Spectroscopic characterization of oceanic dissolved organic matter isolated by reverse osmosis coupled with electrodialysis

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 13C 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).