Tracing the Uptake, Transport, and Fate of Mercury in Sawgrass

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Tracing the uptake, transport, and fate of mercury in sawgrass (Cladium jamaicense) in the Florida Everglades using multi-isotope technique Bo Meng, Yanbin Li, Wenbin Cui, Ping Jiang, Guangliang Liu, Yongmin Wang, Jennifer H Richards, Xinbin Feng, and Yong Cai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04150 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Environmental Science & Technology

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Tracing the uptake, transport, and fate of mercury in sawgrass (Cladium

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jamaicense) in the Florida Everglades using multi-isotope technique

3 4

Bo Meng†,‡✽,Yanbin Li§, Wenbin Cui‡, Ping Jiang‡, Guangliang Liu‡,║, Yongmin

5

Wang‡,#, Jennifer Richards⟘, Xinbin Feng†, Yong Cai‡,║✽

6 7

†

8

Academy of Sciences, Guiyang 550002, P. R. China

9

‡

State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese

Department of Chemistry and Biochemistry, Florida International University, Miami, Florida

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33199, USA

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§

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University of China, Qingdao, 266100, China

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║

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33199, USA

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⟘Department

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#Current address: College of Resources and Environment, Southwest University, Chongqing

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400715, China

18

✽

19

Bo Meng

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Phone: 86-851-84391736. Fax: 86-851-5891609. E-mail: [email protected]

21

Yong Cai

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Phone: 1-305-348-6210. Fax: 305-348-3772. E-mail: [email protected]

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean

Southeast Environmental Research Center, Florida International University, Miami, Florida

of Biological Science, Florida International University, Miami, Florida 33199, USA

Corresponding Authors:

1

ACS Paragon Plus Environment


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Abstract

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The role of macrophytes in the biogeochemical cycle of mercury (Hg) in the Florida

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Everglades is poorly understood. Stable isotope tracer techniques were employed to

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investigate Hg uptake by sawgrass (Cladium jamaicense) from soil and from the

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atmosphere pathways, and the fate of Hg after absorption. Our results suggest that soil

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spiked 201Hg2+ was rapidly taken up by roots and transported to aboveground parts.

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The spiked 201Hg that was transported to the aboveground parts was trapped; no

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release of the spiked 201Hg from the leaf to the air was detected. Atmospheric 199Hg0

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exposure experiments revealed that the majority of the previously deposited 199Hg0

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taken into the leaf was fixed, with a very limited proportion (1.6 %) available for

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re-emission to the atmosphere. The percentage of 199Hg0 fixed in the leaf will help

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reduce the model uncertainty in estimating the Hg0 exchange over air-vegetation

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surface. We propose that sawgrass needs to be viewed as an important sink for

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atmospheric Hg0 in the regional Hg mass balance; this would have important

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implications for the critical loads of Hg to the Everglades. The multi-isotope tracer

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technique could be an effective tool to identify the role of plants in biogeochemical

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cycling of Hg in other ecosystems.

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1. Introduction

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Mercury (Hg) has received great attention as a global pollutant due to its ability

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to undergo methylation, accumulation, and biomagnification in aquatic food chains 1.

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Wetlands are widely known as sites for methylmercury (MeHg) production 2-4, and

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are generally considered net sources of MeHg5. In the Florida Everglades, MeHg is

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biomagnified and can reach high levels in fish and fish-consumers 6, 7. Death of a

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Florida panther, a federally endangered species, had been attributed to Hg poisoning 8,

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9

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between the atmosphere and substrate sediments 4, 10-13. Previous studies showed that

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Hg emission from water and, in particular, transpiration through macrophytes,

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represents an important pathway of Hg removal, with 10% of seasonally deposited Hg

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being evaded out of this ecosystem 4, 12-14. The exact source of the Hg0 emitted from

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macrophytes is unknown, but origins either from soil, water or from atmospheric

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deposition (previously deposited Hg0) are reasonable. Numerous studies have

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suggested that vegetation could assimilate both inorganic and organic Hg from soil

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solution through their roots and transport the Hg to the foliage; this Hg may then be

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emitted to the atmosphere via the stomata 15, 16. While the emission of Hg0 from the

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foliage is undisputed, previous studies have not been able to distinguish the source of

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re-emitted Hg due to technical limitations.

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. Emergent macrophytes in the Everglades are important contributors to Hg exchange

Using stable isotope tracer technique, Mao et al. 17 provides an understanding of

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uptake pathways of Hg within sawgrass in the Everglades. However, it is still

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unknown whether the Hg0 emitted from foliage was derived from the atmospherically

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deposited Hg or from Hg taken up from the soil solution and subsequently converted

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to a volatile form. Based on the measurements of Hg accumulation in foliar over a

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growing season, Rea et al. 18 suggested that only 25% of Hg0 (݂௙௜௫௘ௗ ) through

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atmospheric dry deposition to canopy is permanently fixed into leaf tissues and not

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available for re-emission 18. Recently, an updated model was developed to estimate

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the Hg0 exchange over air-stomata surface 19, 20. It is clear that the estimated Hg0 flux

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over air-stomata surface is dependent on the ݂௙௜௫௘ௗ during the calculations.

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Unfortunately, direct evidence for the fraction of Hg0 fixed into tissues is lacking.

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Therefore, large uncertainty was unavoidable when estimating the global natural

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emission of Hg0 over air-foliar surface 19-21.

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The role of macrophytes within the Everglades as sources or sinks of

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atmospheric Hg is not well understood and several key questions are not answered.

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Do plants simply recycle Hg by taking it up from the soil and then return it by way of

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litterfall, or do leaves directly capture atmospheric Hg and re-emit it to the air as a

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new source? To address these questions, this study focused on the role of sawgrass in

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Hg cycling in the Everglades, as sawgrass is the dominant macrophyte in the

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Everglades wetland, covering approximately 50-75% of the landscape 22, 23. In order

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to characterize the transport of Hg across air-vegetation-soil interfaces, multi-isotope

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tracer techniques were used to trace the different processes. We quantified the uptake

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and transportation of Hg within tissues of sawgrass in a laboratory controlled system

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using an enriched elemental Hg vapor (199Hg0) and a mercuric Hg isotope (201Hg2+) as

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isotopic tracers. The Hg flux between the foliage and atmosphere was measured using

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a dynamic flux bag. The objectives of this study were to 1) assess the uptake and

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transportation of Hg species in sawgrass tissues via root and atmospheric pathways; 2)

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quantify the storage of Hg in tissues of sawgrass; and 3) elucidate whether sawgrass is

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a net source or sink of Hg in the Everglades ecosystem.

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2. Materials and methods

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2.1 Chemicals and Materials

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Enriched 199HgO and 201HgO was purchased from Oak Ridge National

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Laboratory (Oak Ridge, Tennessee). 199HgCl2 and 201HgCl2 solution were prepared by

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dissolving 199HgO and 201HgO in 10% HCl (v/v), respectively. The enriched 199Hg0

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was generated from a Hg vapor generator during the course of atmospheric exposure

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experiments. Other reagents used in this study were at least of analytical grade and

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were purchased from Fisher Scientific Company L.L.C. USA.

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2.2 Soil and sawgrass plants

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Wetland peat soil samples were collected from an Everglades marsh (25°45′57′′ N, 80°29′54.2′′ W). The soil was air-dried, and sieved (4 mm mesh) for homogeneity.

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Sawgrass (Cladium jamaicense) seedlings with 4-5 mature leaves (20-40 cm in length)

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were purchased from Aquatic Plants of Florida, Inc., Sarasota, FL, USA.

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Approximately 290 g (dry weight) homogenized soil was transferred into

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polyethylene pots (15 cm deep x15 cm diameter). Three sawgrass seedlings were

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planted into each pot. In order to simulate natural growing conditions, the sawgrass

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pots were kept outdoor on shelves suspended in a water-filled polyethylene tank

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(3410 L with 1 m deep x 2.1 m diameter) in a fenced yard located at Florida

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International University, Miami, Florida. Flooded soil conditions, with ~2 cm of

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water above soil surface in sawgrass pots, were maintained during the experiment.

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Tap water was used to replenish water in the tanks and in the experimental pots

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throughout the experiment. The plants were allowed to stabilize in the tanks for 1

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month prior to experimentation.

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2.3 Soil 201Hg2+ spiking experiments

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In order to investigate the uptake and transport of Hg across soil-vegetation

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interface and subsequent fates of Hg after absorption, two experimental groups of 9

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pots each (9 pots for control and 9 pots for treatment groups) were carefully designed

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in this study. In detail, enriched isotope of 201Hg2+ tracer solution prepared in

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deionized water (201Hg2+=16 mg L-1as Hg) was spiked into the soil in the pots.

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Spiking solution of 20 mL was applied using a disposable syringe at five positions

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evenly distributed in each pot in soil depth of 5 cm. Previous study confirmed that the

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majority (>90%) of total Hg (THg) in sawgrass leaves was obtained from atmospheric

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Hg, rather than from soil source 17. To obtain sufficient isotopic signal, the

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concentration of spiked 201Hg2+ was designed to be 1100 µg kg-1 for Hg for each of

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the 9 treatment pots, which was approximately 4 times higher than THg levels in

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Everglades soil 4. Immediately after spiking, the surface of the pots was covered with

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plastic lids to reduce the possible loss of Hg through emission from soil, consequently

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changing Hg isotope ratios in above ground part of plant. The lids contained a center

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hole through which the leaves emerged. The center hole was plugged using a

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polyethylene foam collar around the leaves.

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Sampling was initiated the first day after spiking with the Hg isotopes (Day 1),

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and thereafter at 30 and 60 days. Hg vapor flux over sawgrass leaves (1 pot) was

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quantified for 24 hrs at each sampling time as described in Supporting Information

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(SI). After the gas-exchange experiments were complete, the sawgrass plants and all

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the soil from 3 pots per treatment were collected for THg isotopic analysis. Detailed

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information on sample collection and preparation is given in the SI.

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2.4 Atmospheric Hg isotope (199Hg0) exposure experiments

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As shown in SI Figure S1, a flux bag coupled with a stable Hg isotope addition

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technique was used to directly track the short-term uptake and re-emission of Hg0 at

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the air-leaf surface. Plant leaves were exposed to isotopically enriched gaseous

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elemental 199Hg0 (g) for 24 hrs. Exposure experiments were conducted under natural

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plant growth conditions and atmospherically relevant Hg0 (g) concentrations in a

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custom-made flux bag (SI Figure S1). The enriched 199Hg0 stable isotope was pumped

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into the flux bag from a Hg vapor generator (Gas Liquid Separator, M055G003, P S

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Analytical, UK). The generated 199Hg0 was evaded by N2 gas (20 ml min-1) and mixed

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with natural air delivered from a pump, then transferred to a chamber from its top

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through two evenly distributed outlet Teflon pipes at a flow rate of ~11 L min-1 using

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a vacuum pump (P-7902-00, Cole Parmer Instrument Corporation, USA). The target

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enriched 199Hg0 concentration in the flux bag was jointly controlled by the flow rate of

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the carrier gas (ambient air) and the mass of the generated 199Hg0.

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We quantified the mass of enriched 199Hg0 taken up by sawgrass leaves during

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24 hr exposure (SI Figure S2). After exposure for 24 hrs, enriched 199Hg0 flux over

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the leaf-air surface was then measured for 24 hrs using the flux bag to determine the

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release of 199Hg to the atmosphere in the subsequent day (Day=1). Thereafter, the

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re-emission of the enriched 199Hg was sampled again at Day 30 (30 days after the

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exposure experiment) and Day 60. A detailed description of experimental design is

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described in the SI, including quantifying the mass of enriched 199Hg0 taken up by

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sawgrass and the re-emission of the 199Hg from leaf to the air. To determine the

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location and transport of enriched 199Hg within sawgrass plant, sawgrass and

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corresponding soil (3 triplicate pots) were collected following each of the flux

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measurements for enriched THg isotopes analysis, as described in detail in the SI.

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After each of the flux measurements, the flux bag was carefully cleaned and

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stored in a Hg free room (see details in the SI). In all cases, flux measurements for

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each sampling campaign were conducted at 1 hour intervals with fluxes being

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calculated every hour. Mercury captured on gold traps was analyzed for Hg0 isotopes

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by ICP-MS as described in the SI.

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2.5 Analytical methods

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Detailed information concerning analytical methods are described in the SI,

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including Hg0 isotopes captured on gold trap, individual THg isotopes analysis of soil

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samples and sawgrass plant tissues.

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2.6 Quality control

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Quality assurance and control (QA/QC) were carried out throughout the field and

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laboratory sampling and analysis processes. All flux bag components were rigorously

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cleaned before use. Before flux measurements, tests were routinely processed to

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determine the exchange of Hg0 with surfaces of the empty exposure chamber. This

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information is available in the SI. The QA/QC data for the isotopic Hg measurement

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consisted of blanks, triplicate, and the parallel analysis of several certified reference

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materials as described in the SI.

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3. Results and discussion

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3.1 Absorption, transport, and fate of soil Hg in the soil-plant-air system

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3.1.1 Variations of THg in soil. During the growing period, the mean concentrations

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of ambient THg in soil samples collected from control and treatment groups (Hg

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isotope spiked in soil) were 253 ±19 and 269±4.9 µg kg-1, respectively. The

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concentrations of ambient THg in the soil of control and treatment groups showed a

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narrow range of variation with time (Figure S3). Similar to ambient THg, the enriched

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T201Hg in soil samples from the treatment group showed a narrow range of variation

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with time during the growing period (SI Figure S3). During the growing period, the

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levels of T201Hg in soil slightly deceased from Day 1 (1005 ± 118 µg kg-1) to Day 60

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(947 ± 67 µg kg-1), indicating that the majority (94%) of the spiked 201Hg2+ still

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remained in the soil. Because of the reducing conditions in wet land soil, enriched

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201

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studies suggested that wetland plants have the ability to stimulate the release of Hg

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from soil into pore water and the rhizosphere 24, which in turn accelerate the reduction

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of Hg2+ in the rhizosphere. Therefore, the slight decrease in T201Hg in soil during the

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growing period could be attributed to 1) the continual transport of 201Hg2+ from soil to

Hg2+ could be reduced to 201Hg0 by biotic or abiotic processes 12. Moreover, previous

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plant, 2) the emission of formed 201Hg0 over soil-air surface 11, 3) and the adsorption

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of enriched 201Hg on surface of the experimental pot.

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3.1.2 Enriched T201Hg in tissues of sawgrass. If root uptake and translocation of Hg

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was a potential contributing pathway to foliar Hg, the enrichments would be

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detectable in the plant within the time frame of this exposure study. During the

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growing period, the average abundance of 201Hg in plant tissues of control groups was

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13.35 ± 0.12, which comparable with the natural abundance of 201Hg (13.2). The

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measured abundance of 201Hg in samples from control groups indicates that that the

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control groups was not contaminated with the enriched Hg isotopes during growing

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period. Our results showed that T201Hg abundance in sawgrass tissues increased above

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the natural abundances of corresponding individual Hg isotopes throughout the 60

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days of exposure (Figure 1-A, B), indicating that net accumulation was occurring.

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Concentrations of enriched T201Hg in live leaves increased during the growing

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period from Day 1 (5.9±1.9 µg kg-1) to Day 30 (10±2.0 µg kg-1) (Figure 1-A). The

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enrichments of T201Hg in leaves at Day 1 indicated that soil spiked 201Hg was

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translocated to the aboveground parts from root uptake and that root uptake of

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enriched T201Hg was likely a potential source of foliar T201Hg under these

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experimental conditions. Using a stable isotope tracer technique, Mao et al. 17

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suggested that on average 7.4±3.7% of the THg in sawgrass leaves were estimated to

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be from soil, which support our results. Enriched T201Hg concentrations in senescing

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leaves showed a consistent increase from Day 1 (5.0 ± 1.2 µg kg-1) to Day 60 (12 ±

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1.1 µg kg-1) after application. Live leaves showed a decrease in enriched T201Hg

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concentration at Day 60 and enriched T201Hg concentrations in senescing leaves were

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significantly higher than in live leaves at Day 60 (p90%) of uptake was from atmospheric

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sources. The current study, agreeing well with the previous work17, emphasizes that

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sawgrass foliage needs to be viewed as an important sector for atmospheric Hg

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deposition. When leaves senesce and fall off the sawgrass plant, Hg retained in

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litterfall represents a new input of fixed atmospheric Hg to the Everglades. As leaf

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litter is decomposed, fixed Hg can be re-distributed into the soil. Hall and St Louis 36

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observed that Hg in litterfall could be methylated even on dry soils, suggesting that

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litterfall may be a source of MeHg to wetland ecosystems. The results of the present

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investigation did not agree with previous studies by Lindberg et al. 11, 12, where it was

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found that Everglades leaf surfaces acted as biogenic sources of Hg vapor for ambient

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air. Because gas-exchange systems measure only the net direction of Hg flux,

Hg0, together with the observations of soil spiked T199Hg located in sawgrass

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458

previous studies could not distinguish the relative contributions of soil vs.

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atmospheric sources of Hg0.

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This work presents important environmental implications on the biogeochemical

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cycling of Hg in Everglades. If ambient Hg0 in the air interacts with foliage like our

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experimentally applied spiked 199Hg0, the majority of the Hg0 taken up by sawgrass

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leaves would be unavailable for re-emission to the air. As a result, atmospheric Hg

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provided a substantial flux of Hg0 via stomatal and/or nonstomatal absorption when

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compared with inputs from direct soil uptake and transportation. This suggests that

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the input of Hg into the Florida Everglades by sawgrass leaves could be larger than

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previously estimated4. Uptake of the atmospheric Hg0 reservoir by sawgrass leaves

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could substantively increase the Hg load, and thus it is important to evaluate the Hg

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input through litterfall in order to fully understand the biogeochemical cycling of Hg

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in the Florida Everglades.

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We demonstrate in this study the relative importance of sawgrass as receptors for

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Hg in wetland ecosystems and suggest that sawgrass leaves must be considered a sink

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for atmospheric Hg in the regional Hg mass balance. This information is useful for

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accurately quantifying the critical loads of Hg to the Everglades, as currently direct

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wet deposition is often considered being the only major Hg input to the ecosystem 4.

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Furthermore, our study provides useful information for creating models of natural

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deposition of Hg and fills gaps in our understanding of the role of wetland vegetation

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in the biogeochemical cycling of Hg in the Everglades. The data processing

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techniques employed here for constructing a mass inventory and mass budget of Hg

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could be applied to other ecosystems.

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Acknowledgements

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This research was supported by the National Natural Science Foundation of China

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(41473123, 91543103, 21677061, and 41673025). This is contribution # of the

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Southeast Environmental Research Center at FIU.

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Supporting Information Available

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Measurement of Hg flux over leaf-air surface; quantifying the mass of enriched 199Hg0

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untaken by sawgrass and the re-emission of the 199Hg to the atmosphere; sample

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collection and preparation; protocol for individual THg isotopes analysis in soil

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sample and plant sample; the QC/QA protocol and statistical analysis of analytical

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data; The biomass of sawgrass plant in different groups; 2 Tables; 4 Figures. This

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

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597

28


Page 29 of 35


598

Table 1. Hg isotope ratios in inlet and outlet airstreams of the flux bag over time for

599

soil and atmosphere Hg isotopes spike experiments (mean ± SD, range; n=3) 201

Growing periods (day) 1 30 60

Hg/202Hg

199

Hg/202Hg

Inlet

Inlet

Inlet

Outlet

0.45±0.01

0.45±0.01

0.57±0.01

0.64±0.04

(0.44-0.46)

(0.44-0.46)

(0.55-0.58)

(0.59-0.74)

0.44±0.01

0.44±0.01

0.56±0.003

0.56±0.01

(0.43-0.45)

(0.43-0.45)

(0.55-0.57)

(0.55-0.57)

0.44±0.01

0.44±0.01

0.56±0.01

0.56±0.01

(0.42-0.45)

(0.42-0.45)

(0.55-0.57)

(0.54-0.57)

600

29



Page 30 of 35

601

Figure captions:

602

Figure 1. Concentrations (A) and total mass (B) ofenriched T201Hg in sawgrass

603

tissues (n = 3) at different sampling times.

604

Figure 2. Temporal variations over 24 hrs of the hourly surface Hg0 flux over

605

sawgrass leaves (bars) and incoming solar radiation (line) in treatment groups of soil

606

201

607

Figure 3. Temporal variation over 24 hrs of the hourly surface spike 199Hg0 flux over

608

sawgrass leaves (bars) and incoming solar radiation(lines) during the 24 hrs exposure

609

(A) and at Day 1 following the 24 hrs exposure to 199Hg0 (B).

610

Figure 4. Concentrations (A, n = 3) and relative distributions (B) over time

611

ofenriched199Hg in sawgrass tissues.

Hg2+ spiking experiments at Day 1 (A), Days 30 (B) and Days 60 (C).

612 613

30


Page 31 of 35


Figure 1

600

(A)

400 Root

Stem

Senescing leaf

Live leaf

* Data multiplied by five times

201

Enriched T

200

0

615

Root

Stem

Senescing leaf

Live leaf

300

200

201

400

(B)

-1

−1

Hg concentration (µg kg )

800

Enriched T Hg mass (ng plant )

614

** Day=1

**

** Day=30

Growing periods (day)

Day=60

100

0

Day=1

Day=30

Day=60


31



Figure 2

Hg(0) flux(ng m

−2

−2

1000 0

0 -1000

-25 -2000 16:54

-50

(B)

5:56

Hg(0) flux

12:12

-3000 3000

Solar radiation

Hg(0) flux(ng m

−2

−1

hr )

50

23:40

Time of day/hr:m(Day=1)

2000 25

6:00 pm

6:00 am

1000

0

0 -1000

-25 -2000 23:32

-50

25

Hg(0) flux(ng m

−2

−1

hr )

50

617

(C)

5:30

11:35

17:33

Time of day/hr:m(Day=30) Hg(0) flux

Solar radiation

-3000 3000 2000

6:00 am 6:00 pm

1000

0

0 -1000

-25 -2000 -50

18:40

0:37

6:29


Solar radiation (watts m )

2000

6:00 am

6:00 pm

25

3000

Solar radiation

12:26

−2

Hg(0) flux


(A)

−2

−1

hr )

50


616

Page 32 of 35

-3000

618

32


Page 33 of 35


Figure 3 3000

Solar radiation

−2

200

2000

6 pm

6 am

100

−2

Hg(0) flux

1000

0

0

-100

-1000

-200

-2000

−1

2:22

7:20

12:33

Time of day/hr:m

(B)

Hg(0) flux

-3000 3000

Solar radiation

2000

6 pm 5

6 am 1000

0

0

Enriched

199

0

Hg flux(ng m

−2

10

21:25

−2

16:24

-300

hr )

Enriched

199

0

Hg flux(ng m

(A)


−1

hr )

300

620

-1000 -5 -2000 -10

18:26

22:30

4:29

10:43



619

16:35 -3000

621

33



Figure 4

Root

Stem

Senescing leaf

(B)

Live leaf

Hg mass (ng)

(A)

199

10

Enriched T

199

Enriched T

623

80

15

−1

Hg concentration (µg kg )

622

Page 34 of 35

5

0

Day=1

Day=30

Day=60

Root

Stem

Senescing leaf

Live leaf

60

40

20

0

Day=1


Day=30

Day=60


624

34


Page 35 of 35


625

TOC

626 627

35


Tracing the Uptake, Transport, and Fate of Mercury in Sawgrass

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