Identifying the chemical structure of compounds that constitute the refractory dissolved organic matter (RDOM) reservoir will yield new insights into the processes that control the accumulation and removal of organic matter from aquatic environments. A scientist from Scripps Institute of Oceanography will be working to optimize an existing chemical reduction technique to render oxidized aliphatic and aromatic dissolved organic matter (DOM), two compounds which together comprise as much as 60% of the carbon in RDOM, more amenable to gas chromatography (GC) separation. This will allow for the characterization of this important RDOM component.
Much of the time and effort associated with the proposed research will be invested in identifying backbone structures of reduction products. To assist with this structural characterization, the proposal also seeks to adapt and modify existing GC×GC time of flight mass spectrometry techniques to better separate the complex mixture of reduction products. In addition to enabling more accurate compound characterization, this technique will also allow a more rapid determination of structural homogeneity within aquatic DOM. As part of this proposal, a preparatory GC×GC will also be modified such that methods developed using the previous instrument can be easily applied to isolate relevant compounds for further characterization (including isotope measurements). This work will also generate an MS library the aliphatic and aromatic reduction products that will be made available to other investigators interested in pursuing a similar avenue of research.
Besides continuing to further develop the analytical method, the researcher also plans to apply the method to test three hypothesis, namely (1) that reduction will yield several aromatic compounds, some of which resemble degraded lignin; (2) that reduction will yield a range of alicyclic compounds including terpane derivatives; and (3) that GC×GC separation will confirm the presence of structurally related families of aromatic and aliphatic compounds. Samples for this study include Suwannee River fulvic acids and natural organic matter, ultrafiltered DOM isolated from the eastern tropical Pacific Ocean and western tropical Atlantic Ocean, DOM isolated from Circumpolar Deep Water in the Southern Ocean isolated using Agilent Bondesil PPL, and one surface and one deep water sample collected in the North Pacific subtropical gyre obtained using reverse osmosis/electrodialysis that will serve as an ad-hoc marine, reference DOM. Analyses of these four different DOM sample types will help to determine reaction and GC/MS-based analysis parameters to be optimized for each sample type and compound class under study.
The analytical method to be developed and the mass spectrometry library for reduction products would be of interest to the science community. One graduate student would be supported and trained as part of this project. Two high school summer interns chosen with the help of the Ocean Discovery Center would also participate in the study. Students from this program tend to be underrepresented minorities.
Carbon dioxide is taken up during photosynthesis by plants on land and algae in ocean. If this carbon dioxide is returned either to the atmosphere or to the surface ocean through the respiratory activities of plants, animals and bacteria then it does not constitute a sink for atmospheric carbon dioxide. However, some fraction of the carbon dioxide is not respired and can be fixed into biochemical compounds that are not immediately respired. Why these compounds escape respiration on annual to millennial timescales is of interest because this represents a pathway by which biological processes sequester atmosphere carbon dioxide. In the proposed work we were developing a new method to uncover the chemical structures of a subset of compounds that accumulate and escape the process of respiration. Though our new method was not able to preserve as many of the parent structures during analysis as originally expected, it did provide important new insights into a family of chemicals that we could trace from its production source on land to its accumulation on annual timescales in rivers and estuaries. For the first time, we were ably to quantify the importance of condensed lipid molecules in sequestering atmospheric carbon dioxide. Some studies had hinted that these compounds should be accumulating in aquatic environments and reported indirect evidence of their presence. However, until our work, the methods to demonstrate their abundance in the environment were lacking. In addition to identifying and quantifying these compounds, our work also showed that a fixed number of parent compounds can be "multiplied" during their breakdown in the environment, likely by bacteria and light, to produce multiple, related but distinct daughter products. Finally having this compositional handle on some of the compounds that accumulate for long timescales in the environment allows us to understand where these compounds come from and what allows them to persist in the environment. This proposal helped to directly educate and support two graduate students. It also enabled the development of significant analytical infrastructure that indirectly supported the education of several other graduate students.
