Original investigationCoping with environmental stress: The effects of wastewater pollutants on energy stores and leptin levels in insectivorous bats
Introduction
With rapid urbanisation and human expansion, animals are increasingly exposed to anthropogenic environmental stressors (Vitousek et al., 1997). One such stressor is wastewater treatment works (WWTWs), that introduce toxicants and pollutants such as metals, endocrine disrupting chemicals and microplastics into the environment through exposed settlement tanks and released effluent (Naidoo et al., 2013, Park et al., 2009). These pollutants can have severe negative impacts on the fitness of organisms (Bremner, 1998, Sánchez-Chardi et al., 2009). However, organisms that are able to exploit resources at WWTWs, yet tolerate exposure to associated toxicants, often thrive (Park and Cristinacce, 2006, Park et al., 2009). For example, pollutant-tolerant chironomid midges (Diptera) are highly abundant at WWTW sites, which, in turn, attracts insectivorous animals such as bats and birds (Naidoo et al., 2011, Naidoo et al., 2013, Park and Cristinacce, 2006). Although midges can survive in polluted environments, they accumulate toxicants in their bodies (Hare et al., 1991, Krantzberg and Stokes, 1990) that may have lethal or sub-lethal effects on their predators.
Sub-lethal effects of WWTW pollutant exposure can include a more pronounced immune response (Lum et al., 2007), changes in metabolic rate, and an inability to maintain energy stores of glycogen and lipids in exposed organisms (Martin et al., 2003, Zhang et al., 2012). Glucose, the major fuel for metabolic activity, is produced via glycogenolysis (the break-down of glycogen) and gluconeogenesis (the synthesis from pyruvate, glycerol, lactate and alanine) in the liver (Bollen et al., 1998, Srivastava and Krishna, 2010, Wallace and Barritt, 2002). With exposure to stress, these processes are upregulated to accommodate for the increased glucose demands of affected organs (Martin et al., 2003). However, as glycogen stores become depleted, lipid catabolism (lipolysis) is increased to breakdown fats into glycerol and free-fatty acids (Wallace and Barritt, 2002). As fat stores become depleted, muscle protein is catabolised to produce amino acids such as alanine (Wallace and Barritt, 2002). Gluconeogenesis can be upregulated due to stress, and is fuelled by metabolic end products such as alanine, glycerol and lactic acid (Bollen et al., 1998, Wallace and Barritt, 2002). Lactic acid, produced in muscle tissue via anaerobic respiration, may also be converted to glucose via gluconeogenesis in response to stressors (Himwich et al., 1930). Furthermore, the depletion of fat reserves when energy stores are compromised may lead to the down-regulation of the satiation hormone, leptin (Ahima and Flier, 2000, Geiser, 2004, Morris et al., 1994). One of the main functions of leptin in hibernating bats is to regulate fat storage and metabolic rate during the prehibernatory phase (Kronfeld-Schor et al., 2000). Because leptin is responsible for maintaining energy homeostasis, particularly in small mammals such as bats, the down-regulation of this hormone may result in a reduction in metabolism and the onset of torpor to conserve energy until stores can be replenished (Ahima and Flier, 2000, Geiser et al., 1998, Westman and Geiser, 2004, Morris et al., 1994).
Eliciting stress responses, such as a response to the pollutants associated with WWTWs, is energetically costly (Hochachka and Somero, 2002), and may affect key physiological processes of metabolic rate and energy utilisation. Chronic exposure to stressors, such as organochlorine exposure, can significantly increase the food intake and metabolic rate of the insectivorous bat Pipistrellus pipistrellus (Swanepoel et al., 1999). However, it is unknown whether bats may be able to cope with the long term exposure to pollutants whilst exhibiting higher metabolic rates.
The small insectivorous bat Neoromicia nana (Vespertilionidae, Chiroptera) often hunts midges at WWTWs, and consequently is exposed to metals such as cadmium, chromium and nickel (Naidoo et al., 2011, Naidoo et al., 2013, Naidoo et al., 2015). We have previously shown that foraging at WWTWs is detrimental to N. nana. More specifically, they exhibited increased haematocrit and DNA damage as well lesions in their livers and kidneys (Naidoo et al., 2015, Naidoo et al., 2016). This study aimed to determine the effects of foraging at WWTW on the energy stores and leptin levels of N. nana. We predicted that bats exposed to WWTW toxicants would rely on anaerobic respiration and exhibit depleted muscle and liver energy stores. We also predicted lower leptin levels compared to bats foraging at reference sites to facilitate lower energy expenditure during rest as an energy saving mechanism. In adipose tissues, leptin is under direct control of hypoxia inducible factor (Hif), the main modulator of oxygen homeostasis, and we therefore expected Hif-1α levels to be low in WWTW bats at rest.
Section snippets
Material and methods
We collected N. nana from four sites during April 2015: Verulam (29.646241°S 31.063543°E) and Umbilo (29.845283°S 30.890776°E) WWTWs; and Bufflesdrift (29.756730°S 30.678980°E) and Inkunzi Lodge (28.565201°S 31.241312°E) reference sites. Bats were captured with mist nets at WWTWs and by hand from roosts at reference sites. We identified bats to species using a taxonomic key (Monadjem et al., 2010), and released non-target animals at the capture site. Captured N. nana bats were sexed and
Results
Sample size varied between sites (Verulam: 11 ♂ and 2 ♀; Umbilo: 5 ♂ and 8 ♀, Bufflesdrift: 5 ♂ and 7 ♀, Inkunzi: 1 ♂ and 3 ♀). We found no significant differences in forearm length (x2 = 1.798, df = 3, P = 0.615), body mass (x2 = 0.771, df = 3, P = 0.857), or BCI (F(3,26) = 0.381, P = 0.767) between sites. For physiological analyses, we used only animals for which we also had blood samples (Verulam: 6 ♂ and 1 ♀; Umbilo: 2 ♂ and 6 ♀; Buffelsdrift: 5 ♂ and 6 ♀; Inkunzi: 1 ♂ and 3 ♀) and for molecular analyses
Discussion
In correspondence with our predictions, WWTW bats showed increased reliance on anaerobic respiration. Pectoral muscle lactic acid levels were higher in bats from WWTWs, specifically those at Verulam WWTW, than in bats from reference sites. In polluted environments, limited oxygen and increased immune response may result in elevated energy expenditure, which increases the demand for glucose to fuel high ATP turnover (Lum et al., 2007, Martin et al., 2003, Pilosof et al., 2014), and may even
Acknowledgements
This work was funded by the National Research Foundation (grant number CSUR14080687212 to DV and CMS, and scholarships to GM and CM), and supported by the University of KwaZulu-Natal. We thank two anonymous reviewers for their valuable suggestions.
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CM and GM contributed equally to the writing of the manuscript.