Mercury (Hg) is a global pollutant due to its ability to undergo methylation, accumulation, and biomagnification in aquatic food chains. Wetlands are widely known as sites for methylmercury (MeHg) production, and are generally considered as net sources of MeHg.

In Florida Everglades, MeHg is biomagnified and can reach high levels in fish and fish-consumers, posing a potential threat to wild animals. Emergent macrophytes in the Everglades are important contributors to Hg exchange between atmosphere and substrate sediments.

The role of macrophytes within the Everglades as sources or sinks of atmospheric Hg is not well understood due to technical limitations. Therefore, large uncertainty was unavoidable when estimating the global/regional natural emission of Hg⁰ over air-foliar surface.

Dr. MENG Bo from the Institute of Geochemistry，Chinese Academy of Sciences (IGCAS) and Prof. CAI Yong from Florida International University (FIU) characterized the role of sawgrass in Hg cycling in the Everglades. In their study, the stable isotope tracer techniques were employed to investigate Hg uptake by sawgrass (Cladium jamaicense) from soil and from the atmosphere pathways as well as the fate of Hg after absorption.

The results suggested that soil spiked ²⁰¹Hg²⁺ was rapidly taken up by roots and transported to aboveground parts. The spiked ²⁰¹Hg that was transported to the aboveground parts was trapped and no release of the spiked 201Hg from the leaf to the air was detected (Fig.1).

Atmospheric ¹⁹⁹Hg⁰ exposure experiments revealed that the majority of the previously deposited ¹⁹⁹Hg⁰ taken into the leaf was fixed, with a very limited proportion (1.6 %) available for re-emission to the atmosphere (Fig. 1). The percentage of ¹⁹⁹Hg⁰ fixed in the leaf will help reduce the model uncertainty in estimating the Hg⁰ exchange over air-vegetation surface.

The researchers further proposed that sawgrass should be viewed as an important sink for atmospheric Hg⁰ in the regional Hg mass balance. This information would have important implications for the critical loads of Hg to the Everglades.

Furthermore, this study provides useful information for creating models of natural deposition of Hg and fills gaps in understanding of the role of wetland vegetation in the biogeochemical cycling of Hg in the Everglades. The multi-isotope tracer technique could be an effective tool to identify the role of plants in biogeochemical cycling of Hg in other ecosystems.

Their findings were published in Environmental Science & Technology.

Fig.1 Role of sawgrass in Hg cycling in the Everglades (Image by IGCAS & FIU )

Contacts:

MENG　BO 
Institute of Geochemistry, Chinese Academy of Sciences, China
Email: mengbo@vip.gyig.ac.cn

CAI YONG
Department of Chemistry and Biochemistry, Florida International University, USA
Email: cai@fiu.edu 

（By Prof. FENG Xinbin's group）