Aquatic ectotherms under global warming: The hidden role of anaerobic processes in metabolic response

Abstract

In a warming world, acclimation of metabolic rate in aquatic ectotherms has been pointed out as a critical process to offset the temperature-induced increase of energy requirements and the associated higher oxygen demand. However, this paradigm is drawn from measurements of aerobic energy production as a proxy for metabolic rate, thus overlooking the contribution of anaerobic energy production. Here, we combined individual-based respirometric and calorimetric measurements to assess the metabolic rate of the ubiquitous ectotherm Gammarus pulex, a key detritivore species severely impacted by warming of freshwater streams. We assess aerobic and total metabolism after a shift from an optimal temperature (12°C) up to a suboptimal temperature (22°C). We showed that exposure to warming decreases aerobic metabolic rate while total metabolic rate remains constant and is associated with higher lactate body content. Our finding suggests that the reduction of oxygen consumption reflects a shift to anaerobic processes rather than an overall metabolic decrease often referred to as metabolic compensation. Our results urge a critical assessment of anaerobic contribution to the thermal response of ectotherms at the level of populations and communities to enhance prediction of the response of aquatic populations under global change.