A novel technique for the precise measurement of CO2 production rate in small aquatic organisms as validated on aeshnid dragonfly nymphs

Abstract

The present study describes and validates a novel yet simple system for simultaneous in vivo measurements of rates of aquatic CO2 production (M CO2) and oxygen consumption (M O2), thus allowing the calculation of respiratory exchange ratios (RER). Diffusion of CO2 from the aquatic phase into a gas phase, across a hollow fibre membrane, enabled aquatic M CO2 measurements with a highprecision infrared gas CO2 analyser. M O2 was measured with a PO2 optode using a stop-flow approach. Injections of known amounts of CO2 into the apparatus yielded accurate and highly reproducible measurements of CO2 content (R2=0.997, P<0.001). The viability of in vivo measurements was demonstrated on aquatic dragonfly nymphs (Aeshnidae; wet mass 2.17 mg-1.46 g, n=15) and the apparatus produced precise M CO2 (R2=0.967, P<0.001) and MO2 (R2=0.957, P<0.001) measurements; average RER was 0.73± 0.06. The described system is scalable, offering great potential for the study of a wide range of aquatic species, including fish.