Rapid Adjustments in Thermal Tolerance and the Metabolome to Daily Environmental Changes – A Field Study on the Arctic Seed Bug Nysius groenlandicus

Abstract

Laboratory investigations on terrestrial model-species, typically of temperate origin, have demonstrated that terrestrial ectotherms can cope with daily temperature variations through rapid hardening responses. However, few studies have investigated this ability and its physiological basis in the field. Especially in polar regions, where the temporal and spatial temperature variations can be extreme, are hardening responses expected to be important. Here, we examined diurnal adjustments in heat and cold tolerance in the Greenlandic seed bug Nysius groenlandicus by collecting individuals for thermal assessment at different time points within and across days. We found a significant correlation between observed heat or cold tolerance and the ambient microhabitat temperatures at the time of capture, indicating that N. groenlandicus continuously and within short time-windows respond physiologically to thermal changes and/or other environmental variables in their microhabitats. Secondly, we assessed underlying metabolomic fingerprints using GC-MS metabolomics in a subset of individuals collected during days with either low or high temperature variation. Concentrations of metabolites, including sugars, polyols, and free amino acids varied significantly with time of collection. For instance, we detected elevated sugar levels in animals caught at the lowest daily field temperatures. Polyol concentrations were lower in individuals collected in the morning and evening and higher at midday and afternoon, possibly reflecting changes in temperature. Additionally, changes in concentrations of metabolites associated with energetic metabolism were observed across collection times. Our findings suggest that in these extreme polar environments hardening responses are marked and likely play a crucial role for coping with microhabitat temperature variation on a daily scale, and that metabolite levels are actively altered on a daily basis.

Stress Responses and Candidate Gene Expression Investigated Across Species and Ecosystems in the Field

“…To investigate how the environment affects thermal tolerances and potentially induces alterations in gene expression levels in N. groenlandicus, we conducted two experiments. First, a field experiment (section 2.1.1) was carried out to examine how natural temperature dynamics in concert with omnipresent environmental variables affect heat tolerance, measured as 'heat knockdown time' (HKDT), and cold tolerance, measured as 'temperature at chill coma recovery' (T recovery ; procedure described in Noer et al (2022b)). In addition, the underlying gene expression levels were investigated in individuals collected simultaneously in the field using an RNA-seq approach.…”

“…The field work was conducted in July-August 2018 in Narsarsuaq, Greenland (61.160°N, 45.424°W). Insect collections and thermal tolerance assays followed the procedures described in Noer et al (2022b) with small modifications. Briefly, adult individuals of N. groenlandicus were collected at four time points during the day for five days.…”

“…Accordingly, several studies have examined the potential for plasticity through rapid cold hardening (RCH) in polar terrestrial arthropods and they have shown how RCH can improve survival and reproduction after exposure to cold extremes (Lee et al, 1987;Sinclair et al, 2003aSinclair et al, , 2003b. Less is known about rapid physiological adjustments that can alter heat tolerance at short temporal scales, such as heat hardening (HH), despite arctic and subarctic species being frequently exposed to high daily temperature peaks at a microhabitat scale (Mølgaard, 1982;Noer et al, 2022b). Moreover, most knowledge on RCH and HH in arthropods, including species from Arctic and Subarctic regions, is derived from studies conducted under highly controlled laboratory conditions (but see Teets et al, 2020).…”

“…Moreover, most knowledge on RCH and HH in arthropods, including species from Arctic and Subarctic regions, is derived from studies conducted under highly controlled laboratory conditions (but see Teets et al, 2020). However, acclimation and hardening responses to diurnal and seasonal variation in temperature and other environmental variables have been shown to differ between laboratory and field conditions and more field studies are needed to fully understand thermal plasticity under field conditions Noer et al, 2022b).…”

“…The species is univoltine and utilizes high local temperatures to complete its life cycle within the short summer season where daily temperatures can vary by >30°C and reach subzero temperatures at night. Recent work has revealed that this species has a broad thermal tolerance range Noer et al, 2022b) and that it can rapidly increase acute heat tolerance when exposed to high and stressful temperatures under laboratory and field conditions (Noer et al, 2022b). However, we lack a comprehensive understanding of temporal changes in thermal tolerances of this species in the field and the molecular responses associated with diurnal environmental variation.…”