Mercury exposure and mechanism of response in large game using the Almadén mercury mining area (Spain) as a case study

Abstract

Mercury (Hg) accumulation, transfer, defense mechanism and adverse effects were studied in red deer (Cervus elaphus) and wild boar (Sus scrofa) from the Almadén mining district (Spain), the largest (285,000 t of Hg) and the oldest (more than 2000 years) Hg mine/refining operation site in the world. Red deer (n=168) and wild boar (n=58) liver, kidney, bones (metacarpus), testis and muscle were analyzed for total Hg and selenium (Se) within a range of distances to the Almadén mining district. The highest Hg concentrations were found in kidney (0.092 and 0.103 μg/g d.w. for red deer and wild boar, respectively) followed by the levels in liver (0.013 and 0.023 μg/g d.w. for red deer and wild boar, respectively). A significant correlation (r=−0.609, p=0.007) was found between Hg concentrations and distance to the Almadén Hg mining district. However, both red deer and wild boar closest to the mining area still showed mercury concentrations well below the concentration associated with clinical signs of Hg poisoning. Highest Se concentrations were found in kidney (2.60 and 6.08 μg/g in red deer and wild boar, respectively) and testis (2.20 μg/g in red deer). For selenium, differences between red deer and wild boar were statistically significant (p<0.05) in all tissues, concentrations being higher in wild boar than in red deer. In the diagnosis of Se deficiency, the vast majority of the examined red deer livers were deficient. A significant correlation was found between Hg and Se in kidney (r=0.386, p>0.001 for red deer and r=0.567, p=0.005 for wild boar). Liver GSSG concentrations were negatively correlated to total mercury and Hg:Se molar ratio in male deer, which could indicate a hormetic response to Hg exposure. Moreover, a positive association was found between the antioxidant element Se and antioxidant vitamins in red deer tissues.

Introduction

The environmental transport and fate of mercury (Hg) is a matter of global concern because it is a non-essential toxic element that occurs in different chemical and physical forms (Mason and Benoit, 2003, Watras and Huckabee, 1992). The most important forms are elemental Hg (Hg⁰), inorganic Hg (Hg²⁺) and methylmercury (MeHg), which differ with respect to kinetics and toxicology (WHO, 1990, WHO, 1991, Clarkson, 1997, Agency for Toxic Substances and Disease Registry (ATSDR), 1999, Rodrigues et al., 2010). MeHg is known as a very important neurotoxicant that bioacumulates in the aquatic food chain, with fish or seafood being the main source for human exposure (Watras and Huckabee, 1992, Clarkson, 1997). Most studies have been devoted to the aquatic environments, while little attention has been paid to Hg in terrestrial ecosystems until now (Cristol et al., 2008, Gnamus et al., 2000, Wren, 1986, Lodenius, 1994, Lodenius, 1995, Boudou and Ribeyre, 1997). However, the majority of total Hg (approximately 60%) is estimated to be deposited on terrestrial environments close to contamination sources, leading to significant accumulation in local food webs (Fitzgerald and Mason, 1996). Therefore, there are several reasons why the study of Hg in terrestrial animals is of great interest. Firstly, major differences exist compared to the aquatic ecosystem; thus the mechanisms of Hg accumulation and transfer are probably very different. Additionally, part of human exposure to Hg pollution occurs through the consumption of terrestrial animal tissues. Finally, important information about the adverse impact of metal pollution, in terms of lethal and/or sublethal effects, and protection/detoxification mechanisms can be obtained from the study of Hg and Se status of these animals, and may be extrapolated to estimate the impact on humans.

In this context, the study of Hg accumulation and transfer in terrestrial animals in sites with past or active Hg mining constitutes a unique opportunity. Metal mining generated large volumes of crushed waste rock (spoil) and tailings with high concentrations of residual heavy metals. Heavy metal from mining and smelting commonly contaminates air, water and soil, and this contamination affects biota. A special case of interest for the study of Hg in terrestrial biota is the area of the recently closed Almadén Hg mine (Ciudad Real Province, Southern Spain). The Almadén mining district has produced one third of the total Hg production worldwide (285,000 t of Hg). Mining operation began more than 2000 years ago and no other region in the world has been influenced by Hg for such a long time. Therefore it is one of the largest Hg-contaminated sites and it continues to generate significant atmospheric emissions despite its closure in May 2002. Some recent studies have been conducted to evaluate the environmental impact and potential hazards related to Hg contamination in the Almadén ecosystem (Berzas Nevado et al., 2003, Gray et al., 2004, Higueras et al., 2006). Mercury contamination of soil, water, sediment and the atmosphere has been reported at Almadén (Berzas Nevado et al., 2003, Gray et al., 2004, Higueras et al., 2006, Hildebrand et al., 1980, Ferrara et al., 1998, Rodríguez Martin-Doimeadios et al., 2000; Berzas Nevado et al., 2009). On the other hand, the area is largely devoted to and occupied by hunting estates, where several hundred red deer and wild boar are hunted every year. Exposure of wildlife species to Hg can occur through food chains or through the direct ingestion or licking of soil to attain essential elements (Beyer and Fries, 2003, Beyer et al., 2007). The study of Hg distribution in different tissues from these terrestrial animals can provide important information regarding Hg exchanges and transformations since the long-term Hg exposition in Almadén probably yielded a steady cycling state. Moreover, the interactions between Hg and selenium (Se) should be examined, owing to the fact that Se has been described as an antagonist to Hg and the ratio of Hg to Se concentration has been suggested to be a more significant parameter to define the level of intoxication than the Hg concentration alone (Carvalho et al., 2008, Carvalho et al., 2010, Ralston et al., 2008, Ralston and Raymond, 2010, Yang et al., 2008).

Here, we focus on the study of total Hg and Se concentration in red deer (Cervus elaphus) and wild boar (Sus scrofa) from the Ciudad Real Province (Southern Spain), where samples were collected within a range of distances to the Almadén mining district. The selected tissues were liver, kidney, bones (metacarpus), testis and muscle. The objectives of the present study were as follows: (1) to examine the environmental impact of Hg pollution in wildlife and species of interest for hunting; (2) to assess the route of Hg and its transfer into terrestrial organisms tissues and (3) to identify possible toxicity effects and defense mechanisms (Se interaction).