Search

When compositions of an isobaric series of molecular formulae consisting of C, H, and O are displayed in a van Krevelen plot, a remarkable number of nonparallel lines of compositions can be observed. Each group of related lines converge at a point in a negative quadrant of van Krevelen space (e.g., H/C = 4, O/C = -1). These points of convergence have H/C and O/C ratios that correspond to "molecular formulae" in which the stoichiometric coefficients of some of the isotopes are negative. In this manner, a group of related low-mass moieties have been identified (CH4O(-1), C4O(-3), C2H(-8)O(-1), CH(-12), etc.). Each of these moieties has a nominal mass of zero and a very small exact mass. Furthermore, all of these low-mass moieties have compositions that fall on the line H/C = -12 - 16(O/C), which lies entirely in the negative quadrants of van Krevelen space. This paper demonstrates that all low-mass moieties consisting of C, H, and O can be expressed formally as linear combinations of any two moieties. Likewise, all molecular formulae that fall on a line that passes through a given low-mass moiety must differ compositionally by a multiple of the composition of that moiety, and their exact masses must differ by a multiple of the exact mass of the moiety. This latter relationship has been invoked for very rapid assignment of a molecular formula to an exact mass. Finally, a more comprehensive and theoretically based understanding of family scores has been developed around the concept of low-mass moieties.

Here we present results of an initial assessment of the impacts of a water diversion event on the concentrations and chemical composition of dissolved organic matter (DOM) and bacterioplankton community composition in Barataria Bay, Louisiana U.S.A, an important estuary within the Mississippi River Delta complex. Concentrations and spectral properties of DOM, as reflected by UV/visible absorbance and fluorescence, were strikingly similar at 26 sites sampled along transects near two western and two eastern areas of Barataria Bay in July and September 2010. In September 2010, dissolved organic carbon (DOC) was significantly higher (568.1-1043 μM C, x=755.6+/-117.7 μM C, n=14) than in July 2010 (249.1-577.1 μM C, x=383.7+/-98.31 μM C, n=14); conversely, Abs254 was consistently higher at every site in July (0.105-0.314) than in September (0.080-0.221), averaging 0.24±0.06 in July and 0.15±0.04 in September. Fluorescence data via the fluorescence index (FI450/500) revealed that only 30% (8 of 26) of the July samples had an FI450/500 above 1.36, compared to 96% (25 of 26) for the September samples. This indicates a more terrestrial origin for the July DOM. Bacterioplankton from eastern sites differed in composition from bacterioplankon in western sites in July. These differences appeared to result from reduced salinities caused by the freshwater diversion. Bacterioplankton communities in September differed from those in July, but no spatial structure was observed. Thus, the trends in bacterioplankton and DOM were likely due to changes in water masses (e.g., input of Mississippi River water in July and a return to estuarine waters in September). Discharge of water from the Davis Pond Freshwater Diversion (DPFD) through Barataria Bay may have partially mitigated some adverse effects of the oil spill, inasmuch as DOM is concerned., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Oceanic dissolved organic matter (DOM) is one of the largest pools of reduced carbon on Earth, yet DOM remains poorly chemically characterized. Studies to determine the chemical nature of oceanic DOM have been impeded by the lack of efficient and non-fractioning methods to recover oceanic DOM. Here, a DOM fraction (~ 40 to 86% recovery) was isolated using reverse osmosis/electrodialysis (RO/ED) and analyzed by solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy. Samples were obtained from biogeochemically distinct environments: photobleached surface gyre, productive coastal upwelling zone, oxygen minimum, North Atlantic Deep Water, and North Pacific Deep Water. A ubiquitous ‘background’ refractory carbon pool was apparent throughout the ocean and dominated in the deep Pacific Ocean. Advanced NMR spectral editing revealed that condensed aromatic and quaternary anomeric carbons contribute to this deep refractory DOC pool, the quaternary anomeric carbons being a newly identified and potentially important component of bio-refractory carbohydrate-like carbon. Additionally, our results support the multi-pool (e.g. 3-pool: labile, semi-labile, and refractory) conceptual model of marine DOM biogeochemistry. Surface samples, hypothesized to be enriched in labile and semi-labile DOM, were enriched in carbohydrate-like material consistent with prior studies. High carboxyl signals in the deep Pacific support the hypothesis that a major fraction of the refractory pool consists of carboxyl-rich alicyclic molecules (CRAM). [ABSTRACT FROM AUTHOR]

Abstract: Photobleaching of open-ocean dissolved organic matter (DOM) is typically treated as a removal mechanism; however, photobleaching also encompasses a poorly characterized suite of transformative processes. To examine the qualitative changes to DOM optical properties during photobleaching, 674m N. Pacific DOM, concentrated and desalted by reverse osmosis with electrodialysis (RO/ED), was subjected to 68days of continuous irradiation in a UV solar simulator. Approximately 84% of chromophoric and fluorescent DOM (CDOM and FDOM respectively) and 38% of dissolved organic carbon (DOC) were lost during the irradiation. Based on these results the concentration of photochemically refractory DOC in the surface pacific is estimated to be 27μmol of carbon per liter. In addition, the spectra of the remaining CDOM and FDOM were shifted towards shorter wavelengths, a result which has important implications for the interpretation of fluorescence excitation emission matrix (EEM) spectra because the relative positions of fluorescence maxima are often attributed to differences in FDOM source. Qualitative indices derived from CDOM and FDOM spectra for the irradiated deep DOM sample resembled those for surface waters of the North Pacific Ocean indicating that photobleaching has a significant influence upon the optical properties of DOM in the open ocean. [Copyright &y& Elsevier]