frogmetal

Amphibians and Zinc Toxicity in an Urban Environment

Metals are essential to life in trace amounts. As key elements in developmental processes, metals are most commonly obtained through dietary sources and are either readily available for use or are converted to a usable form within the organism. Although heavy metals occur naturally from geologic, volcanic, and marine sources, anthropogenic use of metals has increased the levels and occurrence of these materials throughout the environment. Heavy metals, defined by Linder and Grillitsch (2000) as "a group of metallic elements with atomic weights greater than 40...", are of particular concern as environmental contaminants because small amounts of these metals can be highly toxic to exposed organisms, and can result in bioconcentration, bioaccumulation, and biomagnification.

One source of metals that may lead to problematic exposures to wildlife is urban stormwater runoff, a non-point source of metal pollution originating primarily from vehicles and building materials. Up to 60% of zinc in stormwater runoff has been attributed to tire weathering (for review see Camponelli et al. 2009), but engine oil and brakes, and building materials such as brick, concrete, painted wood and roof materials, may also be a significant source (Davis et al. 2001). Zinc levels in stormwater tend to be higher than other metals, followed by lead, copper, then cadmium, a pattern consistent with findings of metal release from vehicles and building materials (Davis et al. 2001). The removal of lead from petrol has reduced the lead input from automobiles to stormwater, but lead is still released from building materials including brick and painted wood (Davis et al. 2001). Brake pads and linings provide a considerable source of copper, followed by building siding (Davis et al. 2001). The consistent input of metals to the environment leads to an accumulation of metals from stormwater runoff in sediments of stormwater ponds (Bishop et al. 2000, Marsalek et al. 2002). This wikipage discusses the potential physiological and ecological impacts on amphibians developing in stormwater ponds in the context of exposure to metals, specifically zinc, in stormwater runoff.

Stormwater ponds are designed to mitigate flows and contaminants associated with stormwater runoff in urban areas. As one of the few sources of fresh surface water in developed landscapes, stormwater ponds are used by wetland wildlife species, including breeding amphibians. Developing amphibians such as salamander and frog larvae feed on invertebrates or detritus (respectively) in the stormwater ponds. Larval amphibian gas exchange occurs predominantly via gills and skin, with minor if any contribution from pulmonary components (Ultsch et al. 1999). Subsequently these organisms intake metal contaminants through direct exposure in the aquatic environment, and through ingestion of food resources (Linder & Grillitsch 2000).

Zinc concentrations may be of particular concern to amphibians in stormwater ponds. Not only is it received in high levels relative to other metals, it appears to have significant toxicological effects on development. Bishop et al. (2000) found increased time to metamorphosis of leopard frog (//Rana pipiens//) tadpoles in stormwater ponds related to increased zinc concentrations in the sediment. Those findings were repeated by Camponelli et al. (2009) with wood frog (//Rana sylvatica//) tadpoles exposed to two sediment zinc treatments (ZnCl2 and tire particulate matter). Additionally, Camponelli et al. (2009) found decreased mass at time of metamporphosis with increased time to complete metamorphosis. In stormwater ponds, where water levels may drop rapidly after winter rains cease, increased time to metamorphosis can result in the loss of whole cohorts of tadpoles that don't "make it out" in time. Additionally, reduced size may increase the time it takes individuals to cross the mowed, homogenous landscapes typical of stormwater pond facilities and surrounding landscapes, thereby increasing the risk of predation and dessication. Therefore zinc may result in increased production of less fit individuals, leading to a decline or loss of amphibian populations in urban areas.

The specific physiology of zinc toxicity to amphibians has not been fully described. Metallothioneins (MT) are low molecular weight metal-binding proteins with high affinity for cadmium, copper, and zinc that play a role in homeostasis of essential metals and in metal detoxification (for review see Klaasen et al. 1999). Metallothionein production is induced by metals (for example Chen et al. 2007, Saintjacques et al. 1995), and is specifically controlled by a zinc-mediated metallothionein transcription inhibitor (MTI), which disassociates from the zinc-finger transcription factor, MTF-1, in the presence of zinc (Klaasen et al. 1999). The synthesis of MT results in the binding of zinc and the subsequent reformation of the MTF-1/MTI complex (Klaasen et al. 1999), a process which may be disrupted under conditions of chronic zinc exposure. However, results from hepatic MT levels in neotenic salamanders suggest that zinc may also be bound by proteins with higher molecular weights than MT (Dobrovoljc 2003). As with many physiological processes, MT levels are variable according to season, however they are still a viable biomarker for exposure to metal contaminants. Falfushynska et al. (2008) showed seasonal variation in MT levels of marsh frogs (//Rana ridibunda//), however the frogs from their urban (contaminated) site showed consistently higher levels of MT across seasons, indicating a response to contaminant-related stressors, though their results are not implicit to metal contaminants. Finally, Palmiter (2004) shows that zinc toxicity may also be mediated by zinc transporter 1 (ZnT-1), when induced this gene results in greater protection against zinc toxicity and decreased free zinc in baby hamster kidney cells. The role of ZnT-1 in amphibians is not yet known.

Contaminants in stormwater runoff affect the survivability of amphibians in urban environments. Natural selection will certainly play a role in shaping urban amphibian populations, with more resistant individuals shaping future generations (Snodgrass et al. 2008). Although amphibians no longer hold a great economic status, they are important components even of urban ecosystems, where they contribute to the control of mosquitoes (Brodman & Dorton 2006, DuRant & Hopkins 2008), transport nutrients between aquatic and terrestrial environments (Davic & Welsh 2004, Regester et al. 2008), fill an important prey niche (Gonzalez 1997), help educate the public about their surrounding environment (Richter & Ostergaard 1999), and act as sentinels of environmental conditions (Blaustein & Wake 1990). Understanding the ecological and physiological response of amphibians to stressors in urban environments, including ubiquitous heavy metals such as zinc, is key to prediciting population changes and management needs.

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