The Interplay of Phototrophic and Heterotrophic Microbes Under Oil Exposure: A Microcosm Study

Microbial interactions influence nearly one-half of the global biogeochemical flux of major elements of the marine ecosystem. Despite their ecological importance, microbial interactions remain poorly understood and even less is known regarding the effects of anthropogenic perturbations on these microbial interactions. The Deepwater Horizon oil spill exposed the Gulf of Mexico to ∼4.9 million barrels of crude oil over 87 days. We determined the effects of oil exposure on microbial interactions using short- and long-term microcosm experiments with and without Macondo surrogate oil. Microbial activity determined using radiotracers revealed that oil exposure negatively affected substrate uptake by prokaryotes within 8 h and by eukaryotes over 72 h. Eukaryotic uptake of heterotrophic exopolymeric substances (EPS) was more severely affected than prokaryotic uptake of phototrophic EPS. In addition, our long-term exposure study showed severe effects on photosynthetic activity. Lastly, changes in microbial relative abundances and fewer co-occurrences among microbial species were mostly driven by photosynthetic activity, treatment (control vs. oil), and prokaryotic heterotrophic metabolism. Overall, oil exposure affected microbial co-occurrence and/or interactions possibly by direct reduction in abundance of one of the interacting community members and/or indirect by reduction in metabolism (substrate uptake or photosynthesis) of interacting members.

The interplay of extracellular polymeric substances and oil/Corexit to affect the petroleum incorporation into sinking marine oil snow in four mesocosms

Large amounts of oil containing mucous-like marine snow formed in surface waters adjacent to the Deepwater Horizon spill that was implicated in oil delivery to the seafloor. However, whether chemical dispersants that were used increased or decreased the oil incorporation and sedimentation efficiency, and how exopolymeric substances (EPS) are involved in this process remains unresolved. To investigate the microbial responses to oil and dispersants in different oceanic settings, indicated by EPS production, petro- and non-petro carbon sedimentation, four mesocosm (M) experiments were conducted: 1) nearshore seawater with a natural microbial consortia (M2); 2) offshore seawater with f/20 nutrients (M3); 3) coastal seawater with f/20 nutrients (M4); 4) nearshore seawater with a natural microbial consortia for a longer duration (M5). Four treatments were conducted in M2, M3 and M4 whereas only three in M5: 1) a water accommodated fraction of oil (WAF), 2) a chemically-enhanced WAF prepared with Corexit (CEWAF, not in M5), 3) a 10-fold diluted CEWAF (DCEWAF); and 4) controls. Overall, oil and dispersants input, nutrient and microbial biomass addition enhanced EPS production. Dispersant addition tended to induce the production of EPS with higher protein/carbohydrate (P/C) ratios, irrespective of oceanic regions. EPS produced in M4 was generally more hydrophobic than that produced in M3. The P/C ratio of EPS in both the aggregate and the colloidal fraction was a key factor that regulated oil contribution to sinking aggregates, based on the close correlation with %petro-carbon in these fractions. In the short term (4-5 days), both the petro and non-petro carbon sedimentation efficiencies showed decreasing trends when oil/dispersants were present. In comparison, in the longer-term (16 days), petro-carbon sedimentation efficiency was less influenced by dispersants, possibly due to biological and physicochemical changes of the components of the oil-EPS-mineral phase system, which cooperatively controlled the sinking velocities of the aggregates.

Molecular Interaction of Aqueous Iodine Species with Humic Acid Studied by I and C K-Edge X-ray Absorption Spectroscopy

Iodine-129 is one of three key risk drivers at several US Department of Energy waste management sites. Natural organic matter (NOM) is thought to play important roles in the immobilization of aqueous iodide (I^-) and iodate (IO₃^-) in the environment, but molecular interactions between NOM and iodine species are poorly understood. In this work, we investigated iodine and carbon speciation in three humic acid (HA)-I systems using I K-edge XANES and EXAFS and C K-edge XANES spectroscopy: (1) I^- in the presence of laccase (an oxidase enzyme) and a mediator, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in a pH 4 buffer, (2) I^- in the presence of lactoperoxidase (LPO) and H₂O₂ in a pH 7 buffer, and (3) IO₃^- in a pH 3 groundwater. Both oxidase and peroxidase systems could oxidize I^- to I₂ or hypoiodide (HOI) leading to organo-I formation. However, the laccase-ABTS mediator was the most effective and enhanced I^- uptake by HA up to 13.5 mg/g, compared to 1.9 mg/g for the LPO-H₂O₂. IO₃^- was abiotically reduced to I₂ or HOI leading to an organo-I formation. Pathways for HA iodination include covalent modification of aromatic-type rings by I₂ / HOI or iodine incorporation into newly formed benzoquinone species arising from the oxidation of phenolic C species. This study improves our molecular-level understanding of NOM-iodine interactions and stresses the important role that mediators may play in the enzymatic reactions between iodine and NOM.

Nagasaki sediments reveal that long-term fate of plutonium is controlled by select organic matter moieties

Forecasting the long-term fate of plutonium (Pu) is becoming increasingly important as more worldwide military and nuclear-power waste is being generated. Nagasaki sediments containing bomb-derived Pu that was deposited in 1945 provided a unique opportunity to explore the long-term geochemical behavior of Pu. Through a combination of selective extractions and molecular characterization via electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS), we determined that 55 ± 3% of the bomb-derived ^239,240Pu was preferentially associated with more persistent organic matter compounds in Nagasaki sediments, particularly those natural organic matter (NOM) stabilized by Fe oxides (NOM_Fe-oxide). Other organic matter compounds served as a secondary sink of these bomb-derived ^239,240Pu (31 ± 2% on average), and <20% of the ^239,240Pu was immobilized by inorganic mineral particles. In a narrow, ^239,240Pu-enriched layer of only 9-cm depth (total core depth was 600 cm), N-containing carboxyl aliphatic and/or alicyclic molecules (CCAM) in NOM_Fe-oxide and other NOM fractions immobilized the majority of ^239,240Pu. Among the cluster of N-containing CCAM moieties, hydroxamate siderophores, the strongest known Pu chelators in nature, were further detected in these "aged" Nagasaki bomb residue-containing sediments. While present long-term disposal and environmental remediation modeling assume that solubility limits and sorption to mineral surfaces control Pu subsurface mobility, our observations suggest that NOM, which is present in essentially all subsurface systems, undoubtedly plays an important role in sequestrering Pu. Ignoring the role of NOM in controlling Pu fate and transport is not justified in most environmental systems.

Rapid Degradation of Oil in Mesocosm Simulations of Marine Oil Snow Events

Following the Deepwater Horizon oil spill in the Gulf of Mexico, natural marine snow interacted with oil and dispersants forming marine oil snow (MOS) that sank from the water column to sediments. Mesocosm simulations demonstrate that Macondo surrogate oil incorporates into MOS and can be isolated, extracted, and analyzed via Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Up to 47% of the FTICR-MS signal from MOS extracts can be attributed to formulas also found in Macondo surrogate oil demonstrating extensive oil incorporation. Additionally, oxygenation patterns for MOS extracts provide evidence for degraded oil compounds. Formulas having similar double bond equivalents but higher oxygen content (MOS CHO: CHO_2-9, DBE_2-16, MOS CHON: CHO_0-7N₁, DBE_9-18; Macondo CHO: CHO_1-4, DBE_2-15, CHON: CHO_0-3N₁, DBE_9-21) were found in MOS extracts generating isoabundance distributions similar to those of environmentally aged oil. Such shifts in molecular composition are consistent with the transformation of high DBE oil components, unobservable by FTICR-MS until oxygenation in the mesocosms. Low light conditions and the rapid proliferation of hydrocarbon-degraders observed in parallel studies suggest biological activity as the primary cause of oil degradation. MOS may thus represent an important microenvironment for oil degradation especially during its long transit below the euphotic zone to sediments.

Sunlight induced aggregation of dissolved organic matter: Role of proteins in linking organic carbon and nitrogen cycling in seawater

Organic matter export from the euphotic zone is a key component of oceanic carbon (C) and nitrogen (N) cycles. Although interactions between these two cycles are important, studies on geochemical processes to directly connect them are limited. Here we show that sunlight can induce chemical aggregation of dissolved organic matter (DOM) into high N containing photo-aggregates. The size of microgels in natural coastal seawaters increased by18~25% compared to corresponding dark controls. Within a relatively short time (1 h), the C and N sequestered into the photo-aggregates accounted for 10% and 13% of the bulk particulate C and N, respectively. The N/C ratio of the photo-aggregates was two times higher after sunlight irradiation. Furthermore, we show that the aggregation process was dependent on reactive oxygen species (ROS). To accommodate for the different organic material in the marine environment, we monitored the particle size in various extracellular polymeric substances (EPS) and model biopolymers using flow cytometry, dynamic laser scattering, and scanning electron microscopy. We found that proteins play important roles in light-induced aggregation, which is in contrast to previous views that sunlight can break down DOM and interrupt aggregation. The photo-flocculation process involving organic N provides new insights into DOM assembly, bioavailability, and sedimentation, and thus potentially link the C and N cycles.

Extracellular polymeric substances (EPS) producing and oil degrading bacteria isolated from the northern Gulf of Mexico

Sinking marine oil snow was found to be a major mechanism in the transport of spilled oil from the surface to the deep sea following the Deepwater Horizon (DwH) oil spill. Marine snow formation is primarily facilitated by extracellular polymeric substances (EPS), which are mainly composed of proteins and carbohydrates secreted by microorganisms. While numerous bacteria have been identified to degrade oil, there is a paucity of knowledge on bacteria that produce EPS in response to oil and Corexit exposure in the northern Gulf of Mexico (nGoM). In this study, we isolated bacteria from surface water of the nGoM that grow on oil or Corexit dispersant. Among the 100 strains isolated, nine were identified to produce remarkable amounts of EPS. 16S rRNA gene analysis revealed that six isolates (strains C1, C5, W10, W11, W14, W20) belong to the genus Alteromonas; the others were related to Thalassospira (C8), Aestuariibacter (C12), and Escherichia (W13a). The isolates preferably degraded alkanes (17–77%), over polycyclic aromatic hydrocarbons (0.90–23%). The EPS production was determined in the presence of a water accommodated fraction (WAF) of oil, a chemical enhanced WAF (CEWAF), Corexit, and control. The highest production of visible aggregates was found in Corexit followed by CEWAF, WAF, and control; indicating that Corexit generally enhanced EPS production. The addition of WAF and Corexit did not affect the carbohydrate content, but significantly increased the protein content of the EPS. On the average, WAF and CEWAF treatments had nine to ten times more proteins, and Corexit had five times higher than the control. Our results reveal that Alteromonas and Thalassospira, among the commonly reported bacteria following the DwH spill, produce protein rich EPS that could have crucial roles in oil degradation and marine snow formation. This study highlights the link between EPS production and bacterial oil-degrading capacity that should not be overlooked during spilled oil clearance.

Sediment accumulation and mixing in the Penobscot River and estuary, Maine

Mercury (Hg) was discharged in the late 1960s into the Penobscot River by the Holtra-Chem chlor-alkali production facility, which was in operation from 1967 to 2000. To assess the transport and distribution of total Hg, and recovery of the river and estuary system from Hg pollution, physical and radiochemical data were assembled from sediment cores collected from 58 of 72 coring stations sampled in 2009. These stations were located throughout the lower Penobscot River, and included four principal study regions, the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR), and the Penobscot estuary (ES). To provide the geochronology required to evaluate sedimentary total Hg profiles, 58 of 72 sediment cores were dated using the atmospheric radionuclide tracers ¹³⁷Cs, ²¹⁰Pb, and ^239,240Pu. Sediment cores were assessed for depths of mixing, and for the determination of sediment accumulation rates using both geochemical (total Hg) and radiochemical data. At most stations, evidence for significant vertical mixing, derived from profiles of ⁷Be (where possible) and porosity, was restricted to the upper ~1-3cm. Thus, historic profiles of both total Hg and radionuclides were only minimally distorted, allowing a reconstruction of their depositional history. The pulse input tracers ¹³⁷Cs and ^239,240Pu used to assess sediment accumulation rates agreed well, while the steady state tracer ²¹⁰Pb exhibited weaker agreement, likely due to irregular lateral sediment inputs.

Diagnostic tool to ascertain marine phytoplankton exposure to chemically enhanced water accommodated fraction of oil using Fourier Transform Infrared spectroscopy

Phytoplankton alter their macromolecule composition in response to changing environmental conditions. Often these changes are consistent and can be used as indicators to predict their exposure to a given condition. FTIR-spectroscopy is a powerful tool that provides rapid snapshot of microbial samples. We used FTIR to develop signature macromolecular composition profiles of three cultures: Skeletonema costatum, Emiliania huxleyi, and Navicula sp., exposed to chemically enhanced water accommodated oil fraction (CEWAF) in artificial seawater and control. Using a multivariate model created with a Partial Least Square Discriminant Analysis of the FTIR-spectra, classification of CEWAF exposed versus control samples was possible. This model was validated using aggregate samples from a mesocosm study. Analysis of spectra and PCA-loadings plot showed changes to carbohydrates and proteins in response to CEWAF. Overall we developed a robust multivariate model that can be used to identify if a phytoplankton sample has been exposed to oil with dispersant.

Mercury inputs and redistribution in the Penobscot River and estuary, Maine

We examined total mercury (Hg) distributions in sediments from the Penobscot River and estuary, Maine, a site of extensive Hg releases from HoltraChem (1967-2000). Our objectives were to quantify: (1) bottom sediment Hg inventories (upper ~1m; 50-100 y); (2) sediment accumulation rates; and (3) contemporary Hg fluxes to bottom sediments; by sampling the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR) and the Penobscot estuary (ES). Hg was rapidly distributed here, and the cumulative total (9.28 metric tons) associated with sediments system-wide was within the range released (6-12 metric tons). Evidence of sediment/Hg remobilization was observed in cores primarily from the PBR, and to a lesser extent the ES, whereas cores from MM, most of the OR, the ES, and half from the PBR exhibited sharp peaks in Hg concentrations at depth, followed by gradual decreases towards the surface. Based on background PBR sediment Hg concentrations (100ngg^-1), "elevated" (300ngg^-1), or "highly elevated" (600ngg^-1) Hg concentrations in sediments, and resulting inventories, we assessed impact levels ("elevated"≥270, or "highly elevated"≥540mgm^-2). 71% of PBR stations had "elevated", and 29% had "highly elevated" Hg inventories; 45% of MM stations had "elevated", and 27% had "highly elevated" inventories; 80% of OR stations had "elevated" inventories only; and 17% of ES stations had "elevated" inventories only. Most "highly elevated" stations were located within 8km of HoltraChem, in MM, in the PBR, and in the OR. Near-surface sediments in the OR, PBR and MM were all "highly elevated", while those in the ES were "elevated", on average. Mean Hg fluxes to bottom sediments were greatest in the OR (554), followed by the PBR (469), then MM (452), and finally the ES (204ngcm^-2y^-1).

Plutonium Partitioning Behavior to Humic Acids from Widely Varying Soils Is Related to Carboxyl-Containing Organic Compounds

In order to examine the influence of the HA molecular composition on the partitioning of Pu, ten different kinds of humic acids (HAs) of contrasting chemical composition, collected and extracted from different soil types around the world were equilibrated with groundwater at low Pu concentrations (10^-14 M). Under mildly acidic conditions (pH ∼ 5.5), 29 ± 24% of the HAs were released as colloidal organic matter (>3 kDa to <0.45 μm), yet this HA fraction accounted for a vast majority of the bound Pu, 76 ± 13% on average. In comparison, the particulate HA fraction bound only 8 ± 4% on average of the added Pu. The truly dissolved Pu fraction was typically <1%. Pu binding was strongly and positively correlated with the concentrations of organic nitrogen in both particulate (>0.45 μm) and colloidal phases in terms of activity percentage and partitioning coefficient values (logK_d). Based on molecular characterization of the HAs by solid state ¹³C nuclear magnetic resonance (NMR) and elemental analysis, Pu binding was correlated to the concentration of carboxylate functionalities and nitrogen groups in the particulate and colloidal phases. The much greater tendency of Pu to bind to colloidal HAs than to particulate HA has implications on whether NOM acts as a Pu source or sink during natural or man-induced episodic flooding.

Estimates of recovery of the Penobscot River and estuarine system from mercury contamination in the 1960's

Mercury (Hg) was discharged in the late 1960s into the Penobscot River by a chlor-alkali production facility, HoltraChem. Using total Hg concentration profiles from 56 stations (58 sediment cores) in the Penobscot River (PBR), Mendall Marsh (MM), Orland River (OR) and Penobscot Estuary (ES), and sediment accumulation rates derived using detailed profiles of total Hg concentrations and radionuclide activities (¹³⁷Cs, ^239,240Pu, ²¹⁰Pb), recovery from system-wide Hg pollution was assessed. Total Hg concentration profiles showed sharp maxima at depths attributed in time to a 1967 release date, and were divided into two sections: the first 21years (1967-1988; rapid recovery), and the recent 21years (1988-2009; slower recovery). The recent 21years of Hg input were used to estimate 'apparent' recovery rates, yielding exponentially decreasing total Hg concentrations. Apparent recovery half-times (T_1/2=ln2/α) were calculated from an exponential fit of Hg(t)=Hg(t=21)∗exp(-α∗t)+Hg(∞) to total Hg concentration profiles over the past 21years (assuming Hg(∞) of 0, 100, or 400ngg^-1). Mean T_1/2 values were, at PBR 31years (16 of 24 cores), at MM 22years (9 of 11 cores), at ES 20 to 120years (mean of 78years; 12 of 18 cores), and at OR 69years (3 of 5 cores). In 18 out of 57 cores, concentrations either increased towards the surface or remained the same, indicating slower or incomplete 'communication' with the larger system. The Penobscot River and Estuary system has recovered substantially since 1967, and top 1cm sediment Hg concentrations (Hg(0)) from areas in rapid communication with the larger system are converging to 600-700ngg^-1 (1967 maxima of 70,000⁺ngg^-1). However, to recover from Hg(0) of 700ngg^-1 to a Hg(∞) of <100ngg^-1 would require 3 or more half-times.

Light-induced aggregation of microbial exopolymeric substances

Sunlight can inhibit or disrupt the aggregation process of marine colloids via cleavage of high molecular weight compounds into smaller, less stable fragments. In contrast, some biomolecules, such as proteins excreted from bacteria can form aggregates via cross-linking due to photo-oxidation. To examine whether light-induced aggregation can occur in the marine environment, we conducted irradiation experiments on a well-characterized protein-containing exopolymeric substance (EPS) from the marine bacterium Sagitulla stellata. Our results show that after 1 h sunlight irradiation, the turbidity level of soluble EPS was 60% higher than in the dark control. Flow cytometry also confirmed that more particles of larger sized were formed by sunlight. In addition, we determined a higher mass of aggregates collected on filter in the irradiated samples. This suggests light can induce aggregation of this bacterial EPS. Reactive oxygen species hydroxyl radical and peroxide played critical roles in the photo-oxidation process, and salts assisted the aggregation process. The observation that Sagitulla stellata EPS with relatively high protein content promoted aggregation, was in contrast to the case where no significant differences were found in the aggregation of a non-protein containing phytoplankton EPS between the dark and light conditions. This, together with the evidence that protein-to-carbohydrate ratio of aggregates formed under light condition is significantly higher than that formed under dark condition suggest that proteins are likely the important component for aggregate formation. Light-induced aggregation provides new insights into polymer assembly, marine snow formation, and the fate/transport of organic carbon and nitrogen in the ocean.

Recent advances in the detection of specific natural organic compounds as carriers for radionuclides in soil and water environments, with examples of radioiodine and plutonium

Among the key environmental factors influencing the fate and transport of radionuclides in the environment is natural organic matter (NOM). While this has been known for decades, there still remains great uncertainty in predicting NOM-radionuclide interactions because of lack of understanding of radionuclide interactions with the specific organic moieties within NOM. Furthermore, radionuclide-NOM studies conducted using modelled organic compounds or elevated radionuclide concentrations provide compromised information related to true environmental conditions. Thus, sensitive techniques are required not only for the detection of radionuclides, and their different species, at ambient and/or far-field concentrations, but also for potential trace organic compounds that are chemically binding these radionuclides. GC-MS and AMS techniques developed in our lab are reviewed here that aim to assess how two radionuclides, iodine and plutonium, form strong bonds with NOM by entirely different mechanisms; iodine tends to bind to aromatic functionalities, whereas plutonium binds to N-containing hydroxamate siderophores at ambient concentrations. While low-level measurements are a prerequisite for assessing iodine and plutonium migration at nuclear waste sites and as environmental tracers, it is necessary to determine their in-situ speciation, which ultimately controls their mobility and transport in natural environments. More importantly, advanced molecular-level instrumentation (e.g., nuclear magnetic resonance (NMR) and Fourier-transform ion cyclotron resonance coupled with electrospray ionization (ESI-FTICRMS) were applied to resolve either directly or indirectly the molecular environments in which the radionuclides are associated with the NOM.

Evidence for Hydroxamate Siderophores and Other N-Containing Organic Compounds Controlling (239,240)Pu Immobilization and Remobilization in a Wetland Sediment

Pu concentrations in wetland surface sediments collected downstream of a former nuclear processing facility in F-Area of the Savannah River Site (SRS), USA, were ∼2.5 times greater than those measured in the associated upland aquifer sediments; similarly, the Pu concentration solid/water ratios were orders of magnitude greater in the wetland than in the low-organic matter content aquifer soils. Sediment Pu concentrations were correlated to total organic carbon and total nitrogen contents and even more strongly to hydroxamate siderophore (HS) concentrations. The HS were detected in the particulate or colloidal phases of the sediments but not in the low molecular weight fractions (<1000 Da). Macromolecules which scavenged the majority of the potentially mobile Pu were further separated from the bulk mobile organic matter fraction ("water extract") via an isoelectric focusing experiment (IEF). An electrospray ionization Fourier-transform ion cyclotron resonance ultrahigh resolution mass spectrometry (ESI FTICR-MS) spectral comparison of the IEF extract and a siderophore standard (desferrioxamine; DFO) suggested the presence of HS functionalities in the IEF extract. This study suggests that while HS are a very minor component in the sediment particulate/colloidal fractions, their concentrations greatly exceed those of ambient Pu, and HS may play an especially important role in Pu immobilization/remobilization in wetland sediments.

Radioiodine sorption/desorption and speciation transformation by subsurface sediments from the Hanford Site

During the last few decades, considerable research efforts have been extended to identify more effective remediation treatment technologies to lower the (129)I concentrations to below federal drinking water standards at the Hanford Site (Richland, USA). Few studies have taken iodate into consideration, though recently iodate, instead of iodide, was identified as the major species in the groundwater of 200-West Area within the Hanford Site. The objective of this study was thus to quantify and understand aqueous radioiodine species transformations and uptake by three sediments collected from the semi-arid, carbonate-rich environment of the Hanford subsurface. All three sediments reduced iodate (IO3(-)) to iodide (I(-)), but the loamy-sand sediment reduced more IO3(-) (100% reduced within 7 days) than the two sand-textured sediments (∼20% reduced after 28 days). No dissolved organo-iodine species were observed in any of these studies. Iodate uptake Kd values ([Isolid]/[Iaq]; 0.8-7.6 L/kg) were consistently and appreciably greater than iodide Kd values (0-5.6 L/kg). Furthermore, desorption Kd values (11.9-29.8 L/kg) for both iodate and iodide were consistently and appreciably greater than uptake Kd values (0-7.6 L/kg). Major fractions of iodine associated with the sediments were unexpectedly strongly bound, such that only 0.4-6.6 % of the total sedimentary iodine could be exchanged from the surface with KCl solution, and 0-1.2% was associated with Fe or Mn oxides (weak NH2HCl/HNO3 extractable fraction). Iodine incorporated into calcite accounted for 2.9-39.4% of the total sedimentary iodine, whereas organic carbon (OC) is likely responsible for the residual iodine (57.1-90.6%) in sediments. The OC, even at low concentrations, appeared to be controlling iodine binding to the sediments, as it was found that the greater the OC concentrations in the sediments, the greater the values of uptake Kd, desorption Kd, and the greater residual iodine concentrations (non-exchangeable, non-calcite-incorporated and non-Mn, Fe-oxide associated). This finding is of particular interest because it suggests that even very low OC concentrations, <0.2%, may have an impact on iodine geochemistry. The findings that these sediments can readily reduce IO3(-), and that IO3(-) sorbs to a greater extent than I(-), sheds light into earlier unexplained Hanford field data that demonstrated increases in groundwater (127)I(-)/(127)IO3(-) ratios and a decrease groundwater (129)IO3(-) concentrations along a transect away from the point sources, where iodine was primarily introduced as IO3(-). While a majority of the radioiodine does not bind to these alkaline sediments, there is likely a second smaller iodine fraction in the Hanford subsurface that is strongly bound, presumably to the sediment OC (and carbonate) phases. This second fraction may have an impact on establishing remediation goals and performance assessment calculations.

Speciation of iodine isotopes inside and outside of a contaminant plume at the Savannah River Site

A primary obstacle in understanding the fate and transport of the toxic radionuclide (129)I (a thyroid seeker) is an accurate method to distinguish it from the stable isotope, (127)I, and to quantify the various species at environmentally relevant concentrations (~10(-8) M). A pH-dependent solvent extraction and combustion method was paired with accelerator mass spectrometry (AMS) to measure ambient levels of (129)I/(127)I isotope ratios and iodine speciation (iodide (I(-)), iodate (IO3(-)), and organo-I (OI)) in aquatic systems. The method exhibited an overall uncertainty of 10% or less for I(-) and IO3(-), and less than 30% for OI species concentrations and enabled (129)I measurements as low as 0.001 Bq/L (1 Bq/L=10(-13) M). The method was used to analyze groundwater from the Savannah River Site (SRS), South Carolina, USA, along a pH, redox potential (Eh), and organic carbon gradient (8-60 μM DOC). The data confirmed that the (129)I/(127)I ratios and species distribution were strongly pH dependent and varied in a systematic manner from the strongly acidic source. While (129)I speciation in plume samples containing total I concentrations >1.7 Bq/L was similar whether measured by AMS or GC-MS ([I(-)]≫[IO3(-)]=[OI]), AMS enabled (129)I speciation measurements at much lower concentrations than what was possible with GC-MS. AMS analyses demonstrated that groundwater samples minimally impacted by the plume were still orders of magnitude higher than ambient (129)I concentrations typically found elsewhere in the USA groundwaters and rivers. This is likely due to past atmospheric releases of volatile (129)I species by SRS nuclear reprocessing facilities near the study site. Furthermore, the results confirmed the existence of (129)I not only as I(-), but also as OI and IO3(-) species.

Temporal variation of iodine concentration and speciation (127I and 129I) in wetland groundwater from the Savannah River Site, USA

(129)I derived from a former radionuclide disposal basin located on the Savannah River Site (SRS) has concentrated in a wetland 600 m downstream. To evaluate temporal environmental influences on iodine speciation and mobility in this subtropical wetland environment, groundwater was collected over a three-year period (2010-2012) from a single location. Total (127)I and (129)I showed significant temporal variations, ranging from 68-196 nM for (127)I and <5-133 pCi/L for (129)I. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, (129)I levels were significantly correlated with those of (127)I, suggesting that biogeochemical controls on (127)I and (129)I are similar within the SRS aquifer/wetland system. Iodine speciation demonstrates temporal variations as well, reflecting effects from surface recharges followed by acidification of groundwater and subsequent formation of anaerobic conditions. Our results reveal a complex system where few single ancillary parameters changed in a systematic manner with iodine speciation. Instead, changes in groundwater chemistry and microbial activity, driven by surface hydrological events, interact to control iodine speciation and mobility. Future radiological risk models should consider the flux of (129)I in response to temporal changes in wetland hydrologic and chemical conditions.

Plutonium immobilization and remobilization by soil mineral and organic matter in the far-field of the Savannah River Site, U.S

To study the effects of natural organic matter (NOM) on Pu sorption, Pu(IV) and (V) were amended at environmentally relevant concentrations (10(-14) M) to two soils of contrasting particulate NOM concentrations collected from the F-Area of the Savannah River Site. More Pu(IV) than (V) was bound to soil colloidal organic matter (COM). A de-ashed humic acid (i.e., metals being removed) scavenged more Pu(IV,V) into its colloidal fraction than the original HA incorporated into its colloidal fraction, and an inverse trend was thus observed for the particulate-fraction-bound Pu for these two types of HAs. However, the overall Pu binding capacity of HA (particulate + colloidal-Pu) decreased after de-ashing. The presence of NOM in the F-Area soil did not enhance Pu fixation to the organic-rich soil when compared to the organic-poor soil or the mineral phase from the same soil source, due to the formation of COM-bound Pu. Most importantly, Pu uptake by organic-rich soil decreased with increasing pH because more NOM in the colloidal size desorbed from the particulate fraction in the elevated pH systems, resulting in greater amounts of Pu associated with the COM fraction. This is in contrast to previous observations with low-NOM sediments or minerals, which showed increased Pu uptake with increasing pH levels. This demonstrates that despite Pu immobilization by NOM, COM can convert Pu into a more mobile form.

Iodine-129 and iodine-127 speciation in groundwater at the Hanford site, US: iodate incorporation into calcite

The geochemical transport and fate of radioiodine depends largely on its chemical speciation that is greatly affected by environmental factors. This study reports, for the first time, the speciation of stable and radioactive iodine in the groundwater from the Hanford Site. Iodate was the dominant species and accounted for up to 84% of the total iodine present. The alkaline pH (pH ∼ 8) and predominantly oxidizing environment may have prevented reduction of the iodate. In addition, groundwater samples were found to have large amounts of calcite precipitate which were likely formed as a result of CO2 degassing during removal from the deep subsurface (>70m depth). Further analyses indicated that between 7 and 40% of the dissolved (127)I and (129)I that was originally in the groundwater had coprecipitated in the calcite. Iodate was the main species incorporated into calcite and this incorporation process could be impeded by elevating the pH and decreasing ionic strength in groundwater. This study provides critical information for predicting the long-term fate and transport of (129)I. Furthermore, the common sampling artifact resulting in the precipitation of calcite by degassing CO2, had the unintended consequence of providing insight into a potential solution for the in situ remediation of groundwater (129)I.

Novel molecular-level evidence of iodine binding to natural organic matter from Fourier transform ion cyclotron resonance mass spectrometry

Major fractions of radioiodine ((129)I) are associated with natural organic matter (NOM) in the groundwater and surface soils of the Savannah River Site (SRS). Electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) was applied to elucidate the interactions between inorganic iodine species (iodide and iodate) and a fulvic acid (FA) extracted from a SRS surface soil. Iodate is likely reduced to reactive iodine species by the lignin- and tannin-like compounds or the carboxylic-rich alicyclic molecules (CRAM), during which condensed aromatics and lignin-like compounds were generated. Iodide is catalytically oxidized into reactive iodine species by peroxides, while FA is oxidized by peroxides into more aliphatic and less aromatic compounds. Only 9% of the total identified organo-iodine compounds derived from molecules originally present in the FA, whereas most were iodine binding to newly-produced compounds. The resulting iodinated molecules were distributed in three regions in the van Krevelen diagrams, denoting unsaturated hydrocarbons, lignin and protein. Moreover, characteristics of these organo-iodine compounds, such as their relatively low O/C ratios (<0.2 or <0.4) and yet some degree of un-saturation close to that of lignin, have multiple important environmental implications concerning possibly less sterically-hindered aromatic ring system for iodine to get access to and a lower hydrophilicity of the molecules thus to retard their migration in the natural aquatic systems. Lastly, ~69% of the identified organo-iodine species contains nitrogen, which is presumably present as NH2 or HNCOR groups and a ring-activating functionality to favor the electrophilic substitution. The ESI-FTICR-MS technique provides novel evidence to better understand the reactivity and scavenging properties of NOM towards radioiodine and possible influence of NOM on (129)I migration.

Ameliorating effects of extracellular polymeric substances excreted by Thalassiosira pseudonana on algal toxicity of CdSe quantum dots

Quantum dots (QDs) are engineered nanoparticles (ENs) that have found increasing applications and shown great potential in drug delivery, biological imaging and industrial products. Knowledge of their stability, fate and transport in the aquatic environment is still lacking, including details of how these nanomaterials interact with marine phytoplankton. Here, we examined the toxicity of functionalized CdSe/ZnS QDs (amine- and carboxyl-) by exposing them for five days to Thalassiosira pseudonana (marine diatom) grown under different nutrient-conditions (enriched versus nitrogen-limited media). The released polysaccharides and proteins, the major components of extracellular polymeric substances (EPS), were measured to assess their potential effects on the interactions between QDs and T. pseudonana. The partitioning of QDs was analyzed by monitoring the concentration of Cd in different size fractions of the cultures (i.e., filtrate, <0.22 μm and permeate, <3 kDa). We found that the Cd release of QDs in the T. pseudonana culture was dependent on the nutrient conditions and nature of QDs' surface coating. Both amine- and carboxyl-functionalized QDs exhibited higher rates of Cd release in N-limited cultures than in nutrient enriched cultures. The results also showed that amine-functionalized QDs aggregate with minimal Cd release, independent of nutrient conditions. Laser scanning confocal microscopy images confirmed that aggregates are composed of QDs and the culture matrix (EPS). In addition, both types of QDs showed limited toxicity to T. pseudonana. The increasing production of proteins induced by QDs suggests that extracellular proteins might be involved in the detoxification of QDs to T. pseudonana via the Cd release of QDs. Our results here demonstrated that EPS can play an ameliorating role in QD toxicity, fate and transport in the aquatic environment.

Aggregation, dissolution, and stability of quantum dots in marine environments: importance of extracellular polymeric substances

There is an increasing concern that a considerable fraction of engineered nanoparticles (ENs), including quantum dots (QDs), will eventually find their way into the marine environment and have negative impacts on plankton. As ENs enter the ocean, they will encounter extracellular polymeric substances (EPS) from microbial sources before directly interacting with plankton cells. In this study, EPS harvested from four phytoplankton species, Amphora sp., Dunaliella tertiolecta, Phaeocystis globosa, and Thalassiosira pseudonana, were examined for potential interactions with CdSe nonfunctionalized and functionalized (carboxyl- and amine-) QDs in artificial seawater. Our results show that EPS do not reduce the solubility of QDs but rather decrease their stability. The degradation rate of QDs was positively correlated to the protein composition of EPS (defined by the ratio of protein/carbohydrate). Two approaches showed significant inhibition to the degradation of carboxyl-functionalized QDs: (1) the presence of an antioxidant, such as N-acetyl cysteine, and (2) absence of light. Owing to the complexity in evaluating integrated effects of QDs intrinsic properties and the external environmental factors that control the stability of QDs, conclusions must be based on a careful consideration of all these factors when attempting to evaluate the bioavailability of QDs and other ENs in the marine environments.

Bacterial production of organic acids enhances H2O2-dependent iodide oxidation

To develop an understanding of the role that microorganisms play in the transport of (129)I in soil-water systems, bacteria isolated from subsurface sediments were assessed for iodide oxidizing activity. Spent liquid medium from 27/84 bacterial cultures enhanced iodide oxidation 2-10 fold in the presence of H(2)O(2). Organic acids secreted by the bacteria were found to enhance iodide oxidation by (1) lowering the pH of the spent medium, and (2) reacting with H(2)O(2) to form peroxy carboxylic acids, which are extremely strong oxidizing agents. H(2)O(2)-dependent iodide oxidation increased exponentially from 8.4 to 825.9 μM with decreasing pH from 9 to 4. Organic acids with ≥2 carboxy groups enhanced H(2)O(2)-dependent iodide oxidation (1.5-15-fold) as a function of increasing pH above pH 6.0, but had no effect at pH ≤ 5.0. The results indicate that as pH decreases (≤5.0), increasing H(2)O(2) hydrolysis is the driving force behind iodide oxidation. However, at pH ≥ 6.0, spontaneous decomposition of peroxy carboxylic acids, generated from H(2)O(2) and organic acids, contributes significantly to iodide oxidation. The results reveal an indirect microbial mechanism, organic acid secretion coupled to H(2)O(2) production, that could enhance iodide oxidation and organo-iodine formation in soils and sediments.

Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River Site

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.

Factors controlling mobility of 127I and 129I species in an acidic groundwater plume at the Savannah River Site

In order to quantify changes in iodine speciation and to assess factors controlling the distribution and mobility of iodine at an iodine-129 ((129)I) contaminated site located at the U.S. Department of Energy's Savannah River Site (SRS), spatial distributions and transformation of (129)I and stable iodine ((127)I) species in groundwater were investigated along a gradient in redox potential (654 to 360 mV), organic carbon concentration (5 to 60 μmol L(-1)), and pH (pH 3.2 to 6.8). Total (129)I concentration in groundwater was 8.6±2.8 Bq L(-1) immediately downstream of a former waste seepage basin (well FSB-95DR), and decreased with distance from the seepage basin. (127)I concentration decreased similarly to that of (129)I. Elevated concentrations of (127)I or (129)I were not detected in groundwater collected from wells located outside of the mixed waste plume of this area. At FSB-95DR, the majority (55-86%) of iodine existed as iodide for both (127)I and (129)I. Then, as the iodide move down gradient, some of it transformed into iodate and organo-iodine. Considering that iodate has a higher K(d) value than iodide, we hypothesize that the production of iodate in groundwater resulted in the removal of iodine from the groundwater and consequently decreased concentrations of (127)I and (129)I in downstream areas. Significant amounts of organo-iodine species (30-82% of the total iodine) were also observed at upstream wells, including those outside the mixed waste plume. Concentrations of groundwater iodide decreased at a faster rate than organo-iodine along the transect from the seepage basin. We concluded that removal of iodine from the groundwater through the formation of high molecular weight organo-iodine species is complicated by the release of other more mobile organo-iodine species in the groundwater.

Effects of engineered nanoparticles on the assembly of exopolymeric substances from phytoplankton

The unique properties of engineered nanoparticles (ENs) that make their industrial applications so attractive simultaneously raise questions regarding their environmental safety. ENs exhibit behaviors different from bulk materials with identical chemical compositions. Though the nanotoxicity of ENs has been studied intensively, their unintended environmental impacts remain largely unknown. Herein we report experimental results of EN interactions with exopolymeric substances (EPS) from three marine phytoplankton species: Amphora sp., Ankistrodesmus angustus and Phaeodactylum tricornutum. EPS are polysaccharide-rich anionic colloid polymers released by various microorganisms that can assemble into microgels, possibly by means of hydrophobic and ionic mechanisms. Polystyrene nanoparticles (23 nm) were used in our study as model ENs. The effects of ENs on EPS assembly were monitored with dynamic laser scattering (DLS). We found that ENs can induce significant acceleration in Amphora sp. EPS assembly; after 72 hours EN-EPS aggregation reached equilibrium, forming microscopic gels of ∼4-6 µm in size. In contrast, ENs only cause moderate assembly kinetic acceleration for A. angustus and P. tricornutum EPS samples. Our results indicate that the effects of ENs on EPS assembly kinetics mainly depend on the hydrophobic interactions of ENs with EPS polymers. The cycling mechanism of EPS is complex. Nonetheless, the change of EPS assembly kinetics induced by ENs can be considered as one potential disturbance to the marine carbon cycle.

Concentration-dependent mobility, retardation, and speciation of iodine in surface sediment from the Savannah River Site

Iodine occurs in multiple oxidation states in aquatic systems in the form of organic and inorganic species. This feature leads to complex biogeochemical cycling of stable iodine and its long-lived isotope, (129)I. In this study, we investigated the sorption, transport, and interconversion of iodine species by comparing their mobility in groundwaters at ambient concentrations of iodine species (10(-8) to 10(-7) M) to those at artificially elevated concentrations (78.7 μM), which often are used in laboratory analyses. Results demonstrate that the mobility of iodine species greatly depends on, in addition to the type of species, the iodine concentration used, presumably limited by the number of surface organic carbon binding sites to form covalent bonds. At ambient concentrations, iodide and iodate were significantly retarded (K(d) values as high as 49 mL g(-1)), whereas at concentrations of 78.7 μM, iodide traveled along with the water without retardation. Appreciable amounts of iodide during transport were retained in soils due to iodination of organic carbon, specifically retained by aromatic carbon. At high input concentration of iodate (78.7 μM), iodate was found to be reduced to iodide and subsequently followed the transport behavior of iodide. These experiments underscore the importance of studying iodine geochemistry at ambient concentrations and demonstrate the dynamic nature of their speciation during transport conditions.

A novel approach for the simultaneous determination of iodide, iodate and organo-iodide for 127I and 129I in environmental samples using gas chromatography-mass spectrometry

In aquatic environments, iodine mainly exists as iodide, iodate, and organic iodine. The high mobility of iodine in aquatic systems has led to (129)I contamination problems at sites where nuclear fuel has been reprocessed, such as the F-area of Savannah River Site. In order to assess the distribution of (129)I and stable (127)I in environmental systems, a sensitive and rapid method was developed which enables determination of isotopic ratios of speciated iodine. Iodide concentrations were quantified using gas chromatography-mass spectrometry (GC-MS) after derivatization to 4-iodo-N,N-dimethylaniline. Iodate concentrations were quantified by measuring the difference of iodide concentrations in the solution before and after reduction by Na(2)S(2)O(5). Total iodine, including inorganic and organic iodine, was determined after conversion to iodate by combustion at 900 °C. Organo-iodine was calculated as the difference between the total iodine and total inorganic iodine (iodide and iodate). The detection limits of iodide-127 and iodate-127 were 0.34 nM and 1.11 nM, respectively, whereas the detection limits for both iodide-129 and iodate-129 was 0.08 nM (i.e., 2pCi (129)I/L). This method was successfully applied to water samples from the contaminated Savannah River Site, South Carolina, and more pristine Galveston Bay, Texas.

Optimized isolation procedure for obtaining strongly actinide binding exopolymeric substances (EPS) from two bacteria (Sagittula stellata and Pseudomonas fluorescens Biovar II)

Different chemical extractants (NaCl, EDTA, HCl and NaOH) and physical methods (ultrasonication and heating) were examined by their efficacies of extracting "attached" exopolymeric substances (EPS) secreted by marine bacterium Sagittula stellata (SS) and terrestrial bacterium Pseudomonas fluorescens Biovar II (PF). Extraction by 0.5 N HCl for 3 h was best for SS while extraction by 0.05 N NaCl for 3-5 h was regarded as optimal for PF. Improvements in EPS purification included a pre-diafiltration step to remove the broth material and reduce the solution volume, thus the usage of ethanol, and time. The EPS harvested at the optimal time and purified by the improved method were enriched in polysaccharides, with smaller amounts of proteins, thus having amphiphilic properties. Isoelectric focusing of (234)Th or (240)Pu labeled EPS showed both actinides were strongly bound to macromolecules with low pI, similar to reported marine or soil colloidal natural organic matter (NOM).

Organo-iodine formation in soils and aquifer sediments at ambient concentrations

One of the key risk drivers at radioactive waste disposal facilities is radioiodine, especially 129I. As iodine mobility varies greatly with iodine speciation, experiments with 129I-contaminated aquifer sediments from the Savannah River Site located in Aiken, SC, were carried out to test iodine interactions with soils and aquifer sediments. Using tracer 125I- and stable 127I- additions, it was shown that such interactions were highly dependent on I- concentrations added to sediment suspensions, contact time with the sediment, and organic carbon (OC) content, resulting in an empirical particle-water partition coefficient (Kd) that was an inverse power function of the added I- concentration. However, Kd values of organically bound 127I were 3 orders of magnitude higher than those determined after 1-2 weeks of tracer equilibration, approaching those of OC. Under ambient conditions, organo-iodine (OI) was a major fraction (67%) of the total iodine in the dissolved phase and by implication of the particulate phase. As the total concentration of amended I- increased, the fraction of detectable dissolved OI decreased. This trend, attributed to OC becoming the limiting factor in the aquifer sediment explains why at elevated I-concentrations OI is often not detected.

129I/(127)I as a new environmental tracer or geochronometer for biogeochemical or hydrodynamic processes in the hydrosphere and geosphere: the central role of organo-iodine

Iodine is a biophilic element, with several short-lived isotopes (e.g. (131)I, t(1/2)=8 days), one long-lived isotope, (129)I (t(1/2)=15.6 million years) and one stable isotope, (127)I. The inventory of (129)I in surface environments has been overwhelmed by anthropogenic releases over the past 50 years. Iodine and its isotopes are important for a number of reasons: (1) The largest fraction of the short-term and long-term dose from accidental releases and fallout from atomic bomb tests was from iodine isotopes. (2) (129)I is one of the two long-lived nuclides with highest mobility in stored radioactive waste. (3) (129)I could provide the scientific community with a new geochemical tracer and new geochronological applications in environmental science. (4) A better assessment of iodine deficiency disorders, mineralization in exploration geochemistry, and the transfer of volatile organic greenhouse-active and ozone-destroying iodine species from the oceans to the atmosphere is needed. One of the most promising future applications for the (129)I/(127)I ratio is not only as a new geochronometer, but also as a new source tracer for terrestrial organic matter with ages of 50 years or less. This is especially attractive, since radiocarbon can be, at times, an ambiguous chronometer for the 50-year time-scale, whereas (129)I concentrations during this time are overwhelming previous levels by orders of magnitude. Iodine is to a significant extent involved in the cycle of organic matter in all surface environments. Its biophilic nature is demonstrated by a relative enrichment of iodine in seaweed and dissolved macromolecular organic matter. Because of the close coupling of iodine and organic carbon cycles, our understanding of the underlying molecular mechanisms of the processes regulating iodination reactions in aquatic systems is still limited. The binding of iodine by organic matter has the potential to modify the transport, bioavailability and transfer of iodine isotopes to man. Equilibration times for (129)I in many reservoirs are likely long enough that (129)I could be used as a new source tracer for organic matter of terrestrial origin, and as a geochronometer. Current tracer applications of (129)I are limited by our knowledge of the effects of UV-radiation, microbial activity and geochemical redox conditions on organo-I compounds and overall iodine speciation. The biogeochemical behavior of iodine and its isotopes appears to be different in North America and European waters, possibly due to climatic, source and speciation differences.