Effect of repeated freeze-thaw cycles on geographically different populations of the freeze tolerant wormEnchytraeus albidus(Oligochaeta)

Abstract

Freeze-tolerant organisms survive internal ice formation; however, the adaptations to repeated freeze-thaw cycles are often not well investigated. Here we report how three geographically different populations of Enchytraeus albidus (Germany, Iceland and Svalbard) respond to three temperature treatments -constant thawed (0°C), constant freezing (−5°C) and fluctuating temperature (0 to −5°C) -over a period of 42 days. Survival varied between treatments and populations such that enchytraeids from arctic locations had a higher survival following prolonged freeze periods compared with temperate populations. However, enchytraeids from temperate locations had the same survival rate as arctic populations when exposed to repeated freeze-thaw events. Across all populations, metabolic rate decreased markedly in frozen animals (−5°C) compared with thawed controls (0°C). This decrease is likely due to the lower temperature of frozen animals, but also to the transition to the frozen state per se. Animals exposed to repeated freeze-thaw events had an intermediate metabolic rate and freeze-thaw events were not associated with pronounced excess energetic costs. Overwintering under either condition was not associated with a decrease in lipid content; however, during exposure to constant freezing and repeated freeze-thaw events there was a noticeable decrease in carbohydrate stores over time. Thus, animals exposed to constant freezing showed a decrease in glycogen stores, while both glucose and glycogen content decreased over time when the organisms were exposed to repeated freezing. The results therefore suggest that carbohydrate resources are important as a fuel for E. albidus during freezing whereas lipid resources are of marginal importance.

Population‐specific transcriptional differences associated with freeze tolerance in a terrestrial worm

“…The strain from Greenland tolerates much lower temperatures and endures freezing for much longer periods than worms from Germany (Fisker et al. ; Silva, Holmstrup, Kostal, & Amorim, ). Henceforth, we term the Greenland strain “freeze tolerant,” while the Germany strain is termed “freeze sensitive.” Our main aim was to identify key transcriptomic pathways linked to physiological alterations associated with freeze tolerance.…”

“…It should be noted that both strains survive this freezing exposure (Fisker et al. ). Each replicate contained ten pooled animals to obtain sufficient RNA for sequencing.…”

“…This species is freeze tolerant and has a wide physiological tolerance range to freezing temperatures in its habitat (Slotsbo et al., ; Fisker et al. ). In general, freeze tolerance of ectothermic animals is promoted by adaptive modifications of membrane lipid composition that reduce the risk of deleterious phase transitions of cellular membranes during cold exposure, that is, homeoviscous adaptation (Kostal, ), and the accumulation of low molecular weight cryoprotectants, that can reduce the ice fraction at sub‐zero temperatures (Zachariassen, ).…”

“…In E. albidus , freeze tolerance involves active mobilization of glucose (from glycogen reserves) starting when the freezing process initiates and is therefore more akin to freeze tolerance of vertebrate ectotherms like frogs than of insects where cryoprotectant loading takes place months before the actual freezing of hemolymph starts (Fisker et al. ; Holmstrup, Costanzo, & Lee, ; Slotsbo et al., ). Strains of this species inhabiting different geographic locations have different freeze tolerance capacities.…”

Adaptations of enchytraeids to single and combined effects of physical and chemical stressors

“…al., 2009; Crowe et al, 1987; Fisker et al, 2015; Hazel and Williams, 1990) Avoid Ice formation, by removing ice-nucleating particles and producing antifreeze proteins, which associated with cryoprotectants leads to a depression of the freezing point (Ramløv, 2000; Zachariassen, 1985; Zachariassen and Husby, 1982; Zachariassen and Kristiansen, 2000).Allow cryoprotective dehydration i.e., losing water (dehydrate) until the melting point of their body fluid and the water vapor pressure in the surrounding atmosphere reaches an equilibrium, so that no freezing can occur.Decrease the cellular metabolism to save energy(Calderon et al, 2009;Fisker et al, 2014a;Irwin and Lee, 2003;Storey and Storey, 1988) No (low) mortality of inoculative freezing(Holmstrup and Zachariassen, 1996; Patrício Silva et al, 2013b) …”