Properties influencing flux and diatom uptake of mercury and methylmercury from estuarine sediments

Abstract

Mercury (Hg) is a conspicuous and persistent global pollutant. Ionic Hg can be methylated into noxious methylmercury (CH₃Hg), which biomagnifies in marine tropic webs and poses a health risk to humans and organisms. Sediment Hg methylation rates are variable, and the output flux of created CH₃Hg are dependent on sediment characteristics and environmental factors. Thus, uncertainties remain about the formation and flux of CH₃Hg from sediment, and how this could contribute to the bioaccumulative burden for coastal organisms in shallow ecosystems. Cores were collected from 3 estuarine locations along the Eastern USA to examine how sediments characteristics influence the introduction of Hg and CH₃Hg into the base of the food chain. Stable isotopes of inorganic ²⁰⁰Hg and CH₃¹⁹⁹Hg were injected into sediments of individual cores, with cultured diatoms constrained to overlying waters. Five different treatments were done on duplicate cores, spiked with: (1) no Hg isotopes (control); (2) inorganic ²⁰⁰Hg; (3) CH₃¹⁹⁹Hg; (4) both ²⁰⁰Hg and CH₃¹⁹⁹Hg isotopes, (5) both ²⁰⁰Hg and CH₃¹⁹⁹Hg into overlying waters (not sediment). Experimental cores were incubated for 3 days under temperature and light controlled conditions. These results demonstrate that upper sediments characteristics lead to high variability in Hg cycling. Notably, sediments which contained abundant and peaty organic material (∼28 %LOI), had the highest pore water DOC (3206 μM) and displayed bands of sulfur reducing bacteria yielded the greatest methylation rate (1.97 % day^-1) and subsequent diatom uptake of CH₃²⁰⁰Hg (cell quota 0.18 amol/cell) in the overlying water.

Keywords: DOC; Methylation; Organic matter; Phytoplankton; Stable isotopes; Sulfide.