Abstract
Anthropogenic pressures, such as contaminant exposure, may affect stable isotope ratios in biota. These changes are driven by alterations in the nutrient allocation and metabolic pathways induced by specific stressors. In a controlled microcosm study with the amphipod Gammarus spp., we studied effects of the β-blocker propranolol on stable isotope signatures (δ15N and δ13C), elemental composition (%C and %N), and growth (protein content and body size) as well as biomarkers of oxidative status (antioxidant capacity, ORAC; lipid peroxidation, TBARS) and neurological activity (acetylcholinesterase, AChE). Based on the known effects of propranolol exposure on cellular functions, i.e., its mode of action (MOA), we expected to observe a lower scope for growth, accompanied by a decrease in protein deposition, oxidative processes and AChE inhibition, with a resulting increase in the isotopic signatures. The observed responses in growth, biochemical and elemental variables supported most of these predictions. In particular, an increase in %N was observed in the propranolol exposures, whereas both protein allocation and body size declined. Moreover, both ORAC and TBARS levels decreased with increasing propranolol concentration, with the decrease being more pronounced for TBARS, which indicates the prevalence of the antioxidative processes. These changes resulted in a significant increase of the δ15N and δ13C values in the propranolol-exposed animals compared to the control. These findings suggest that MOA of β-blockers may be used to predict sublethal effects in non-target species, including inhibited AChE activity, improved oxidative balance, and elevated stable isotope ratios. The latter also indicates that metabolism-driven responses to environmental contaminants can alter stable isotope signatures, which should be taken into account when interpreting trophic interactions in the food webs.
Highlights
In human physiology, the natural variations of the isotopic ratios of carbon, nitrogen and other major elements comprising biomass (δ15N, δ13C, δ18O, δ2H, δ44/40Ca, etc.) are attracting increasing attention since they offer a new means to study the imbalances linked to pathological conditions [1,2]
The between-group principal component analysis (bgPCA) suggested a lower similarity between the animals exposed to the high propranolol concentration (PH) and the control compared to that for the low propranolol (PL) and control groups (Fig 2)
The projection of PH treatment on PC1 suggested that high propranolol exposure coincided with increased %N yet decreased protein content and lowered oxidative damage, i.e., low levels of TBARS and high levels of oxygen radical absorbance capacity (ORAC): TBARS ratio (Table 2)
Summary
The natural variations of the isotopic ratios of carbon, nitrogen and other major elements comprising biomass (δ15N, δ13C, δ18O, δ2H, δ44/40Ca, etc.) are attracting increasing attention since they offer a new means to study the imbalances linked to pathological conditions [1,2]. There is ample evidence that consumer Δ-values and the isotopic signatures may vary depending on various endogenous and environmental factors via their effects on metabolism and growth. These factors include variations in moulting status [3], food quantity [4,5] and quality [6], temperature [7,8], and contaminant exposure [9]. Isotope signatures have been reported to respond to changes in oxidative status [10], suggesting that both δ15N and δ13C values can reflect the diet and shifts in the balance between antioxidative and pro-oxidative processes. Under toxic exposure variations in δ-values may be explained by changes in the relative abundance of heavy and light isotopes due to the elevated oxidation
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