Metabolic rate can be measured in a number of ways. Measuring heat production, known as “direct calorimetry,” is the most straightforward theoretically. Indirect methods, such as using respirometry to measure oxygen consumption or carbon dioxide production, can be simpler to implement. The method in use here at the lab is known as coulometric respirometry.

Coulometric Respirometry: Principles

The general idea has been around since at least the 1930s. The apparatus consists of a sealed chamber that contains an organism, a pressure sensor (P Sensor), something that absorbs exhaled carbon dioxide (CO₂ Absorb), and an electrolyte solution to generate oxygen (O₂ Gen). As the organism breathes, it consumes oxygen and releases carbon dioxide. Because the CO₂ is removed by the absorbent, pressure in the chamber decreases steadily.

Outside the chamber is a controller (symbolized by an X inside a circle),which compares the pressure inside the chamber with a pre-set target pressure (Set Point). When the pressure inside the chamber decreases below the Set Point, the controller activates the Current Source to send current into the electrolyte solution in the O₂ Generator. The current through the O₂ Generator releases oxygen from the solution in proportion to the number of electrons that pass through the circuit.

Measuring oxygen production (and therefore consumption) is a simple matter of measuring the current used by the O₂ generator then using fundamental chemistry to convert current into the number of electrons, and then number of electrons into number of O₂ molecules.

Details regarding the chambers, pressure sensors, controllers, and recording devices can differ dramatically (see the selected references below). For my system, I use the Bosch BME280 miniature pressure, temperature, and humidity sensor controlled by an Arduino Nano Every, with all data sent to a desktop computer via USB.

Advantages and Limitations

Coulometric respirometry offers a number of advantages compared to other respirometry methods used for small organisms. An important advantage is cost. Each channel (chamber, sensor, controller) costs less than $100 US, and a system capable of measuring eight organisms at once, with cables, USB hub, and racks, can be assembled for less than $1000 US.

The system is also modular. Each channel is self contained, with a chamber and controller, and because it connects to the computer via USB, a single computer can operate many chambers. The number of channels is essentially limited by the capacity of the climate control system.

Oxygen consumption is measured by the amount needed to maintain pressure, so a chamber of any shape can be used and it is not necessary to know the exact volume. One should be mindful to match the chamber size to the animal, so that the subject fits comfortably but the chamber is small enough for oxygen consumption to to generate a measurable signal.

One final, but important advantage of this respirometer is that the accuracy of the measurement depends only on the signal of the BME 280 sensor that measures pressure inside the chamber and the sense resistor that measure the current through the oxygen generator. Unlike respirometers that depend on gas sensors, for either oxygen or carbon dioxide, neither the pressure sensor nor the sense resistor require calibration over the course of months and possibly years. This means that the respirometer can be packed up and used anywhere without worrying about accuracy.

The primary limitations of the coulometric respirometer stem from the need for stability of temperature and humidity inside the chamber. If there is drift of temperature or pressure, the pressure sensor will interpret this as respiration, and the result will be inaccurate. Therefore, experiments need to be performed with adequate temperature control (usually an incubator or water bath), and chambers need to be equilibrated (about 90 minutes) before measurements begin.

Learn More

Selected References

Heusner, A. A., Hurley, J. P. and Arbogas, T. R. (1982). Coulometric Microrespirometry. Am. J. Physiol. 243, 185–192.

Hoegh-Guldberg, O. and Manahan, D. T. (1995). Coulometric measurement of oxygen-consumption during development of marine invertebrate embryos and larvae. J. Exp. Biol. 198, 19–30.

Lighton, J. R. B. (2019). Measuring Metabolic Rates. 2nd ed. Oxford: Oxford University Press.

Sandstrom, D. J. and Offord, B. W. (2022). Measurement of oxygen consumption in Tenebrio molitor using a sensitive, inexpensive, sensor-based coulometric microrespirometer. Journal of Experimental Biology 225, jeb243966.

Tartes, U., Kuusik, A. and Vanatoa, A. (1999). Diversity in gas exchange and muscular activity patterns in insects studied by a respirometer-actograph. Physiological Entomology 24, 150–157.

Villasenor, J., Perez, M., Fernandez, F. and Puchalski, C. (2011). Monitoring respiration and biological stability during sludge composting with a modified dynamic respirometer. Bioresource Technology 102, 6562–6568.

Werthessen, N. T. (1937). An apparatus for the measurement of the metabolic rate of small animals. J. Biol. Chem. 119, 233–239.