The importance of experimental time when assessing the effect of temperature on toxicity in poikilotherms

Abstract

Article (refereed) -postprintNorhave, Nils J.; Spurgeon, David; Svendsen, Claus; Cedergreen, Nina. 2014. The importance of experimental time when assessing the effect of temperature on toxicity in poikilotherms. Environmental Toxicology and Chemistry, 33 (6). 1363-1371. 10.1002/etc.2563 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner.Acc e pted P r e pr i nt Acc e p ted P r e p r i nt Abstract: Temperature is an important factor affecting toxicity, determining chemical toxicokinetics and toxicodynamics in poikilothermic organisms. As also metabolic rates are affected by temperature, interactions between the emergence of toxic effects and time are very likely. The aim of this study was to investigate how temperature affected the toxicity of copper towards the nematode Caenorhabditis elegans when measured during short fixed timeframes compared to during full life-cycles. Copper toxicity was tested in two experiments at four or six temperatures in the range of 11 to 24˚C, with copper concentrations spanning from one to 40 mg Cu/L agar, respectively. Reproduction and mortality were determined across the entire life-cycle, and the time to production of first egg and the population growth rate were calculated. The results showed that the 50% effect concentrations (EC50) of copper increased 1.5 to 2.5-fold with increasing temperature within the tested range, depending on endpoint. When calculating EC50 on reproduction after 24 or 96 hours, as is the typical setup for temperature/chemical interaction studies, results ranged from no temperature effect to effects much larger than for the full life cycle. Studies of temperature effects on toxicity must therefore be carefully designed in relation to the research question investigated.

Variable Temperature Stress in the Nematode Caenorhabditis elegans (Maupas) and Its Implications for Sensitivity to an Additional Chemical Stressor

“…Reproduction, final body length, lifespan and PGR could be described with concentration-response curves for all studies under the different temperature regimes ( Table 3 ). Consistent with our hypothesis, increasing temperature to constant 24°C resulted in a lower EC 50 for all responsive traits than at 12, 16 and 20°C, consistent with previous work and expectations [ 22 ]. The largest shift in sensitivity was seen for PGR for which the EC 50 reduced from 61.2 mg Cu L -1 at 12°C to 25.5 mg Cu L -1 at 20°C.…”

“…elegans to Cu-stress at the different constant temperatures partly confirmed the study of Nørhave et al [ 22 ], who found a 1.5 to 2.5-fold decrease in Cu EC 50 increasing temperatures from 11 to 24°C, and in addition found stimulations of reproduction and PGA at low Cu concentrations indicative of hormesis. However, going beyond Norhave et al [ 22 ], the inclusion of a lower temperature treatment at 8°C (as compared to the lowest temperature treatment of 11°C used by [ 22 ]) showed an increase in sensitivity to Cu at low temperature, based on the EC 50 values for the different endpoints ( Table 3 ). Hence, an innately stressful temperature regime was shown to increase the sensitivity of C .…”

“…That the warmest temperature of 24°C did not in itself prove stressful was itself quite surprising as this temperature is close to temperatures that have previously been shown to affect traits, such as reduced brood size at 25°C [ 22 ] and growth, reproduction and survival at 25.5°C [ 21 ]. Transition from optimal temperature to stressful temperature at the higher range, therefore, seems to occur within a small temperature range for the species.…”

“…The largest shift in sensitivity was seen for PGR for which the EC 50 reduced from 61.2 mg Cu L -1 at 12°C to 25.5 mg Cu L -1 at 20°C. Additional inclusion of a treatment at 8°C, which was not included in the study of Norhave et al [ 22 ], identified an increased Cu sensitivity of nematodes kept at this lower temperature, most notably for broodsize ( Table 3 ). This sensitivity was not consistent with our initial hypothesis of increased sensitivity with increasing temperature.…”

“…Temperatures were changed with a rate of 4°C per hour to simulate changes at two points within a 24 hr period separating two stable temperature periods to provide an simulation of a daily temperature regime under condition of high diurnal variation ( Fig 1 ). Separate toxicity tests with copper were conducted under all temperature regimes using concentration known to result in changes in measured traits [ 22 ]. Copper concentrations used were 0, 1, 3, 8, 20 and 40 mg Cu/L agar.…”