Research Areas

Electrodialysis

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The large size and dynamics of the ocean dissolved organic matter (DOM) pool have made it an important focus of many studies of global elemental cycles. These studies are motivated by the direct and indirect influences of DOM cycling on processes such as global warming and marine productivity. Over the last three decades, numerous compositional studies of various size fractions and chemical extracts have significantly increased our understanding of the origin and cycling of marine DOM. A large fraction of marine DOM, however, still remains compositionally uncharacterized. A major obstacle in the study of marine DOM has been isolating from seawater sufficient quantities for analysis of this highly dilute and chemically complex material. One of my new lines of research, funded by NSF, is to develop, test, and apply a process using reverse osmosis in combination with electrodialysis for the concentration and practical retrieval of significant quantities of essentially unaltered DOM from seawater. Up to now the oceanographic community has had to focus their studies on high molecular weight (HMW) organic compounds, which are the only compounds recoverable using current technologies. These HMW compounds only represent a small fraction of the total marine DOM pool (less than 30% at best). Even with this limitation, studies of HMW compounds have been the subject of many papers published in the journals Science and Nature. Preliminary results suggest the electrodialysis technique in combination with reverse osmosis methods will be able to recover greater than 80% of the dissolved organic matter in seawater including small molecules. As such, scientists using a variety of analytical approaches to examine organic matter (isotopic, chemical, radioisotopic) will be interested in both the samples we extract from seawater and the methodology developed as part of this research.

The success of our research activities so far has given us the confidence to build a larger ED system in order to recover analytically significant quantities of dissolved organic matter. The construction of this much larger system is complete and we have just begun testing. We have also constructed a new RO system with a greatly reduced dead volume that will be necessary for effective processing of seawater samples.

Phosphorus & Carbon Spectroscopy

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Another general theme of my research has been the application of spectroscopic methods such as nuclear magnetic resonance (NMR) and XANES (X-ray Absorption Near Edge Structure) to study the composition and cycling of phosphorus and carbon.

XANES

I was recently funded by NSF with Jay Brandes and Claudia Benitez-Nelson to explore the use of XANES spectroscopy to obtain compositional information at the microscale in natural materials. This research is a focused effort employing recently developed phosphorus (P) specific X-ray spectro-microscopic techniques along with traditional NMR and chemical methods to examine P compound-classes within sinking particles, sediments and porewaters from a range of oxic to anoxic environments. Our overall goal is to relate the composition of particulate and dissolved P to remineralization and sequestration processes in marine particulates and sediments. These x-ray techniques require the use of synchrotron radiation sources (There are only 2 synchrotron in the US capable of P XANES). This study will yield maps of P composition and concentrations within particulates at scales relevant to microbially-mediated storage and degradation mechanisms. To our knowledge, this is the first study to develop XANES for the study of phosphorus in marine systems. Additionally, this is the first study to target patterns of P speciation across a wide range of environments and within particles and porewaters. Results from this research will not only lead to a better understanding of cycling of the vital nutrient element P but also demonstrate the potential of XANES spectroscopy for the analysis of natural samples.

Composition of Atmospheric Aerosols

Another application of spectroscopic techniques that I am particularly excited about is collaborative work with Dr. Rodney Weber to use 13C NMR to characterize water-soluble organics in aerosols. Water-soluble organics in aerosols are of interest because their abundance, composition and physical properties have implications for regional air quality and global climate. For example, these compounds can produce adverse health responses when inhaled. The soluble organic fraction can also influence hygroscopicity of aerosols. Particle hygroscopicity plays a major role in aerosol radiative properties by altering particle light absorption and scattering characteristics, and also affects cloud-nucleating properties. While detailed information on a wide range of specific compounds in aerosols can be obtained with techniques such as gas chromatography coupled with mass spectrometry (GC-MS), only a small fraction of aerosol OC has been identified in the form of specific compounds. One reason for this is that a substantial fraction of the polar oxygenated organic compounds present in water-soluble organic aerosols, cannot be put in a form that is readily analyzable by GC-MS. NMR provides a new method for the atmospheric community to characterize the chemical composition of aerosols.

Cycling of Carbon in Marine Dissolved and Particulate Matter

Organic Matter (OM) in the oceans is one of the largest dynamic reservoirs of organic carbon (C) on Earth. The marine OM reservoir contains approximately 730Gt of C, which is comparable in size to the atmospheric CO2 (750Gt) and land biota (570Gt) reservoirs. The sizes of these reservoirs relative to the fluxes between them indicate that changes in marine OM cycling in the water column can significantly influence the global C cycle on relatively short time scales. While, there has been considerable progress in our understanding of the origin, composition and cycling of marine OM, a large fraction still remains compositionally uncharacterized. A recent publication by my group in Deep-sea research, “Cycling of dissolved and particulate organic matter at station Aloha: Insights from 13C NMR spectroscopy coupled with elemental, isotopic and molecular analyses” by Sannigrahi, Ingall and Benner, develops a picture of carbon cycling using a combination of solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopy, isotopic, elemental and molecular analyses on dissolved and particulate organic matter in the ocean. To the best of my knowledge, this is the first use of solid-state 13C NMR to characterize suspended particulate organic matter in the ocean and also the first direct comparison of NMR results to molecular analyses on different size fractions from the same site. Direct comparison of the contribution of amino acids and carbohydrates to C in DOM and POM as obtained by 13C NMR to molecular analyses yielded insights into the composition of the large fraction of marine organic matter that is compositionally uncharacterized.

Phosphorus Cycling in Marine Sediments

Phosphorus (P) is an essential and in many cases limiting nutrient sustaining marine primary productivity. Burial of P compounds resistant to remineralization during diagenesis is a significant sink in the global marine P budget. A large body of evidence suggests that the presence of anoxic bottom waters enhances the release of P (as compared to N and C) from marine sediments. The exact mechanisms behind the enhanced release of P have been a mystery and a source of some controversy in the oceanographic community. A recent paper “Polyphosphates as a source of enhanced P fluxes in marine sediments overlain by anoxic waters: Evidence from 31P NMR” by Sannigrahi and Ingall, provides evidence for a new and potentially significant redox sensitive P cycling mechanism in sediments. This paper provides evidence showing that the varying stability of polyphosphates in microorganisms under different redox conditions can explain many observations of enhanced P flux under low oxygen conditions.

Nitrogen Cycling

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Nitrogen like phosphorus is a key nutrient element in terrestrial and aquatic systems whose abundance plays a significant role in controlling productivity. Recent studies have reported imbalances in global marine fixed N budgets with rates of N loss exceeding rates of N input. These imbalanced budgets reflect the difficulties in making estimations given the many uncertainties in the pathways and rates of key N supply and removal reactions. In particular, the relatively few measurements of pathways and rates of denitrification reactions, the largest sink term in the global N budget, confound estimations of global N losses. Denitrification in continental shelf sediments is one of the largest sinks of oceanic N, accounting for up to 67% of estimates of total global denitrification. Most direct denitrification rate measurements for continental shelves have been made on fine-grained, muddy sediments, which cover only 30% of global shelf area. Sandy sediments cover the remaining 70% of continental shelf area. These sandy sediments are generally characterized by low organic matter and high pore water dissolved oxygen concentrations, properties typically considered unfavorable for heterotrophic denitrification. The lack of data in this widespread environment raises the issues of the correlation between organic matter content and denitrification, and the presence of alternative pathways to N2, which may not be limited by organic matter content, oxygen, or observed dissolved inorganic N levels. In a recent paper, “Denitrification pathways and rates in the sandy sediments of the Georgia continental shelf, USA” by Vance-Harris and Ingall we examined denitrification in the coarse-grained, sandy sediments of the Georgia continental shelf providing new information in an often overlooked but potentially significant sediment type. This work utilized a specialized device, a membrane inlet mass spectrometer, constructed in my lab.

Phosphorus Anoxia Feedback

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My most cited work examines the effect of bottom water oxygen concentration on seafloor diagenetic processes and elemental fluxes of phosphorus in marine sediments. The overall burial and preservation of phosphorus in marine sediments is strongly dependent on the concentration of oxygen in waters overlying the sediments. These studies indicate that oxygen depleted bottom waters enhance phosphorus regeneration from sediments, diminishing overall burial efficiency. This enhanced regeneration has important implications for eutrophication in coastal regions and for the long term cycling of P especially during periods of widespread ocean anoxia. These implications were highlighted in two papers demonstrating through simple models the possibility of feedback mechanisms linking ocean ventilation, dissolved P concentration, marine productivity and atmospheric oxygen levels (Science 271:493-496; Paleoceanography 9:677-692). These models show that atmospheric oxygen levels can be stabilized over geology time scales through the feedback between phosphorus burial and anoxia. I recently published a paper providing further evidence for this feedback derived from measurements at a unique study site, Effingham Inlet (American Journal of Science 305:240-258). At this site, benthic phosphorus fluxes could be compared at sites that were nearly identical except for the presence or absence of oxygen at the sediment water interface. A subsequent recent paper identified one potential mechanism to explain oxygen sensitive burial and preservation of phosphorus in sediments (Geochemical Transactions 6:52-59). Based in part on the work presented in both papers, we will be returning to Effingham Inlet as part of the research funded in the NSF proposal, “Collaborative Research: Examining redox-sensitive phosphorus speciation and remineralization using X-ray and NMR spectroscopic methods.”