur species. Iron isotope determination (56Fe/54Fe ratio; expressed as δ56Fe) of both dissolved (δ56Fedissolved) and particulate (δ56Feparticulate) iron reveals a sharp transition within the chemocline with a heavy δ56Fedissolved value (+0.75‰) at 11.5 m and light δ56Fedissolved value (−0.61‰) below at 12 m. The large shift in δ56Fedissolved occurs where anoxygenic phototrophic bacteria become abundant. Modelling of the dissolved iron isotope fractionation within the chemocline produced fractionation factors of −0.60‰ (kinetic model) and −1.52‰ (equilibrium model) assuming a two-step process involving iron oxidation followed by precipitation. Changes in the isotope composition of particulate iron with depth are subtler to that of dissolved iron, with slightly lighter values (−0.18‰) above the chemocline and slightly heavier values (+0.13‰) below. The production of reduced iron(II) within and below the chemocline is likely coupled to dissimilatory iron(III) reduction and dissolution reactions associated with sinking iron(III) oxy and hydroxy species. These processes lead to a peak in dissolved iron concentrations at 12.5 m. The decline in dissolved iron concentration below this depth and its change in isotope composition is consistent with the formation of iron sulphide species, e.g. mackinawite, under mildly euxinic conditions. Overall our results indicate that variations in δ56Fedissolved likely involve a combination of biotic and abiotic processes with biological mixing enhancing the rate at which iron(II) is transferred across the chemocline. The observed isotope transformation of dissolved iron across the chemocline of Lake Cadagno may make it a reasonable analogue to past shallow-water systems with mild euxinia.