Amanda M. Grannas

Origin and sources of dissolved organic matter in snow on the East Antarctic ice sheet.

Polar ice sheets hold a significant pool of the worlds carbon reserve and are an integral component of the global carbon cycle. Yet, organic carbon composition and cycling in these systems is least understood. Here, we use ultrahigh resolution mass spectrometry to elucidate, at an unprecedented level, molecular details of dissolved organic matter (DOM) in Antarctic snow. Tens of thousands of distinct molecular species are identified, providing clues to the nature and sources of organic carbon in Antarctica. We show that many of the identified supraglacial organic matter formulas are consistent with material from microbial sources, and terrestrial inputs of vascular plant-derived materials are likely more important sources of organic carbon to Antarctica than previously thought. Black carbon-like material apparently originating from biomass burning in South America is also present, while a smaller fraction originated from soil humics and appears to be photochemically or microbially modified. In addition to remote continental sources, we document signals of oceanic emissions of primary aerosols and secondary organic aerosol precursors. The new insights on the diversity of organic species in Antarctic snowpack reinforce the importance of studying organic carbon associated with the Earths polar regions in the face of changing climate.

Role of Dissolved Organic Matter in Ice Photochemistry

In this study, we provide evidence that dissolved organic matter (DOM) plays an important role in indirect photolysis processes in ice, producing reactive oxygen species (ROS) and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin. Rates of DOM-mediated aldrin loss are between 2 and 56 times faster in ice than in liquid water (depending on DOM source and concentration), likely due to a freeze-concentration effect that occurs when the water freezes, providing a mechanism to concentrate reactive components into smaller, liquid-like regions within or on the ice. Rates of DOM-mediated aldrin loss are also temperature dependent, with higher rates of loss as temperature decreases. This also illustrates the importance of the freeze-concentration effect in altering reaction kinetics for processes occurring in environmental ices. All DOM source types studied were able to mediate aldrin loss, including commercially available fulvic and humic acids and an authentic Arctic snow DOM sample isolated by solid phase extraction, indicating the ubiquity of DOM in indirect photochemistry in environmental ices.

Photochemical Production of Singlet Oxygen from Dissolved Organic Matter in Ice

Dissolved natural organic matter (DOM) is a ubiquitous component of natural waters and an important photosensitizer. A variety of reactive oxygen species (ROS) are known to be produced from DOM photochemistry, including singlet oxygen, 1O2. Recently, it has been determined that humic-like substances and unknown organic chromophores are significant contributors to sunlight absorption in snowpack; however, DOM photochemistry in snow/ice has received little attention in the literature. We recently showed that DOM plays an important role in indirect photolysis processes in ice, producing ROS and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin.1 ROS scavenger experiments indicated that 1O2 played a significant role in the indirect photodegradation of aldrin. Here we quantitatively examine 1O2 photochemically produced from DOM in frozen and liquid aqueous solutions. Steady-state 1O2 production is enhanced up to nearly 1000 times in frozen DOM samples compared to liquid samples. 1O2 production is dependent on the concentration of DOM, but the nature of the DOM source (terrestrial vs microbial) does not have a significant effect on 1O2 production in liquid or frozen samples, with different source types producing similar steady-state concentrations of 1O2. The temperature of frozen samples also has a significant effect on steady-state 1O2 production in the range of 228-262 K, with colder samples producing more steady-state 1O2. The large enhancement in 1O2 in frozen samples suggests that it may play a significant role in the photochemical processes that occur in snow and ice, and DOM could be a significant, but to date poorly understood, oxidant source in snow and ice.

Molecular Insights on Dissolved Organic Matter Transformation by Supraglacial Microbial Communities

Snow overlays the majority of Antarctica and is an important repository of dissolved organic matter (DOM). DOM transformations by supraglacial microbes are not well understood. We use ultrahigh resolution mass spectrometry to elucidate molecular changes in snowpack DOM by in situ microbial processes (up to 55 days) in a coastal Antarctic site. Both autochthonous and allochthonous DOM is highly bioavailable and is transformed by resident microbial communities through parallel processes of degradation and synthesis. DOM thought to be of a more refractory nature, such as dissolved black carbon and carboxylic-rich alicyclic molecules, was also rapidly and extensively reworked. Microbially reworked DOM exhibits an increase in the number and magnitude of N-, S-, and P-containing formulas, is less oxygenated, and more aromatic when compared to the initial DOM. Shifts in the heteroatom composition suggest that microbial processes may be important in the cycling of not only C, but other elements such as N, S, and P. Microbial reworking also produces photoreactive compounds, with potential implications for DOM photochemistry. Refined measurements of supraglacial DOM and their cycling by microbes is critical for improving our understanding of supraglacial DOM cycling and the biogeochemical and ecological impacts of DOM export to downstream environments.

Partial Decay of Thiamine Signal Transduction Pathway Alters Growth Properties of Candida glabrata.

The phosphorylated form of thiamine (Vitamin B1), thiamine pyrophosphate (TPP) is essential for the metabolism of amino acids and carbohydrates in all organisms. Plants and microorganisms, such as yeast, synthesize thiamine de novo whereas animals do not. The thiamine signal transduction (THI) pathway in Saccharomyces cerevisiae is well characterized. The ~10 genes required for thiamine biosynthesis and uptake are transcriptionally upregulated during thiamine starvation by THI2, THI3, and PDC2. Candida glabrata, a human commensal and opportunistic pathogen, is closely related to S. cerevisiae but is missing half of the biosynthetic pathway, which limits its ability to make thiamine. We investigated the changes to the THI pathway in C. glabrata, confirming orthologous functions. We found that C. glabrata is unable to synthesize the pyrimidine subunit of thiamine as well as the thiamine precursor vitamin B6. In addition, THI2 (the gene encoding a transcription factor) is not present in C. glabrata, indicating a difference in the transcriptional regulation of the pathway. Although the pathway is upregulated by thiamine starvation in both species, C. glabrata appears to upregulate genes involved in thiamine uptake to a greater extent than S. cerevisiae. However, the altered regulation of the THI pathway does not alter the concentration of thiamine and its vitamers in the two species as measured by HPLC. Finally, we demonstrate potential consequences to having a partial decay of the THI biosynthetic and regulatory pathway. When the two species are co-cultured, the presence of thiamine allows C. glabrata to rapidly outcompete S. cerevisiae, while absence of thiamine allows S. cerevisiae to outcompete C. glabrata. This simplification of the THI pathway in C. glabrata suggests its environment provides thiamine and/or its precursors to cells, whereas S. cerevisiae is not as reliant on environmental sources of thiamine.

High-resolution mass spectrometric characterization of dissolved organic matter from warm and cold periods in the NEEM ice core

Dissolved organic matter (DOM) is an important component of ice cores but is currently poorly characterized. DOM from one Holocene sample (HS, aged at 1600–4500 B.P.) and one Last Glacial Maximum sample (LS, aged at 21000–25000 B.P.) from the North Greenland Eemian Ice Drilling (NEEM) ice core were analyzed by ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). CHO compounds contributed 50% of the compounds identified in negative-ionization mode in these two samples, with significant contributions from organic N, S, and P compounds, likely suggesting that marine DOM was an important source in these samples. Overall, the chemical compositions are similar between these two samples, suggesting their consistent DOM sources. However, subtle differences in the DOM between these two samples are apparent and could indicate differences in source strength or chemistry occurring through both pre- and post-depositional processes. For example, higher relative amounts of condensed carbon compounds in the HS DOM (5%), compared to the LS DOM (2%), suggest potentially important contributions from terrestrial sources. Greater incorporation of P in the observed DOM in the LS DOM (22%), compared to the HS DOM (13%), indicate more active microbiological processes that likely contribute to phosphorus incorporation into the DOM pool. Although these two samples present only a preliminary analysis of DOM in glacial/interglacial periods, the data indicate a need to expand the analysis into a broader range of ice-core samples, geographical locations, and glacial/interglacial periods.

Metal sorption studies biased by filtration of insoluble metal oxides and hydroxides

Toxic metals in the environment are often remediated using sorption techniques, particularly in aquatic and drinking water systems. However, a review of over 30 published sorption studies in the past two years alone revealed that the use of filtration to separate sorbed from unsorbed metals do not take into account metal hydroxide and oxide formation, and thus likely produce erroneous results. We quantified the effect of filtration on the removal of metal oxide/hydroxides from solution using a 0.45 μm filter as a function of pH, initial metal concentration and ionic strength for As, Be, Cd, Cu, Cr, Pb and Zn. We found that even when the initial metal concentration was as low as 0.1 mg/L, up to 93% of metals in solution were removed and up to 100% removal was observed when the initial metal concentration was 5 mg/L at a pH of 7. If this was unaccounted for, precipitated metal oxide/hydroxide removed via filtration will be inaccurately attributed to metal sorption. Additionally, we demonstrate that speciation modeling can underestimate the pH at which insoluble metal species form and therefore can only be used to approximate metal precipitation, especially in complex matrices. Overestimating the sorption capacity of sorbent materials has major implications if these sorbents are used for the purification of drinking water or other vital environmental remediation efforts. We recommend sorption studies using filtration prepare the appropriate matrix-matched control samples to quantify potential metal oxide/hydroxide formation.

Surface-promoted hydrolysis of 2,4,6-trinitrotoluene and 2,4-dinitroanisole on pyrogenic carbonaceous matter

This study investigates the fate of sorbed nitroaromatics on the surface of pyrogenic carbonaceous matter (PCM) to assess the feasibility of a PCM-promoted hydrolysis. The degradation of two nitroaromatic compounds, 2,4,6-trinitrotoluene (TNT) and 2,4-dinitroanisole, was observed at pH 7 in the presence of graphite powder, a model PCM. By contrast, no decay occurred without graphite. Using TNT as a model compound, our results suggest that TNT decay demonstrated a strong pH dependence, with no reaction at pH 3-5 but rapid degradation at pH 6-10. Moreover, by fitting TNT decay at different pH conditions along with its sorption kinetics to the Langmuir Kinetic Model, our results suggest that the base-catalyzed hydrolysis was important. The activation energy for TNT decay was obtained by measuring reaction rates at different temperatures with or without graphite and no significant difference was observed. However, the addition of tetramethylammonium cation was able to promote TNT decay possibly due to its ability to attract more OH- from the aqueous solution, leading to an increase in the sorbed OH- concentrations. Nitrite and a Meisenheimer complex were identified as degradation products for TNT. Other PCM, such as biochar, also demonstrated a comparable ability in promoting TNT decay at pH 7. Furthermore, a rapid degradation of TNT at pH 7 was observed when biochar was used as a soil amendment (4% by weight). Our results suggest that PCM can facilitate TNT and 2,4-dinitroanisole decay via a surface-promoted hydrolysis at neutral pH conditions, suggesting a promising alternative for in situ soil remediation.

Photochemistry of Organic Pollutants in/on Snow and Ice

Organic pollutant cycling and fate is impacted by the presence of snow and ice, which can serve as a repository for deposited species and also as a chemical reaction medium. Photochemistry (light-induced chemistry) occurring in/on snow and ice at Earth’s surface is now known to play an important role in a variety of environmentally relevant processes including the production and release of atmospherically relevant species such as halogens, nitrogen oxides, and volatile organic compounds. Less is known about the role of snow and ice photochemistry in organic pollutant fate, but increasing recent evidence points to the potential importance of photochemical alteration of organic pollutants in sunlit snowpacks. This chapter describes recent work during and since International Polar Year (2007) aimed at investigating the potential importance of photochemistry to organic pollutant processing in snow and ice.