Impacts of redox conditions on dissolved organic matter (DOM) quality in marine sediments off the River Rhone, Western Mediterranean Sea

Sedimentary dissolved organic matter (DOM) is an important pool of intermediates produced and consumed during early diagenesis of organic matter in the anoxic subseafloor. Rapid degradation of organic matter in the coastal sediment results in stratification of redox zones. However, to date little is known about the selectivity with respect to organic matter of the initial microbially mediated anaerobic degradation processes under contrasting redox conditions and how these affect the composition of DOM. In order to study the effect of sulfate reducing vs. methanogenic conditions on DOM quality and degradation, sediments (0-18 cm) from the Rhone River Delta were incubated, with redox conditions being controlled by sulfate amendment. The progress of incubation was monitored by H-2, CH4, sulfate, DIC, DOC and acetate production. DOM composition was determined by 3D Fluorescence Spectroscopy, i.e., Excitation Emission Matrix Spectroscopy (EEMs), and ultrahigh-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Parallel factor analysis (PARAFAC) of EEMs was used to distinguish different groups of DOM (humic-like and protein-like compounds) to evaluate composition, conjugation and size of DOM. Prior to incubation, humic-like sedimentary DOM predominated; nearly half of the molecular formulae (>5000) identified by FT-ICR-MS were CHO and one third were CHNO compounds. During incubation, protein-like DOM and CHNO formulae with 3 and 4 N atoms formulae were rare under both sulfate reducing and methanogenic conditions. Incubation under sulfate reducing conditions resulted in rapid release and net accumulation of dissolved organic carbon (DOC). Protein-like DOM was rapidly cycled and humic-like DOM accumulated. Consistently, CHNO formulae with 3 and 4 N atoms decreased faster, whereas formulae with one N and oxygen-rich unsaturated compounds became more concentrated. In contrast, during incubation under methanogenic conditions, there was no net accumulation of DOC; blue-shift of humic-like peaks suggest the transformation possibly associated with loss of oxygen-bearing functional groups in conjugated structures of humic substances. This interpretation is consistent with the relative decrease of oxygenation and carbon number in the pool of aromatic and highly unsaturated compounds observed by FT-ICR-MS analysis. Approximately 90% of molecular formulae that were lost under methanogenic conditions were accumulated under sulfate reducing conditions. Our results suggest that under sulfate reducing conditions degradation of organic matter results in the accumulation of highly oxidized DOM, while protein-like compounds are selectively consumed. When the redox regime changes to methanogenic conditions, microbes apparently utilize the humic-like and oxygen-rich compounds of the oxidized DOM pool that accumulated under sulfate reducing conditions. Consequently, redox regimes and the associated biogeochemical processes influence rate and fractions of DOM released by and consumed in the deep biosphere, which could ultimately shape the composition of the preserved sedimentary organic matter and the DOM released to the ocean. (C) 2020 Elsevier Ltd. All rights reserved.