Physio-chemical effects of freshwaters on the dissolution of elementary mercury

Heavy metal movement through insect food chains in pristine thermal springs of Yellowstone National Park

Yellowstone National Park thermal features regularly discharge various heavy metals and metalloids. These metals are taken up by microorganisms that often form mats in thermal springs. These microbial mats also serve as food sources for invertebrate assemblages. To examine how heavy metals move through insect food webs associated with hot springs, two sites were selected for this study. Dragon-Beowulf Hot Springs, acid-sulfate chloride springs, have a pH of 2.9, water temperatures above 70 °C, and populations of thermophilic bacterial, archaeal, and algal mats. Rabbit Creek Hot Springs, alkaline springs, have a pH of up to 9, some water temperatures in excess of 60 °C, and are populated with thermophilic and phototrophic bacterial mats. Mats in both hydrothermal systems form the trophic base and support active metal transfer to terrestrial food chains. In both types of springs, invertebrates bioaccumulated heavy metals including chromium, manganese, cobalt, nickel, copper, cadmium, mercury, tin and lead, and the metalloids arsenic, selenium, and antimony resulting from consuming the algal and bacterial mat biomass. At least two orders of magnitude increase in concentrations were observed in the ephydrid shore fly Paracoenia turbida, as compared to the mats for all metals except antimony, mercury, and lead. The highest bioaccumulation factor (BAF) of 729 was observed for chromium. At the other end of the food web, the invertebrate apex predator, Cicindelidia haemorrhagica, had at least a 10-fold BAF for all metals at some location-year combinations, except with antimony. Of other taxa, high BAFs were observed with zinc for Nebria sp. (2180) and for Salda littoralis (1080). This accumulation, occurring between primary producer and primary consumer trophic levels at both springs, is biomagnified through the trophic web. These observations suggest trace metals enter the geothermal food web through the microbial mat community and are then transferred through the food chain. Also, while bioaccumulation of arsenic is uncommon, we observed five instances of increases near or exceeding 10-fold: Odontomyia sp. larvae (13.6), P. turbida (34.8), C. haemorrhagica (9.7), Rhagovelia distincta (16.3), and Ambrysus mormon (42.8).

Dark oxidation of mercury droplet: Mercurous [Hg(I)] species controls transformation kinetics

Liquid elemental mercury droplet (Hg(0)l) is an important species in heavy Hg-contaminated environments. The oxidation processes of Hg(0)l and its related mechanisms are still poorly understood. Herein, for the first time, it was verified that mercurous species [Hg(I)] was an important species in natural water contaminated by Hg(0)l as well as in the simulated dark oxidation of Hg(0)l. The formation and further transformation of Hg(I) controlled the overall oxidation process of Hg(0)l and were affected by different environmental factors. Through kinetic modeling using ACUCHEM program, oxidation of Hg(0) to Hg(I) (Hg(0) → Hg(I)) was determined to be the rate-limiting step in Hg(0)l oxidation because its k value ((8.7 ± 0.21) × 10-11s-1) is seven orders of magnitude lower than that of Hg(I) oxidation (Hg(I) → Hg(II), (4.7 ± 0.15) × 10-4s-1). Ligands like OH-, Cl-, and natural organic matter enhanced the formation of Hg(I) via promoting the constants of comproportionation (up to (9.5 ± 0.78) × 10-4s-1). These findings highlight the importance of Hg(I) in Hg(0)l oxidation process by controlling the transformation kinetics of Hg species, facilitating an improved understanding of the environmental redox cycles of Hg.