Influence of dissolved organic matter (DOM) characteristics on dissolved mercury (Hg) species composition in sediment porewater of lakes from southwest China
Graphical abstract
Introduction
Dissolved organic matter (DOM) plays an important role in many biogeochemical processes including the global carbon cycle and fate of pollutants (e.g., trace heavy metals and organic containments) (Aiken et al., 2011a; b; Bolan et al., 2011; Nelson and Siegel, 2013; Mopper et al., 2015). As an important fraction of natural organic matter (Aiken et al., 2011b; Mostofa et al., 2013), DOM is widely recognized as a key environmental parameter, influencing the ecology of both aquatic and terrestrial environments (Tranvik et al., 2009; Aiken et al., 2011a; b). Sediments may function as a sink or source of natural organic matter and many types of pollutants (Förstner, 2004), including Hg (Isidorova et al., 2016). Pollutants are released from sediment particles into its porewater through desorption and dissolution processes, as well as by biological decomposition of NOM. Linking sediments to overlying water, DOM has a key role in the transfer of pollutants from sediments to the water column (Ziegelgruber et al., 2013; Abbott et al., 2015). Determination of porewater DOM characteristics in sediment and water is one of the fundamentals to improve our understanding of how DOM structures and DOM reactivity influence the fate of pollutants in the aquatic environment (Chen and Hur, 2015). The recent advancement of DOM characterization (Leenheer and Croué, 2003), UV–Vis and excitation-emission matrices (EEMs) fluorescence spectroscopy provides convenient and powerful tools to analyze DOM composition, widely used to track sources of DOM (e.g., autochthonous versus allochthonous) and DOM processing (Fellman et al., 2010; Coble et al., 2014). Because spectral characteristics of DOM in porewater can be used to track the composition and origin of DOM in the bulk sediment (Bravo et al., 2017), and reflect the microbial activity by its composition (Ziegelgruber et al., 2013), they have been proven useful tools for explaining the environmental fates of contaminants, such as mercury (Herrero Ortega et al., 2017; Kim et al., 2017; Lescord et al., 2018).
Mercury (Hg) contamination is a global environmental issue stemming from the severe damage it may cause humans and ecosystem health. Its methylated chemical form, methylmercury (MeHg), is of special concern because it accumulates in aquatic food webs through biomagnification processes (Lavoie et al., 2013; Lehnherr, 2014). The methylation of inorganic Hg, henceforth denoted Hg(II), to MeHg occurs under oxygen-limited conditions such as those found in submerged soils (e.g. wetlands, (Tjerngren et al., 2012a; b), lake sediments (Bravo et al., 2017), stratified water columns (Eckley et al., 2005) and even in micro-environments of settling particles (Gascón Díez et al., 2016). The formation of MeHg is ascribed to the activity of anaerobic microorganisms (e.g., iron- or sulfate-reducing bacteria, archaea) (Zhang et al., 2012, 2014; Hammerschmidt and Fitzgerald, 2004; Tjerngren et al., 2012a; b; Parks et al., 2013). Indeed, Hg(II) methylation processes are governed by the activity of the microbial community (Bravo et al., 2017), as well as the availability of Hg(II) to methylating microorganisms, which in turn is dependent on the Hg(II) speciation (Gerbig et al., 2011, 2012; Jonsson et al., 2012, 2014; Zhang et al., 2012, 2014). The Hg mobilization, speciation, and methylation have been widely studied across various environments (Skyllberg, 2008, 2012; Graham et al., 2012, 2013).
Of all the biogeochemical variables affecting Hg transformations, DOM is one of the most important factors (Aiken et al., 2011a; Ravichandran, 2004; Gerbig et al., 2012; Bravo et al., 2017; Herrero Ortega et al., 2017). There are many reports on the interactions and correlations between DOM and Hg, and comprehensive reviews are available elsewhere (Ravichandran, 2004; Aiken et al., 2011a; b; Skyllberg, 2012; Gerbig et al., 2012; Hsu-Kim et al., 2013). In field investigations, correlations between concentrations of DOC and Hg have been established to indicate the role of DOM (Mierle and Ingram, 1991; Driscoll et al., 1995; Hurley et al., 1998; Dittman et al., 2009; Bergamaschi et al., 2012). However, across aquatic systems, influenced by diverse biogeochemical processes and sources of DOM, the correlation between concentrations of Hg and DOC is not always consistent. Lab-scale experiments have revealed a dual effect of DOM on MeHg net production, by i) acting as a stimulatory electron donor, and by ii) modulating the chemical speciation of Hg(II). DOM also serve as a substrate for microorganisms and stimulate Hg methylation (Hsu-Kim et al., 2013; Schartup et al., 2013; Gerbig et al., 2012; Kim et al., 2011). Indeed, most groups of microbes containing phyla of Hg methylators are key anaerobic decomposers of organic materials (Gilmour et al., 2013; Reddy and Delaune, 2008; Logue et al., 2016). Under sulfidic conditions, a stimulatory effect of DOM on MeHg production is attributed to the disintegration or dissolution of HgS-minerals (Waples et al., 2005), resulting in enhanced Hg(II) bioavailability to methylating bacteria (Gerbig et al., 2011; Graham et al., 2012, 2013). In contrast, chemisorption to DOM functional groups may limit Hg(II) bioavailability for methylating microorganisms when the molecular size of DOM is too large to cross the bacterial cell membranes (Hammerschmidt and Fitzgerald, 2004; Hammerschmidt et al., 2008). Recent studies (Schartup et al., 2013; Chiasson-Gould et al., 2014; Bravo et al., 2017; Herrero Ortega et al., 2017; Lescord et al., 2018; Noh et al., 2018) demonstrate that differences in the composition of DOM may largely explain the variability in Hg methylation. Indeed, it is known that specific DOM components regulate speciation and dynamics of Hg through numerous mechanisms, including chemical complex formation (Ravichandran, 2004; Skyllberg, 2008, 2012), redox processes (Gu et al., 2010) and microbial activity (Bravo et al., 2017). Thus, all the mentioned studies underscore the complexity of the interactions between Hg and DOM, and more importantly, highlight the potential to perform comprehensive studies to identify the Hg risk, especially the MeHg net production in the environment using DOM composition.
Despite the recently highlighted importance of DOM composition and quantity, environmental biogeochemical studies of Hg still poorly or incompletely take DOM characterization into account, especially outside the boreal zone. Here we use the variability in DOM characteristics to improve our understanding about how concentrations of DOM and Hg may be related (or not related) at different study sites in subtropical China. We hypothesize that, beyond DOC concentration, DOM composition characteristic of terrigenous and aquatic sources are important drivers behind Hg concentration levels and the transformation of Hg to MeHg in different types of lakes located at southern latitudes. Based on the “structure-reactivity” concept of DOM biogeochemistry (Senesi et al., 2009; Aiken et al., 2011a; b), we: 1) characterized the properties of porewater DOM in three lakes of southwest China, and 2) tracked DOM sources to elucidate their influence on Hg concentration levels and transformation to MeHg in lake porewaters. Our results demonstrate optical characterization of DOM as a useful tool to improve our understanding of the causality behind DOM-Hg relationships established by field observations of three different lakes.
Section snippets
Study sites
On the Southwestern China Plateau, there are more than 30 lakes with a water depth of 10–50 m. Three of these lakes, Caohai Lake (CH), Hongfeng Lake (HF) and Wujiangdu Lake (WJD), were selected for this study (Fig. 1). Caohai Lake (CH) (26°50′50″N, 104°14′27″E) is in southwestern Weining County of the Guizhou Province. It is a pristine shallow lake located in the Caohai State Nature Reserve. The catchment of CH includes evergreen broad-leaf forests with riparian soils exporting terrestrial DOM
DOC and CDOM
The porewater DOC concentration ranged from 30.3 to 90.4 mg. L−1 (Supporting Information 4, Table S3). By the one-way ANOVA, the difference in DOC concentration between CH and the other two lakes was significant (p = 0.031), but no significant difference was observed between HF and WJD. The lowest DOC concentration was observed in CH (Fig. 2-a). The CDOM/DOC ratio was calculated to estimate the relative contribution of CDOM to the bulk DOC. In the three lakes, the average CDOM/DOC (L mg-C−1 m−1
Conclusion
Based on the spectral analysis of DOM, a mixing model consisting of two end-members of terrestrial and aquatic origins showed that porewater DOM characteristics in three Chinese lakes varied, from predominantly a terrestrial origin in the pristine lake (CH) to more algal- and microbial-derived DOM in the HF and WJD lakes. Our results highlight that while the relationships between DOC and Hg were inconsistent among lakes, DOM optical properties were useful in elucidating the sources of DOM and
Acknowledgments
This work is financially supported by National Natural Science Foundation of China (41273099, 41403079), Sino-Swedish Mercury Management Research Framework (SMaRef) of Swedish Research Council (contract number D697801), and the National Key Basic Research Program of China (973 Program) (2013CB430004). Dr. Tao Jiang would personally thank Swedish Research Council to US (No. 621-2014-5370) for generously sponsoring his research position in Swedish University of Agricultural Sciences. Dr. Andrea G
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