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Photochemical alteration of dissolved organic sulfur from sulfidic porewater Gonzalo V Gomez-Saez, Anika M Pohlabeln, Aron Stubbins, Chris M Marsay, and Thorsten Dittmar Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03713 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Environmental Science & Technology

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Photochemical alteration of dissolved organic sulfur from sulfidic porewater

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Gonzalo V Gomez-Saeza*, Anika M Pohlabelna, Aron Stubbinsb, Chris M Marsayb, Thorsten

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Dittmara

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a) Research Group for Marine Geochemistry (ICBM - MPI Bridging Group), Institute for

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Chemistry and Biology of the Marine Environment, Carl von Ossietzky University of

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Oldenburg, D-26111, Oldenburg, Germany

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b) Skidaway Institute of Oceanography, Department of Marine Sciences, University of Georgia,

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Savannah, GA 30602-3636, USA

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*) corresponding author: [email protected]

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Abstract

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Sulfidic sediments are a source of dissolved organic sulfur (DOS) to the ocean but the fate of

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sedimentary DOS in the oxic, sunlit water column is unknown. We hypothesized that

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photodegradation after discharge from the dark sedimentary environment results in DOS

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molecular transformation and decomposition. To test this hypothesis, sulfidic porewater from a

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saltmarsh was exposed to potential abiotic transformations of dissolved organic matter (DOM) in

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the water column. We quantitatively investigated DOM transformations via elemental analysis

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and molecularly via ultrahigh-resolution mass spectrometry. Our study indicated that

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photoreactivity is dependent on DOM elemental composition as DOS molecular formulas were

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more photo-labile than those without sulfur. Prior to solar irradiation, of the 6451 identified

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molecular formulas in sulfidic porewater, 39 % contained sulfur. After 29 days of irradiation, the

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DOS concentration was depleted from 13 to 1 µM, together with a 9 % decrease in the number

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of DOS molecular formulas. Comparing porewater and oceanic DOS molecular formulas, solar

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irradiation increased the similarity due to the removal of photo-labile DOS formulas not present

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in the ocean. In conclusion, DOS from sulfidic sediments is preferentially photo-labile and solar

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irradiation can be a potential mechanism controlling the stability and fate of porewater DOS.

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Keywords: dissolved organic sulfur (DOS), dissolved organic matter (DOM), FT-ICR-MS,

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photodegradation, sulfidic sediments, saltmarsh


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Visual abstract - Table of Contents (TOC) Art

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1. Introduction

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Dissolved organic matter (DOM) is a complex mixture of hundreds of thousands of organic

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compounds with an essential role in global biogeochemistry.1 DOM is important as it contains an

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enormous amount of carbon dissolved in the oceans, more than 200 times the carbon of all living

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marine biomass,2 and similar to all atmospheric CO2.3 DOM is often operationally divided into

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reactivity fractions depending on its turnover time: minutes to days for labile DOM, weeks to

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one year for semi-labile DOM, and thousands of years for refractory DOM.2,4 However, the

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reasons behind DOM stability remain unknown.5

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Many compounds within the DOM mixture contain sulfur (dissolved organic sulfur, DOS), and

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in sum they make up the largest reservoir of organic sulfur in the ocean (global inventory of >6.7

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Pg S).6 While a small fraction of DOS is rapidly cycled and representative of the labile DOS

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pool,6 the majority of DOS compounds in the ocean are highly oxidized and presumably

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refractory.7 Knowledge on DOS molecular composition, sources and turnover is scarce, and

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therefore the connection between the labile and non-labile DOS pools remains unclear.8,9

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Sulfidic environments represent a source of potentially labile, reduced DOS to the ocean, as

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inorganic sulfur species get abiotically incorporated into DOM producing DOS compounds.10

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Benthic porewater fluxes, and potentially submarine groundwater discharge, are major pathways

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for the transport of sulfidic porewater and DOS from sediments into the water column,

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representing at least five times the annual export of riverine organic sulfur into the ocean.6,10

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Once DOS compounds leave dark, anoxic sediments, different biogeochemical transformations

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may change their properties and structures.11–13 Nevertheless, the fate of the sulfidic porewater

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DOS when exported to the oxic, sunlit water column is unknown.10

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One factor controlling oxidation states and organic matter transformations are photochemical

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reactions induced by solar irradiation.14 The optical properties of DOM in aquatic environments 4 ACS Paragon Plus Environment

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are categorized into two groups. Chromophoric or colored DOM (CDOM) absorbs sunlight,

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influencing ocean color15 and attenuating ~90 % of potentially harmful UV radiation in the

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global ocean.16 Upon absorbing UV light, CDOM undergoes a variety of photochemical

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reactions that lead to the photobleaching of CDOM and are of significance to marine

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biogeochemistry.17 A fraction of the CDOM, the fluorescent DOM (FDOM), emits light upon

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absorption and can be even more sensitive to photodegradation than CDOM.18 Photochemical

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reactions result in molecular DOM alteration and partial to full oxidation of DOM to CO2.18 The

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influence of photochemistry in the ocean is not restricted to the photic zone due to the vertical

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export of photoaltered DOM.19 Photodegradation of DOS compounds can affect global climate

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by releasing climate impacting gases, including carbonyl sulfide and dimethylsulfide.20

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Furthermore, DOS compounds bind and alter the solubility and bioavailability of metals like

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mercury that could be released to aquatic systems as a consequence of photochemical

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degradation.17 The DOS metal binding capacity depends on the type of chemical bonding and

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oxidation state of sulfur. For example, reduced sulfur or thiol functional groups have

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exceptionally strong binding affinities with mercury.21–23

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Recent advances in mass-spectrometry allow molecular characterization of the complex mixture

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of DOM. The use of Fourier transform ion-cyclotron resonance mass spectrometry (FT-ICR-MS)

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brings a major fraction of DOM into the analytical window, allowing the observation of diverse

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biogeochemical transformations.24–27 Linkages between photochemical reactivity and molecular

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composition of DOM have been reported.19,28–36 The photo-labile fraction of DOM is

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predominantly aromatic, including lignin, a vascular plant biomarker, and condensed aromatic

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compounds.28–31,36 As a consequence, a shift from terrestrial to marine molecular signatures of

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DOM during solar irradiation experiments on riverine DOM has been observed.30,36,37 Besides

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aromatics, other sulfur-containing compounds can be photo-labile.34 However, to the best of our



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knowledge there has not been a study quantifying and characterizing the photochemical

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transformations of DOS from sulfidic porewaters.

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In this study, we quantitatively and molecularly investigated the impact of solar irradiation on

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porewater DOM from a sulfidic saltmarsh system during its simulated way to the open ocean,

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focusing on the fate of DOS components. We hypothesized that photodegradation after discharge

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from the dark sedimentary environment results in DOS molecular transformation and

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decomposition. To test this hypothesis, sulfidic porewater from a subtropical saltmarsh system

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was exposed to oxygen, metal co-precipitation and simulated solar irradiation to mimic potential

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abiotic transformations in the water column. Samples were molecularly characterized using

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ultrahigh-resolution mass spectrometry and organic sulfur was quantified via elemental analysis.

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The results were then compared to natural samples from the surrounding saltmarsh and the open

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ocean.

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2. Materials and methods

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2.1. Study site

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Porewater sampling was conducted at the Saltmarsh Ecosystem Research Facility (SERF), a

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subtropical saltmarsh system on the campus of the Skidaway Institute of Oceanography (SkIO)

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on Skidaway Island, Georgia, USA. This facility is a 213 m long boardwalk providing direct

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access to the saltmarsh. The environments surrounding SERF include an inland saltmarsh

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meadow covered with Spartina alterniflora, areas of bare sediment, and a tidal creek.38 The

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sampling location was an unvegetated flat on the tidally inundated marsh around 30 m from the

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island’s shore (31°58’31.4”N, 81°01’50.5”W). Sediments were anoxic after a few millimeters

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depth and highly sulfidic after a few centimeters. Tides are semi-diurnal at SERF with the

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sediment at the sampling location being exposed to air for 5 to 7 h at low tide38 and covered by


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up to 60 cm of water at high tide. The surficial water above the porewater sampling location was

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sampled twice during high tide: once, pre-dawn before direct solar irradiation; and, once at noon

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when sunlight was shining directly on the water (“surficial water dawn and noon” samples;

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Table 1). Estuarine samples from the Skidaway River were accessed from a dock (31°59’24.5”N,

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81°01’20.4”W) on the SkIO campus (“estuary” sample; Table 1). Creek water samples were

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taken at low and high tides in Groves Creek (31°58’16.8”N, 81°01’37.5”W), an intertidal

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saltmarsh system close to Wilmington River on Skidaway Island with tidal range of up to 3 m

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(“creek LT and HT” samples; Table 1). Open ocean samples were taken during a cruise with the

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RV Savannah. Surface seawater (80

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% reduction of CDOM and >60 % depletion of SPE-DOC concentrations in the porewater (Fig.

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1a, b). This is consistent with previous studies which report CDOM photobleaching and DOC



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loss for freshwater,56–58 estuarine or riverine,36,59 plant leachate-derived,33,57 and deep-sea DOM

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samples.31,34

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Aromatic compounds are the primary chromophores and initiators of photoreactions in natural

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waters.42,60 In our study, photodegradation preferentially removed aromatic compounds as

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indicated by >50 % decrease in SUVA254, almost total photo-bleaching of humic-like FDOM,

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and the loss of 80 % of aromatic molecular formulas and the accompanying decrease in

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aromaticity and DBE indices (Table 1; Fig. 1b; Fig. 2). Many aromatic terrigenous compounds

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can also be degraded by aquatic microbes,61 but for our experiments, microbial degradation can

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be ruled out as flow cytometry cell counts were below detection limits during solar irradiation,

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and CDOM and SPE-DOC of the dark control did not change during incubation (Fig. 1a, b). The

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preferential photodegradation of aromatic compounds was consistent with previous FT-ICR-MS

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studies in which the photo-labile fraction was predominantly aromatic.28–30,33,36

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3.2. Preferential photodegradation of sulfur-containing compounds

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Prior to solar irradiation, of the 6451 identified molecular formulas in sulfidic porewater 39 %

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contained sulfur, which was 15 % higher than DOS percentage in seawater (Fig. 2a) and one of

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the highest values reported in the literature for natural DOM samples.13,55,62 The initial

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concentration of SPE-DOS in the sulfidic porewater was 10 µM, and the ratio of sulfur-per-

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carbon concentration SPE-DOS/SPE-DOC was 0.014 (Table 1; Fig. 1a, c). These values were

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one order of magnitude higher than open ocean seawater concentrations measured with the same

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methodology.7,63 After 29 days of solar irradiation, bulk concentrations of SPE-DOS, SPE-

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DOS/SPE-DOC ratios, and the relative abundance of SPE-DOS molecular formulas in total SPE-

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DOM had decreased, suggesting preferential photochemical losses of sulfur-containing

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compounds (Table 1; Fig. 1a, c; Fig. 2a). This was consistent with the number of photo-labile

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molecular formulas containing sulfur (CHOS1-2) being more than double the number of those 14 ACS Paragon Plus Environment

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containing only CHO, and one order of magnitude higher than N-containing formulas (CHON1-4;

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CHON1-4S1-2; Fig. 2c). Thus, the photoreactivity of DOM was clearly dependent on its elemental

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composition with sulfur-containing compounds being more reactive than those without sulfur.

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The photo-labile DOS molecular formulas were predominantly aromatic or highly unsaturated

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molecules (Fig. 2b, c), which are typical for vascular-plant derived DOM.22,36 Possibly, sulfur

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was preferentially introduced into terrigenous, aromatic compounds in a secondary reaction in

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the sulfidic environment.10 Most aromatic compounds absorb UV light and are more photo-labile

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than other DOM compounds,36 which could explain the observed loss of DOS in our

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experiments. However, most aromatic compounds that contained nitrogen did not degrade in our

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experiment, which indicates that the presence of sulfur itself may have increased photo-lability

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while the presence of nitrogen in a molecule enhanced photo-stability (Fig. 2c). At this point, we

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can only speculate about the reasons behind this observation, but because of the obvious

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biogeochemical implication, future studies should further explore mechanistic aspects of DOS

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photo-lability.

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DOS from the deep sea 31,34 or in acid mine drainage64 has also been shown to be photo-labile. A

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potential product of DOS photodegradation could be the climate impacting gases carbonyl

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sulfide or dimethyl sulfide.34 Organic sulfur compounds that contain N or P were identified as

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precursors for the photo-production of carbonyl sulfide, while compounds that did not contain

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heteroelements beyond S and O were not.20 In our study, the photo-lability of molecular

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formulas containing both S and N was not accentuated compared to those containing only CHOS

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(Fig. 2a, c). Whether the observed stoichiometry of photo-labile DOS compounds indicates the

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absence of carbonyl sulfide production is speculative but merits further research.

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DOS plays a key role in metal complexation depending on the type of chemical bonding and

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oxidation state of sulfur. For instance, reduced sulfur or thiol functional groups have remarkably 15 ACS Paragon Plus Environment


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strong binding affinities with mercury.21,22 Metallo-sulfur complexes likely stabilize sulfur

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compounds to thermal oxidation, but destabilize them to photodegradation due to ligand-metal-

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charge-transfer reactions.17 Therefore, the photodegradation of metallo-sulfur complexes could

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lead to a release of the free toxic metals to the environment.17,65 In our study, thiols were below

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detectable concentrations (Table 1) suggesting that they are not the dominant functionality of

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porewater SPE-DOS, which is consistent with previous observations using FT-ICR-MS.7

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However, even a minor percentage of reduced sulfur moieties in total DOS would be sufficient

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to significantly influence mercury and trace metal mobility in coastal marine systems.17 The

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presence of thiols in porewaters have been consistently reported in studies utilizing other

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analytical methods.22,23 Therefore, it is possible that chemical changes during SPE formed

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products outside the detectable range of the FT-ICR-MS and/or too polar to be retained by

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SPE.66 Furthermore, photochemical reactions could have produced low molecular weight sulfur

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products that fall outside of the SPE/FT-ICR-MS window. Low molecular weight compounds

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such as pyruvate, maleic or fumaric acid have been previously reported after cleavage of

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covalent bonds due to photochemical degradation.14,67 In our study, the decrease in S/O ratios

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during photo-incubation suggests that there might be a quantitative conversion of reduced to

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oxidized DOS driven by light, together with a diminished retention efficiency by SPE of polar

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compounds (Table 1). Previous experimental studies using FT-ICR-MS and exposing DOM

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samples to photodegradation prior to bio-incubation,36 hydrothermal conditions,66 or abiotic

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sulfurization10 also detected a decrease in SPE extraction efficiency. Our observation on the

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photo-lability of DOS should motivate future studies on the structural characteristics of

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porewater DOS and the potential complexation of mercury and its release upon

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photodegradation.

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In contrast to DOS, nitrogen-containing compounds were more resistant to photodegradation

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than the bulk of DOM and DOS in our experiment (Fig. 2a, c). They showed very different 16 ACS Paragon Plus Environment

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trends during solar irradiation: the SPE-DON/SPE-DOC concentration ratio, as well as the

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number of nitrogen-containing molecular formulas increased during photodegradation (Fig. 1d;

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Fig. 2c), similar to previous FT-ICR-MS studies.33,36 The observed molecular diversification of

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DON may partially be due to photo-nitration, as a result of the formation of nitrogen dioxide

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radicals derived from irradiation,68 or photochemical reactions incorporating ammonium into

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dissolved organic forms.69 However, the fact that there was a net-loss of SPE-DON (Fig. 1a)

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indicates the formation of new DON compounds was minor or at least occurred at a lower rate

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than the photochemical loss of DON.

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3.3. Solar irradiation increased the similarity between porewater and oceanic DOS

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During photoalteration, SPE-DOS molecular signatures converged upon those found in oxic,

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sunlit surface waters. From porewater to the ocean, just as in the irradiations, SPE-DOS/SPE-

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DOC ratios and the percentage of SPE-DOS decreased (Fig. 1a, c; Fig. 2a). A Bray-Curtis

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molecular dissimilarity analysis of the SPE-DOM molecular formulas was performed

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considering not only the presence and absence of molecular formulas, but also their signal

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intensity distribution (Fig. 3).36 Comparing porewater, saltmarsh and ocean samples, molecular

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dissimilarities >80 % between SPE-DOS formulas were initially detected (red colors of CHOS,

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CHONS; Fig. 3), which illustrates that porewater DOS was very different in its molecular

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composition from DOS in the oxic water column. In contrast, molecular formulas containing

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only CHO or CHON were considerably less dissimilar (blue-purple colors of CHON, CHO; Fig.

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3), indicating that porewaters and surface waters contained DOM with similar CHO and CHON

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formulas.

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Solar irradiation reduced the differences in SPE-DOS molecular formulas. During the irradiation

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(yellow squares; Fig. 3), the similarity of porewater SPE-DOS molecular signatures to those of

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saltmarsh and oceanic SPE-DOS increased, so that after just 7 days, photo-altered SPE-DOS was 17 ACS Paragon Plus Environment


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more similar to the oxic saltmarsh samples than to the precursor and dark porewater SPE-DOS

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samples (t3 - 15d, t4 - 29d, dendogram CHOS; Fig. 3). This molecular transformation was not

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observed in SPE-DOS formulas after oxidation, co-precipitation with metals or incubation in the

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dark (+O2, metals precip., dark - 29d; Fig. 3), indicating that photochemical processes were

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responsible for the transformation of porewater DOS molecular signatures.

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Photochemical reactions resulted in homogenization of the molecular fingerprint of DOM. It has

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been proposed that microbial and photodegradation of DOM may result in the accumulation of

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refractory compounds that share very similar molecular structures, independent of their original

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source.70 This was consistent with studies of the optical characteristics of extensively bleached

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DOM collected at the thermocline level of the North Pacific that were comparable to those of

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surface waters,71 or studies considering extensive microbial and photodegradation, where a high

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percentage of shared formulas were found between deep-sea and wetland plant-derived SPE-

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DOM.33 However, a recent study suggested that convergence into an universal molecular

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fingerprint of DOM in the course of degradation requires processes in addition to microbial and

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photodegradation.36 In contrast, our study indicated high molecular similarity between

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photodegraded porewater DOS and water column DOS. Therefore, photochemical alteration of

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porewater DOS appears to result in DOS molecules with the same elemental formulas as those

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found to be persistent and ubiquitous in the ocean.7

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Comparing porewater DOS to NEqPIW sample, representative of long-term persistent DOM in

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the deep ocean,50 36 % of the photo-resistant DOS formulas also represented 43 % of total DOS

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compounds in NEqPIW (Fig. 2d). Therefore, it is unclear whether ocean DOS molecular

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formulas were photo-products of porewater DOS irradiation or represent photo-refractory,

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survivor molecules that were already present in the porewater. In contrast, only 1 % of the photo-

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labile formulas with sulfur from our experiment were present in NEqPIW (Fig. 2d), indicating


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that the prominent increase in similarity between porewater and oceanic DOS was due to the

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removal of photo-labile DOS formulas not present in the ocean. Our findings are in accordance

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with previous studies suggesting that intensively photodegraded DOM shares common molecular

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properties independent of its origin.33,70 We conclude that DOS from sulfidic porewater is

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preferentially photo-labile and solar irradiation can be a potential mechanism controlling the

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stability and fate of DOS, when exported to the oxic, sunlit water column.



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Acknowledgments

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The authors are most thankful to A. Goranov, E. Palmer, L. Zhu, M. Liao and R. Nicholson for

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assistance while sampling and help in the laboratory at SkIO and T. Ferdelman (MPI) for very

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useful advice and geochemical analyses support. Furthermore, we thank T. B. Bittar (SkIO) for

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help with cell count analyses, N. Castellane (SkIO) for taking the ocean samples, C. Buck

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(SkIO), E. Gründken and B. Schnetger (ICBM), K. Klaproth and I. Ulber (ICBM-MPI) and K.

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Imhoff (MPI) for laboratory assistance and B.E. Noriega-Ortega (ICBM-MPI) for statistical

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guidance. We also thank the editor and three anonymous reviewers whose comments helped to

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improve an earlier version of this manuscript. This work was funded by the German Academic

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Exchange Service (DAAD, PhD Student Stipend) and the DFG project (DI 842/6-1).

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Supporting Information

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DOM datasheet including all samples with formulas detected by FT-ICR-MS and normalized

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intensities. This information is available free of charge via the Internet at http://pubs.acs.org.


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Table:

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Table 1: Physicochemical composition of DOM including general geochemistry, optical and FT-

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ICR-MS data in all samples from laboratory porewater incubations and natural saltmarsh and

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ocean samples, with “-” representing not analyzed and “

Photochemical Alteration of Dissolved Organic ... - ACS Publications

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