DOI: 10.5061/dryad.7m0cfxpz7
title: Dataset and scripts from: Dietary sterol availability modulates heat tolerance of Daphnia

description: The increasing frequency and intensity of summer heat waves is pushing
 freshwater zooplankton towards their upper thermal tolerance limits. At
 the same time, higher temperatures and prolonged water column
 stratification can favor the dominance of cyanobacteria in phytoplankton.
 Even when not toxic or grazing resistant, these prokaryotes lack
 phytosterols as essential precursors for cholesterol, the main sterol in
 animal tissues. Cholesterol plays a crucial role in the physiological
 adaptation of ectotherms to high temperature. Therefore, the shift to
 cyanobacteria-dominated systems may increase the vulnerability of
 zooplankton to heatwaves by intensifying cholesterol
 limitation. Here, we used death time curves that take into
 consideration the intensity and duration of a thermal challenge and a
 dynamic model to study the effects of cholesterol limitation on the heat
 tolerance of the keystone species Daphnia magna and to simulate the
 cumulative mortality that could occur in a fluctuating environment over
 several days of heatwave.  We show that increasing cholesterol
 limitation does not affect the slope between time-to-immobilization and
 temperature, but does decrease the maximal temperature that Daphnia can
 withstand by up to 0.74°C. This seemingly small difference is sufficient
 to halve the time individuals can survive heat stress.  Our
 simulations predicted that, when facing heatwaves over several days, the
 differences in survival caused by cholesterol limitation build up rapidly.
 Considering the anticipated intensity and duration of future (2070-2099)
 heatwaves, cholesterol limitation could increase mortality by up to 45%
 and 72% under low- and medium-greenhouse-gas-emission scenarios,
 respectively. These results suggest that the increasing risk of
 cholesterol limitation due to more frequent cyanobacterial blooms could
 compromise the resistance of zooplankton populations to future heatwaves.
 More generally, this study shows the importance of considering the
 nutritional context in any attempt to predict ectotherm mortality with
 increasing temperatures in the field.
description: We used death time curves to study the effects of cholesterol
 limitation on the heat tolerance of the keystone species Daphnia magna and
 to simulate the cumulative mortality that could occur in a fluctuating
 environment over several days of heatwave. In that aim, D. magna neonates
 were grown under controlled laboratory condition on a sterol-free,
 non-toxic cyanobacterial diet (Synnechococcus obliquus) supplemented with
 different amounts of cholesterol-containing liposomes to obtain four
 cholesterol concentrations of 0.25, 2.5, 5, and 8 µg per mg C. In all
 treatments, a small amount of eicosapentaenoic acid (20:5ω3) was also
 added via liposomes (resulting in 0.25 µg 20:5ω3 per mg C) in order to
 prevent severe 20:5ω3 limitation, thereby aiding the development of
 daphnids. The daphnids were grown at 27°C, a non-lethal yet super-optimal
 temperature. Heat tolerance (time to immobilization) of the daphnids from
 the different cholesterol treatments was tested on day seven at five
 different constant temperatures (34.5°, 35.5°, 36.5°, 37.5°, 38.5°C).
 Here, we provide the raw data, as well as the R scripts used to analyze
 the data.