Anaerobic Metabolism at Thermal Extremes: A Metabolomic Test of the Oxygen Limitation Hypothesis in an Aquatic Insect

Abstract

Thermal limits in ectotherms may arise through a mismatch between supply and demand of oxygen. At higher temperatures, the ability of their cardiac and ventilatory activities to supply oxygen becomes insufficient to meet their elevated oxygen demand. Consequently, higher levels of oxygen in the environment are predicted to enhance tolerance of heat, whereas reductions in oxygen are expected to reduce thermal limits. Here, we extend previous research on thermal limits and oxygen limitation in aquatic insect larvae and directly test the hypothesis of increased anaerobic metabolism and lower energy status at thermal extremes. We quantified metabolite profiles in stonefly nymphs under varying temperatures and oxygen levels. Under normoxia, the concept of oxygen limitation applies to the insects studied. Shifts in the metabolome of heat-stressed stonefly nymphs clearly indicate the onset of anaerobic metabolism (e.g., accumulation of lactate, acetate, and alanine), a perturbation of the tricarboxylic acid cycle (e.g., accumulation of succinate and malate), and a decrease in energy status (e.g., ATP), with corresponding decreases in their ability to survive heat stress. These shifts were more pronounced under hypoxic conditions, and negated by hyperoxia, which also improved heat tolerance. Perturbations of metabolic pathways in response to either heat stress or hypoxia were found to be somewhat similar but not identical. Under hypoxia, energy status was greatly compromised at thermal extremes, but energy shortage and anaerobic metabolism could not be conclusively identified as the sole cause underlying thermal limits under hyperoxia. Metabolomics proved useful for suggesting a range of possible mechanisms to explore in future investigations, such as the involvement of leaking membranes or free radicals. In doing so, metabolomics provided a more complete picture of changes in metabolism under hypoxia and heat stress.

Warm acclimation and oxygen depletion induce species-specific responses in salmonids

“…The oxygen and capacity limited thermal tolerance (OCLTT) model was developed to accommodate such observations (Pörtner, 2010). In agreement with the model, there are findings that temperature tolerance decreases in hypoxic conditions (Verberk et al, 2013) and that high temperatures reduce hypoxia tolerance (e.g. Capossela et al, 2012;McBryan et al, 2013;Lapointe et al, 2014).…”

Metabolomics reveal physiological changes in mayfly larvae (Neocloeon triangulifer) at ecological upper thermal limits

“…We found malic acid, an intermediate metabolite in the TCA cycle increased in response to thermal challenge. The disruption of TCA cycle under thermal challenge was also found in other published research (Michaud et al, 2008; Verberk et al, 2013b). This perturbation indicates glycolysis as an important energy source under thermal challenge for maintenance as oxidative phosphorylation from TCA cycle was contributing less energy.…”

A role for haemolymph oxygen capacity in heat tolerance of eurythermal crabs

“…Beyond the optimal range, warming causes a reduction of oxygen availability to tissues and constrains cellular aerobic metabolism. The augmented demand, mainly in mitochondria, exceeds the rate of oxygen supply, and at the critical temperature (Tc), is increasingly covered by anaerobic metabolism leading to end products such as succinate and L-lactate (Zielinski and Pörtner, 1996; Sommer et al, 1997; Pörtner et al, 1999; van Dijk et al, 1999; Frederich and Pörtner, 2000; Peck et al, 2004; Verberk et al, 2013). Within the thermal range the oxygenation levels of arterial and venous blood are maintained supporting the sustenance of functions.…”