Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

Abstract

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4000-6000 years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular dataset of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50°N to 70°S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising D 14 C values from À229& to À495&) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24 ka for the most persistent DOM fraction. First order kinetic degradation rates for 1557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of up to~100 ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment ("island of stability"). These most persistent compounds encompass only a narrow range of the molecular elemental ratios H/C (average of 1.17 ± 0.13), and O/C (average of 0.52 ± 0.10) and molecular masses (360 ± 28 and 497 ± 51 Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3 ± 3.3 lM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, representing average surface water DOM ages of 4920 ± 180 a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index I DEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate.

Pacific carbon cycling constrained by organic matter size, age and composition relationships

“…This model result is broadly consistent with an overall size age trend but also suggests that the oldest, most abundant DOC in the surface ocean may not be the material with the smallest molecular weight but rather LMW material between~200 and 500 Da. Whereas this modeling exercise can only suggest this as a hypothesis, such a negative Δ 14 C size offset is consistent with recent work proposing an "island of stability" for DOC molecules of sizes~330-550 Da, which can be produced by bacterial degradation in days but persist for millenia [Lechtenfeld et al, 2014].…”

Molecular insights into the microbial formation of marine dissolved organic matter: recalcitrant or labile?

“…However, for specific DOM fractions and molecular formulas, the residence time can be substantially longer (Loh et al, 2004;Lechtenfeld et al, 2014). It is not clear how this carbon buffer will evolve in the future global biogeochemical cycle and how it affects the climate system (Denman et al, 2007).…”

Seasonal variability of dissolved organic matter in the Columbia River: In situ sensors elucidate biogeochemical and molecular analyses

“…After analysis, mass spectra were recalibrated internally with a set of marine DOM molecules (Koch et al, 2014 formulas with a mass tolerance of ± 0.5 ppm were considered. Peak intensities were normalized to the highest ion peak in each spectrum, and the dataset was restricted to a relative peak intensity of ≥ 0.3 %.…”

“…By referring to each molecular formula's abundance as the ratio of its maximum seasonal abundance (Koch et al, 2014), we gained valuable insights into seasonal trends of DOM dynamics. Another goal was to establish a set of molecular markers that are closely tied to the biogeochemistry of the Columbia River.…”

“…The application of this technique has led to an increased understanding of the processing of DOM in the ocean (e.g. Koch et al, 2014;Kujawinski et al, 2004;Lechtenfeld et al, 2014) and in estuaries (e.g. Lechtenfeld et al, 2013;Sleighter and Hatcher, 2008).…”