The Fate of Terrestrial Dissolved Organic Matter in Ocean Margins Investigated Through Coupled Microbial-Photochemical Incubations of Vascular Plant Leachates

Rivers are the primary link between terrestrial and marine carbon reservoirs. Dissolved organic matter (DOM) contributes the majority of carbon flux between these environments. Understanding the influence of source, availability and transformations of dissolved organic carbon (DOC) in rivers and coastal ocean systems is important to determine the fate of DOM in the marine environment. Coupled microbial-photochemical incubations were used to analyze microbial and photochemical decomposition of plant leachates and to investigate DOM cycling in the Sacramento-San Joaquin River Delta/San Francisco Bay estuary. A wide range of chemical and optical parameters were tracked during coupled incubations including absorbance, fluorescence, enantiomeric amino acids, and neutral sugars measurements. Vascular plant leachates were characterized by high neutral sugar yields and low amino acid yields with variable THAA and THNS composition. Biomarkers that most accurately tracked vascular plant DOM and microbial DOM during coupled incubations were selected to apply to seasonal transects collected from the Sacramento-San Joaquin River Delta and San Francisco Bay, California. Enantiomeric amino acids and neutral sugars were used to investigate the composition and bioavailability of riverine DOM in this natural system. Biochemical trends in the Delta-Bay system were influenced by source inputs, wetlands, and environmental processes. Differences in DOM composition and concentration related to differences in regions, highlighting heterogenetic inputs, hydrology of the system, and in situ production of DOM. Furthermore, terrestrial DOM was already extensively degraded prior to entering the Delta and was largely refractory throughout the Bay. Neutral sugar yields in the transect were used to determine a median of ~10% terrestrial DOC labile fraction, and amino acid yields were used to determine a median of 5% in situ DOM labile fraction. Median total labile fraction in the Delta and Bay was 21% and was influenced by hydrological conditions in the estuary. Our study demonstrated the complexity of river delta and estuarine systems integrating complex varying sources and decomposition trends connected to seasons and flow regime. Bay systems were recognized as efficient filters of terrestrial DOM limiting its flux to the ocean and exerting a major control on air-sea CO2 fluxes, acidification and nutrient budgets in the estuary

Superoxide Production by a Manganese-Oxidizing Bacterium Facilitates Iodide Oxidation

The release of radioactive iodine (i.e., iodine-129 and iodine-131) from nuclear reprocessing facilities is a potential threat to human health. The fate and transport of iodine are determined primarily by its redox status, but processes that affect iodine oxidation states in the environment are poorly characterized. Given the difficulty in removing electrons from iodide (I(−)), naturally occurring iodide oxidation processes require strong oxidants, such as Mn oxides or microbial enzymes. In this study, we examine iodide oxidation by a marine bacterium, Roseobacter sp. AzwK-3b, which promotes Mn(II) oxidation by catalyzing the production of extracellular superoxide (O(2)(−)). In the absence of Mn(2+), Roseobacter sp. AzwK-3b cultures oxidized ∼90% of the provided iodide (10 μM) within 6 days, whereas in the presence of Mn(II), iodide oxidation occurred only after Mn(IV) formation ceased. Iodide oxidation was not observed during incubations in spent medium or with whole cells under anaerobic conditions or following heat treatment (boiling). Furthermore, iodide oxidation was significantly inhibited in the presence of superoxide dismutase and diphenylene iodonium (a general inhibitor of NADH oxidoreductases). In contrast, the addition of exogenous NADH enhanced iodide oxidation. Taken together, the results indicate that iodide oxidation was mediated primarily by extracellular superoxide generated by Roseobacter sp. AzwK-3b and not by the Mn oxides formed by this organism. Considering that extracellular superoxide formation is a widespread phenomenon among marine and terrestrial bacteria, this could represent an important pathway for iodide oxidation in some environments

Temporal Variation of Iodine Concentration and Speciation (¹²⁷I and ¹²⁹I) in Wetland Groundwater from the Savannah River Site, USA

¹²⁹I 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 ¹²⁷I and ¹²⁹I showed significant temporal variations, ranging from 68–196 nM for ¹²⁷I and <5–133 pCi/L for ¹²⁹I. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, ¹²⁹I levels were significantly correlated with those of ¹²⁷I, suggesting that biogeochemical controls on ¹²⁷I and ¹²⁹I 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 ¹²⁹I in response to temporal changes in wetland hydrologic and chemical conditions

Iodine-129 and Iodine-127 Speciation in Groundwater at the Hanford Site, U.S.: 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 ¹²⁷I and ¹²⁹I 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 ¹²⁹I. 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 ¹²⁹I