Mercury bioaccumulation in fishes of a paddy field in southern of Brazil

Abstract

The aim of present study was to evaluate the Hg concentration in two species of fish (Astyanax sp and Corydoras paleatus) and its potential use as a biomonitor, in order to know if the use of pesticides and fertilizers in paddy can enhance the Hg contamination to adjacent aquatic environment. Methods: Soil, suspended particulate matter and fish samples were sampled in a paddy field in South Brazil. A cold vapor system, coupled with a GBC 932 atomic absorption spectrophotometer was used for total Hg determinations in samples. Results: The paddy soil shows Hg concentration 2-fold higher (mean 31 ng g-1) in comparison to background areas (not cultivated). Suspended particle matter Hg concentration in paddy channels (mean 232.5 ± 44.2 ng g–1) are 1.5 times higher than the regional background. The analyzed fish specimens Astyanax sp in paddy showed Hg concentration 4-fold higher and significant different to background area. The mean Hg concentration in fish was: 51.7 ± 19.5 ng g–1 in Astyanax sp and 156.8 ± 44.0 ng g–1 in Corydoras paleatus. Conclusions: Considering the linear regression and Man whitney test hypothesis to Hg concentration in fish tissue from paddy suggests that Astyanax sp. can be a good biomonitor of Hg contamination, whereas Corydoras paleatus is a potential biomonitor. However, more studies with Corydoras are necessary in order to aggregate consistency to this hypothesis

O objetivo do presente estudo foi avaliar a concentração de Hg em duas espécies de peixes (Astyanax sp e Corydoras paleatus) e seu uso potencial como biomonitor, a fim de saber se o uso de pesticidas e fertilizantes na lavoura pode levar a contaminação por Hg ao ambiente aquático adjacente. Métodos: Solo, material particulado em suspensão e amostras de peixes foram coletadas em um campo de arroz no sul do Brasil. Um sistema de vapor frio acoplado a um espectrofotômetro de absorção atômica (GBC 932) foi usado para as determinações de Hg total nas amostras. Resultados: O solo da lavoura de arroz apresentou concentração de Hg 2 vezes mais elevada (média de 31 ng g–1), em comparação com a área controle (não cultivado). A concentração de Hg no material particulado em suspensão dos canais de irrigação do arroz (média de 232,5 ± 44,2 ng g–1) é 1,5 vezes mais elevada do que na área controle. Os espécimes de peixes Astyanax sp analisados na área de cultivo mostraram concentração de Hg 4 vezes maior e, significativamente diferentes da área controle. A concentração média de Hg em peixes foi: 51,7 ± 19,5 ng g–1 em Astyanax sp e 156,8 ± 44,0 ng g–1 em Corydoras paleatus. Conclusões: Considerando a regressão linear e o teste de hipótese de 192 Kütter, V.T. et al. Acta Limnologica Brasiliensia Since 1908, the rice production is cultivated in paddies in the Southern Brazil (Beskow, 1986). Nowadays, Brazil is the ninth largest rice producer in the world (Wong, 2004). The Rio Grande do Sul State produces 68% of the national production (CONAB, 2013). This region is part of the Pampa Biome that covers about 63% of Rio Grande do Sul State (IBAMA, 2004). The Pampa has a great biodiversity including endemic species, some of them endangered or at risk of extinction (Marques et al., 2002). Notwithstanding the considerable number of endangered species, this region has received little attention in terms of environmental research (Bencke, 2010). The paddy has a considerable impact on aquatic animals, since many species that inhabit nearest areas, swamps and lagoons, move to them after the flood. Furthermore, this environment has great ecological significance because it represents an important feeding, resting and breeding area for birds, including migratory species that comes from North and South America’s (Dias & Burger, 2005). Studies with fish demonstrated that this animal group is a good indicator of environmental health (Van der Oost et al, 2003; Raimundo et al., 2011). In this group, the main Hg incorporation in tissue occurs through feeding (Kidd et al., 1995; Snodgrass et al., 2000). The MeHg is absorbed by the gut and accumulated in tissues, where it can be biomagnified because of the long half-life of this compound (Lacerda & Malm, 2008). Therefore, as predator fish are in the top of the food web, they have the highest concentrations of this metal in relation to species which are at the bottom. Mirlean et al. (2005), who investigated Hg levels in lakes near the paddy region in the Southern Brazil, found that the main source of Hg in this area is the atmospheric deposition. In this same study, the authors have proposed the use of Astyanax sp as a bioindicator of Hg contamination in studied area, once these species have shown a good correlation with Hg levels in the environment. The genus Astyanax has been appointed by several authors as an excellent biomonitor of aquatic ecosystems, responses of countless biomarkers, such as histopathological assays (Prado et al., 2011; 1. Introduction Mercury (Hg) is considered a highly toxic metal and has been used in the composition of pesticides utilized in rice fields (Smart & Hill, 1968). In Brazil, the use of pesticides containing Hg was banned in the 70’s. Although the input of anthropogenic Hg had been stopped at these sites after the banishment, its effects persist in the environment and biota. Studies on paddies in Japan have reported an insignificant reduction of Hg soil contamination over eight years of study (Nakagawa & Yumita, 1998). Furthermore, more recent studies in paddies reported high concentration of methylmercury (MeHg) in rice grains (Zhang et al., 2010a,b; Zhao et al., 2010; Zhu et al., 2011; Peng et al., 2012; Rothenberg et al., 2012; Li et al., 2013). In Brazil, the study developed by Silva et al. (2010) found Hg concentrations ranging from 2.2 to 4.4 ng g-1 in rice grains. Actually the large use of fertilizers in the rice production can intensify the methylation process in paddy fields due to bacteria growth stimulation. Amongst the three Hg forms, the organic form MeHg is the most toxic to humans even in low concentrations (Zahir et al., 2005). Although Hg presents low concentration in water, it can be potentially biomagnified, reaching very higher concentrations in predatory fish, which can result in an increased risk of developing health problems when consumed by humans (NRC, 2000). According to the report of the Committee on the Toxicological Effects of Methylmercury in the United States, the major source of human exposure to MeHg is the consumption of contaminated fish (NRC, 2000). Innumerous studies have been demonstrated that more than 80% of the total Hg present in fish tissue is in the form of MeHg (Silva et al., 2011; Kannan et al., 1998). The characteristics of paddy are favorable to the process of Hg conversion into MeHg by anaerobic microorganisms (Rothenberg & Feng, 2012). Moreover, some studies have demonstrated that this process can also occur in the intestine of some fish species when they are fed with inorganic mercury (Rudd et al., 1980; Leaner & Mason, 2002). de Man Whitney para a concentração de Hg em tecidos de peixes da lavoura de arroz, sugere que Astyanax sp. pode ser uma bom biomonitor de contaminação Hg, enquanto Corydoras paleatus é um biomonitor potencial. No entanto, mais estudos com Corydoras são necessárias, a fim de agregar consistência a este estudo