The good, the bad, and the toxic: approaching hormesis in Daphnia magna exposed to an energetic compound

Abstract

A hormetic response is characterized by an opposite effect in small and large doses of chemical exposure, often resulting in seemingly beneficial effects at low doses. Here, we examined the potential mechanisms underlying the hormetic response of Daphnia magna to the energetic trinitrotoluene (TNT). Daphnia magna were exposed to TNT for 21 days, and a significant increase in adult length and number of neonates was identified at low concentrations (0.002-0.22 mg/L TNT), while toxic effects were identified at high concentrations (0.97 mg/L TNT and above). Microarray analysis of D. magna exposed to 0.004, 0.12, and 1.85 mg/L TNT identified effects on lipid metabolism as a potential mechanism underlying hormetic effects. Lipidomic analysis of exposed D. magna supported the hypothesis that TNT exposure affected lipid and fatty acid metabolism, showing that hormetic effects could be related to changes in polyunsaturated fatty acids known to be involved in Daphnia growth and reproduction. Our results show that Daphnia exposed to low levels of TNT presented hormetic growth and reproduction enhancement, while higher TNT concentrations had an opposite effect. Our results also show how a systems approach can help elucidate potential mechanisms of action and adverse outcomes.

Figure 1

a) Number of neonates per survivor (as % of control treatment) in Daphnia exposed to TNT from range-finding exposure A; b) Daphnia length (as % of control treatment) in Daphnia exposed to TNT from range-finding exposure A; c) Daphnia length (as % of control treatment) in Daphnia exposed to TNT from range-finding exposure B.

Figure 2

Results from the transcriptional Benchmark Concentration analysis showing significant toxicological functions (a) and pathways (b) used for functional enrichment after TNT exposure.

Figure 3

Concentration of different lipids (mg lipid/daphnia) at different TNT concentration. Letters indicate statistical significance after pairwise comparison (p<0.05). The lipids shown are: a) Total sphingomyelin (SM) and dihydrosphingomyelin (DSM), b) Total lysophosphatidylethanolamine (LPE), c) Total phosphatidylglycerol (PG), d) Total phosphatidic acid (PA), e) Total lysophosphatidylcholine (LPC), f) Total lipids

Figure 4

Concentration of different lipid isomers (mg lipid/daphnia) at different TNT concentration (significant changes indicated by *, p<0.05). A) phosphatidic acid isomers; b) lysophosphatidylethanolamine isomers; c) lysophosphatidylcholine isomers.

Figure 5

concentration of different free fatty acids (mg lipid/daphnia) at different TNT concentrations. Letters indicate statistical significance after pairwise comparison (p<0.05). Letters are not added to FFA without any significant changes. The free fatty acids are: a) total free fatty acids; b) 14:0, 14:1, 14:2 free fatty acids; c) 16:0, 16:1, 16:2, 16:3 free fatty acids; d) 18:0, 18:1, 18:2, 3:3 free fatty acids, e) 20:3, 20:4, and 20:5 free fatty acids.